Czech Technical University In Prague

Faculty of Mechanical Engineering

Summary of Dissertation

Vapor Phase Soldering Device

Department of Instrumentation and Control Engineering

Supervisor : Prof.Ing IVAN UHLIR.DrSc

Ing. Do Mai Lam

Prague 6/2011

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ACKNOWLEDGEMENTS

Thanks parents who gave countless sacrifice and did every

effort in reach to nurture me and provided the highest moral values.

My sincere regards to my advisor in the present work, Professor Ing.

Uhlíř Ivan DrSc, Ing. Novák Martin Ph.D, doc. Ing. Chyský Jan CSc

who always helped and supported me in the time when I needed

them. Thanks Professor doc. Ing. Janovec Jiří CSc who helps me to

check quality of my products. I am also thankful to the committee

members for their valuable suggestions in bringing out the present

work. This letter would remain incomplete if I did not mention the

support, sacrifice and affection of my friends. Their presence always

gave me strength and courage to overcome every hurdle I faced in

these years.

Ing.Do Mai Lam

10/06/2011

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Table of ContentsAbstract 8Chapter I 8Introduction 8

1.1 Soldering..................................................................................................81.2 Vapor phase soldering.............................................................................9

Chapter II 11Literature review 11

2.1 Fact and fiction in lead free soldering.....................................................112.3 High temperature effect to solders.........................................................132.4 Comparative Wetting Ability of Lead-Free Alloys...................................152.5 Soldering in oxygen and soldering in moisture.......................................202.6. Process Conditions of Electronic Assembly..........................................212.7 Method for constructing electronic circuits.............................................212.7.2 Surface-mount technology..................................................................232.8 Soldering process method......................................................................252.9. Types of Soldering Method...................................................................282.10 Heating/Cooling Rate of Assembly.......................................................36

Chapter III 40List of goals 40

3.1 Global goal.............................................................................................403.2 List of goals............................................................................................42

Chapter IV 43Model heat transfer in vapor phase soldering 43

4.1 Choice of vapor phase soldering method...............................................434.2 Model heat transfer in preheat process..................................................444.3 Heat transfer at reflow process..............................................................49

Chapter V 59Vapor phase soldering device with peltier heater 59

5.1 Peltier.....................................................................................................595.2 Structure of soldering device with peltier heater.....................................615.3 Supply energy calculation for heater......................................................635.4 Running and result.................................................................................65

Chapter VI 69Vapour phase soldering with resistance heater 69

6.1 Structure.................................................................................................696.2 Supply energy calculation for heater......................................................736.3 Performance test....................................................................................74

Chapter VII 76Measurements of heat transfer by thermal camera 76

Chapter IIX 80Experiment with electronic elements and printed circuit board. 80

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8.1 Experiment soldering at good profile temperature..................................818.2 Experiment at slow rate and maintaining the peak temperature for a long time..............................................................................................................828.3 Experiment at fast rate and maintaining the peak temperature for a short time..............................................................................................................838.4 Experiment soldering both sides............................................................848.5 Experiment to measure the difference in temperature at the center and corner of printed cicuit board........................................................................868.6 Experiment measure temperaure at different distance from liquid.........888.7 Soldering small electronic elements.......................................................898.8 Experiment bridging, wetting force, de-wetting.......................................90

Chapter IX 92Checking quality of solder joint 92

Chapter X 95List of result and novelty 95

Chapter XI 97Conclusion 97Chapter XII 99Future Work 99

LIST OF PUBCLICATION 100REFERENCES 101

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List of Figure

Figure 1 Vapor phase soldering..........................................................................10Figure 2 Liquidus temperature of some lead-free solders compared with that of Sn63 Pb37 ..........................................................................................................12Figure 3 Thermal Fatigue of Solder Joints .........................................................13Figure 4 Poor solderability...................................................................................15Figure 5 Good solderability..................................................................................16Figure 6 Wetting time of lead-free solders at 245°C ...........................................17Figure 7 Wetting force of lead-free solders at 245°C..........................................18Figure 8 Comparison of maximum wetting force of lead-free solders.................19Figure 9 Dross formation in the solder pot as a function of oxygen concentration............................................................................................................................20Figure 10 Through-hole technology.....................................................................22Figure 11 Surface-mount technology..................................................................23Figure 12 Schematic diagram showing the principle of reflow soldering.............25Figure 13 Schematic diagram showing the sequence of wave soldering ...........26Figure 14 Soldering Iron Method.........................................................................29Figure 15 Hot air soldering method.....................................................................30Figure 16 Laser soldering method.......................................................................30Figure 17 pulse soldering method.......................................................................31Figure 18 IR method . Sketch of an IR-soldering furnace...................................32Figure 19 Convection Reflow Method.................................................................33Figure 20 Vapor phase soldering........................................................................35Figure 21 Lead-free soldering profile...................................................................37Figure 22 Typical setup for Plastic Ball Grid Array (PBGA) ................................41Figure 23 Heat transmission paths for difference methods ...............................43Figure 24 Vapor phase soldering device.............................................................45Figure 25 Heat transfers in preheat process at 0 second....................................45Figure 26 Heat transfers in preheat process at 40 seconds................................46Figure 27 Heat transfers in preheat process at 80 seconds................................46Figure 28 Heat transfers in preheat process at 120 seconds..............................47Figure 29 Heat transfers in preheat process at 160 seconds..............................47Figure 30 Contour temperature at 160 seconds..................................................48Figure 31 The point temperature graph at PCB in preheat process....................48Figure 32 PCB with electronic element and solder..............................................50Figure 33 Heat transfer in reflow process at 0 second........................................50Figure 34 Heat transfer in reflow process at 1 second........................................51Figure 35 Heat transfer in reflow process at 2 seconds......................................51Figure 36 Heat transfer in reflow process at 5 seconds......................................52Figure 37 Heat transfer in reflow process at 10 seconds....................................52

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Figure 38 Temperature at the center of a slice of electronic element and a solder joint at 10 seconds..............................................................................................53Figure 39 The temperature profile of the solder and the PCB in reflow process. 54Figure 40 Heat transfers in cooling process at 0 second.....................................55Figure 41 Heat transfers in cooling process at 4 seconds...................................56Figure 42 Heat transfers in cooling process at 8 seconds...................................56Figure 43 Heat transfers in cooling process at 16 seconds................................57Figure 44 Heat transfers in cooling process at 30 seconds.................................57Figure 45 The point temperature in cooling process...........................................58Figure 46 Semiconductor combination................................................................59Figure 47 Peltier Effect........................................................................................60Figure 48 Series of peltier...................................................................................60Figure 49 Structure soldering device with peltier heater......................................61Figure 50 Real device in the lab..........................................................................62Figure 51 Temperature at different voltage and current......................................66Figure 52 Experiment increases temperature to boiling temperature..................66Figure 53 structure of soldering device with resistance heater............................69Figure 54 Solder tank..........................................................................................70Figure 55 Real device in the laboratory...............................................................71Figure 56 Block diagram of the device................................................................71Figure 57 Experiments at different heating up rate.............................................74Figure 58 Heat transfer at 0 sencond..................................................................76Figure 59 Heat transfer in PCB when center of PCB is 35oC and 39oC...............76Figure 60 Heat transfer in PCB when center of PCB are 51oC and 59oC............77Figure 61 Heat transfer in PCB when center of PCB are 84.7oC and 99.6oC......77Figure 62 Heat transfer in PCB when center of PCB are 120oC and 124oC........77Figure 63 Heat transfer in PCB when center of PCB are 139oC..........................78Figure 64 Heat transfer in PCB when center of PCB are 160oC and 170oC........78Figure 65 Soldering at good profile temperature.................................................81Figure 66 Solder joint as good profile temperature.............................................82Figure 67 Long peak temperature profile............................................................82Figure 68 Solder joint as maintaining the peak temperature for a long time.......83Figure 69 Keep Peak temperature time less than 4 seconds..............................83Figure 70 Soldering with short peak temperature................................................84Figure 71 Position of thermocouple.....................................................................84Figure 72 Difference temperature at top side and bottom side............................85Figure 73 Bottom side.........................................................................................85Figure 74 Top side..............................................................................................86Figure 75 Position of thermocouple.....................................................................86Figure 76 Difference temperature in center and corner of PCB and the liquid....87Figure 77 soldering at difference posiion.............................................................87Figure 78 Position of thermocouple.....................................................................88Figure 79 Soldering small electronic elements....................................................89Figure 80 Experiment bridging joint.....................................................................90Figure 81 De-wetting experiment........................................................................91Figure 82 Left and right sections of the solder joint after cut, a step is 500µm....92

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Figure 83 Left and right sections of the solder joint after cut, a step is 100µm....92Figure 84 Solder joint bettwen solder and copper on the left and solder on the right. A step is 50 µm...........................................................................................93Figure 85 Solder joint bettwen solder and electronic element on the left and solder on the right at scale 50 µm.......................................................................93Figure 86 Solder layer on the top of the electronic element on the left and on the right. A step is 50 µm...........................................................................................93

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Abstract

Soldering has become the major and conventional method in the electronic assembling industry for joining electronic components. The industry is seeking for the use of environment friendly lead-free solders.

A system is provided for vapor phase soldering of components and printed circuit board(PCB), system controls increasing speed and cooling speed temperature of the components and PCB lands that are to be soldered together.

Vapor phase soldering (VPS) uses the latent heat of liquid vaporization to provide heat for soldering. This latent heat is released as the vapor of the inert liquid condenses on components and PCB lands. The peak soldering temperature is the boiling temperature of the inert liquid at atmospheric pressure.

Chapter I

Introduction1.1 Soldering

Soldering is one of the oldest methods of joining two pieces of metal together.Historically, the technique dates back more than two thousand years. In a very simple soldering process, the space between the metals to be joined is filled with an alloy, normally a mixture of two or more pure metals, which has a lower melting temperature than the metals to be joined. Heat is applied so that the alloy melts around the metals to be joined and, upon solidification, forms a permanent joint between them.

The alloy, called the solder, plays an important role in deciding the quality of the resulting joint. Some years ago, the soldering industry usually utilized lead-based solders for majority of applications. Traditionally, the lead-based solders are tin-lead alloys of eutectic composition. These solders have been in use in the soldering industry for many decades and proved to be an excellent soldering material to suit the majority of industrial requirements. Owing to the fact that lead-based solders have a very high concentration of toxic lead, they are seen as hazardous to the environment and must, therefore, be changed for eco-friendly

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materials. Alternate lead-free solders have been immensely studied over the last two decades, because of the environmental and health concerns.

The reflow process window for traditional tin-lead solder is relatively wide. The melting point of tin-lead solder is 183oC. The lower temperature limit for reflow is 200oC. The upper limit is generally 235oC, which is the maximum temperature that most components can be exposed to. These high and low temperature limits provide a process window of over 30oC.

The most common lead-free solder alloy in the United States and Europe consists of 95.6 percent tin, 3.7 percent silver and 0.7 percent copper. This alloy has a melting point of 217oC. According to current practice, it needs a minimum reflow temperature of 245oC to ensure good wetting. The maximum reflow temperature is 260oC. This leaves a process window of only 20oC. [24]

Despite some components performing satisfactorily after being exposed to a temperature of 260oC, there are many reasons to limit reflow temperatures. Limiting reflow temperature minimizes thermal stress on boards and components, reducing the possibility of manufacturing defects. High temperatures can cause enormous stress on plated through-holes and barrels, resulting in cracking. High first-pass temperatures on double-sided assemblies increase the amount of second-side oxidation, causing solderability problems on the second pass. Limiting peak temperatures reduces intermetallic growth. It also limits the possibility of “popcorning” of components with high moisture content. Presently, almost all soldering machines are optimized for tin-lead solder but current industrial requirements seek the use of lead-free solder. This therefore is the reason which has pressed researchers to develop the soldering process to meet the requirements of lead-free solder, to reduce the maximum temperature and soldering defects.

In the industrial world of today, soldering has become vital for theinterconnection and packaging of almost all electronic devices and circuits. The fast changing technology and increasing miniaturization of electronic devices places a challenge for finding reliable and successful component joints. This is one reason which has forced researchers to study and develop the soldering process to meet the needs and goals of modern day solder interconnects.

1.2 Vapor phase soldering

Vapor phase soldering (VPS), sometimes referred to as condensation soldering, has gone through changes in popularity. It was the method of choice in the early 1980s.

VPS uses the latent heat of liquid vaporization to supply heat for soldering. This latent heat is released as the vapor of the inert liquid condenses on

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component leads and PCB lands. The liquid creates a dense, saturated vapor that displaces air and moisture in VPS. The temperature of the saturated vapor zone is equal to the boiling point of the vapor phase liquid. The fluid does not cause any environmental problems. The peak soldering temperature is the boiling temperature of the inert liquid at atmospheric pressure.

VPS heats uniformly, and no part on the board goes beyond the fluid-boiling temperature. The method is appropriate for soldering odd-shaped parts, flexible circuits, pins, connectors and for reflowing tin/lead and lead-free surface mount package leads [3].

Figure 1 Vapor phase soldering

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HEATER

VAPOR

PCB

LIQUID

COOLING TUBE

Chapter II

Literature review2.1 Fact and fiction in lead free soldering

The electronics industry’s transition to lead-free soldering has been marked by misunderstandings about the properties that are key in a lead-free solder and about the consequences for soldering processes and solder joint reliability. The realities that have materialized from the practical implementation of lead-free soldering are reviewed and what that means for the considered alloy selection. Particular attention is given to the advantages of replacing tin-lead, which behaves as almost perfect eutectic, with lead-free solders that also behave as a eutectic.

The change from tin-lead to lead-free has been upsetting for the electronics industry both in expectation and in implementation and perhaps as a on sequence has been marked by misunderstandings and misinformation.

This is perhaps comprehensible given that the industry has been asked to give up the material that has been the foundation for the assembly of electronic circuitry since there was such a thing as electronic circuitry. Although far from a perfect material for creating joints between the individual parts of a circuit it was for the electronics industry “the devil you know” and in the many years of its use techniques were discovered for compensating for or accommodating its weaknesses as well as benefiting from its strengths. It might be said in retrospect, nevertheless, that the misunderstandings and misinformation were a result of the absence of understanding of some of the essential features of solders and soldering processes. It is for that reason that many people feel that the electronics industry will come out from the transition to lead-free stronger than it went in it; the change has compelled the industry to look at solders and soldering processes more carefully than they have before and as a result to learn more about what is important. It has to be acknowledged too that some of the misunderstandings and misinformation were the result of deliberate mischief on the part of those who opposed the change as just another non-tariff barrier created by European bureaucrats. It was in the interest of such people to make that change appear to be as difficult, expensive, and dangerous as possible. By contrast, although the challenge that it faced was no less daunting, the Japanese electronics industry willingly decided to get rid of lead from its solders, not because of any legislation that needed it but because it realized that if electronics were going to be economically recycled, the presence of lead would be an expensive complication. In a country such as Japan with limited waste disposal capacity there was general support in the electronics industry for the legislation on recycling that was implemented in 2002 in spite of the consequent need to get

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rid of lead. It is critical that the melting point of lead-free solder be as close as it can to that of the tin-lead solder it replaces.

As the industry began to consider the problem of finding a lead-free solder some alarm was caused by the awareness that there were no element in the Periodic Table that would decrease the melting of tin as low as did lead, i.e. from 232°C to 183°C, that did not have some undesirable complications. Those complications include limited availability, which is reflected in cost, reduced recyclability and reduced reliability. There was really no choice for the primary constituent of lead-free solder- its relatively low melting point and its ability to form intermetallic compounds with all of the metals that the industry needed to solder made tin the obvious choice. Bismuth at the level of 57% could lower

The melting point further to 139°C even though the resulting alloy is difficult to use. Zinc at the level of 9% lowers the melting point to 198°C and while there has been some successful commercial application of alloys based on this eutectic problems arise from the relatively high reactivity of that element.

The addition with least complications that produces an alloy with many properties very similar to those of tin-lead, copper reduces the melting point only a few degrees to 227°C, still 44°C higher than that of tin-lead solder. The addition of silver to tin-copper reduces the melting point a further 10°C to 217°C but that is still 34°C higher than that of tin-lead solder. Those concerned that 217°C was still too high for a practical tin-lead replacement made further additions of elements such as bismuth and indium but these increased the cost. Practical experience has made it known however, that a melting point close to that of tin-lead was not as vital as first thought.

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Figure 2 Liquidus temperature of some lead-free solders compared with that of Sn63 Pb37 [11]

2.3 High temperature effect to solders

Lead-free solder alloys normally have melting points that are 30-70°C higher than the melting point of lead alloys. When using the lead-free solder alloys, we have to use higher temperatures in soldering, which influence the reliability of components, PCB, and solder joints.

Higher melting point of lead-free solder affects reliability of electronic equipment manufacturing through a number of issues such as:

Thermal fatigue of solder joints Delamination of multi-layer PCBs Popcorn failures of IC packages Degradation/Damage to heat sensitive components[54]

2.3.1. Thermal Fatigue of Solder Joints

Thermal fatigue is defined as thermal expansion of a section or the entirety of parts put together leading to degradation or cracking of materials brought about by repeated heating and cooling.

Thermal cycling during the testing or normal working life of boards can cause thermal fatigue of solder joints. Thermal fatigue reduces the life of lead-free solder joints. If thermal fatigue is too much, it can damage or crack the lead-free solder joints.

Figure 3 Thermal Fatigue of Solder Joints [54]

Thermal fatigue leads to serious reliability issues in case of solder joints of surface mounts components.

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2.3.2. Delamination of Multi-layer PCBs

Delamination, unfortunately, is a common problem for nowadays circuit boards, especially high-density, multi-layer boards or those composed of high profile materials. Boards are probably delaminating because, during processing, they are thermally and/or mechanically stressed beyond their limits of adhesion.

The risk of delamination increases with moisture absorption by PCBs.[54]

2.3.3 Popcorn failures

Bad storage, handling, or packaging of plastic encapsulated semiconductor devices can cause the introduction of moisture. Moisture trapped inside plastic encapsulated packages can damage them during soldering, as the moisture vaporizes and tries to expand. The expansion of trapped moisture can lead to internal separation (delamination) of the plastic from the die or lead-frame, wire bond damage, die damage, and internal cracks. Most of this damage is not visible on the component surface. In extreme cases, cracks will extend to the component surface. In the most severe cases, the component will bulge and pop. This is known as the “popcorn” effect. Plastic package containing moisture can also result in external steam jets from the package, which may displace other nearby components on the circuit board during the solder process.[54]

2.3.4. Degradation/Damage to Heat Sensitive Components

Due to the fact that most lead-free solder alloys have higher melting point (30-70oC more) than conventional lead based solders, more temperature is required during component soldering using lead-free alloys. Applying high temperature at component leads can create thermal stress on components [54].

Thermal stress on heat sensitive components can lead to:

Component failures Variance in component parameter values Reduction in component life expectancy, ie., components wear out

early

Therefore to decrease the effect of high temperature, I designed the machine to reduce the peak temperature during the soldering process.

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2.4 Comparative Wetting Ability of Lead-Free Alloys

Wetting Properties of SolderWetting is an important feature of solders, because reliable

interconnection requires good wetting. The liquid solder is sometimes kept from spreading away by surrounding the surface around the confines of the joint with a solder mask. The capability of liquid to wet a solid surface is measured in terms of the contact angle. The contact angle is the angle that the liquid solder makes with the solid base metal surface. Figure below illustrates the relationship between the contact angle, θ and the degree of wetting. A contact angle of more than 90 degrees shows lack of wetting affinity. To ensure good wetting, the molten solder must wet the contact surface with a contact angle that is not greater than 10 degrees.

Figure 4 Poor solderability

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Figure 5 Good solderabilityVery good wetting when θ → 0o

Figure 4,5 Representation of the degree of wetting in terms of the contact angle, θ

Majority of lead-free solders seem to demonstrate inferior wetting on Cu than Sn-Pb near eutectic alloy solders. It is well known fact that small amounts of certain impurities in the solder alloy influence the degree of wetting of the molten solder and the mechanical properties of an interconnection. While addition of Ag slightly promotes wetting on Cu, addition of Bi betters wetting significantly. Improved fluxes have also demonstrated enhanced wetting behavior of solders [5,6].

Usually the wetting force of lead-free solder is not as strong as for the tin-lead solder. Comprehending the wetting kinetics of lead-free alloys becomes vital in selecting an appropriate lead-free composition for assembling PCBs. For the last two decades, the intrinsic wetting ability of solder alloys has not been the subject of study and discussion for the reason that SnPb eutectic (63Sn37Pb) has been a commonly used composition and no choice needed to be made. With the advent of lead-free alloys, the understanding of the wetting kinetics of lead-free alloys becomes crucial to the selection of a suitable lead-free composition for assembling circuit boards, thus the assurance of overall quality of the solder joints as well as the yield of circuit board manufacturing. Intrinsic wetting ability of solder alloys is an important performance property, which directly affects the integrity of solder interconnections. This intrinsic wetting ability also controls the production yield and throughput under the dynamic soldering process — wave soldering or reflow soldering. Additionally, it is well known that the solderability of substrates for both PCBs and components by a given solder alloy in conjunction with the effectiveness of flux chemistry contributes to the quality of solder joints.

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The test below evaluates the relative wetting performance among other selected lead-free alloys, which are seen as the most viable candidates based on combined performance properties. The alloys under this study include: 99.3Sn/0.7Cu, 96.5Sn/3.5Ag, 93.5Sn/3.5Ag/3.0Bi,95.5Sn/4.0Ag/0.5Cu, 96Sn/2.5Ag/1.0Bi/0.5Cu, 88Sn/3.5Ag/4.5Bi/4.0In, 95Sn/0.5Cu/0.5Ga/4In, 91.4Sn/4.1Ag/0.5Cu/4.0In with Sn63/Pb37.

In this test, wetting time (t), wetting force (F), the effect of temperature on wetting time and wetting force were studied for the lead-free alloys of interest. Wetting time < one second for wave soldering; wetting time < two seconds for reflow, the wetting time tested under the above described conditions being less than one second is regarded as a good wetting phenomenon corresponding to a quality wave soldering process.[12]

Figure 6 Wetting time of lead-free solders at 245°C [12]

Figure above evaluates the relative wetting time performance among the selected lead-free alloys. It is noted that at a temperature of 245°C, 63Sn/37Pb has the wetting time less than lead-free.

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Figure 7 Wetting force of lead-free solders at 245°C[12]

Wetting force measurements were achieved at 2.0 seconds at soldering temperature of 245°C. It is worthy to note that Lead-free solder have wetting force less than tin-lead solder

Wetting time of lead-free alloys varius soldering temperature.

Table 1 Wetting Time of Lead-free Alloys various Temperature [12]

Solder tamp °C 235 245 255 265Wetting time (seconds)

63Sn/37Pb 0.767 0.606 0.546 0.4699.3Sn/0.7Cu 1.411 1.034 0.682 0.16596.5Sn/3.5Ag 2.189 1.352 1.05 0.7495.5Sn/4.0Ag/0.5Cu 3.368 1.946 1.284 1.04896.5Sn/3.5Ag/3.0Bi 3.173 1.669 0.814 0.65395Sn/0.5Cu/0.5Ga/4In 1.758 1.542 1.502 0.85188Sn/3.5Ag/4.5Bi/4In 0.949 0.791 0.569 0.47691.4Sn/4.1Ag/0.5Cu/4In 1.156 0.716 0.544 0.24496Sn/2.5Ag/1.0Bi/0.5Cu 1.86 1.235 0.824 0.668

Table 1 summarizes the wetting ability of the selected lead-free alloys in response to soldering temperature. It notes that tin-lead soldering wetting time is less than lead-free at various temperatures.

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Figure 8 Comparison of maximum wetting force of lead-free solders.[12]

Maximum wetting force: Figure above shows the maximum wetting force at two seconds over the temperature range of 235°— 265°C. Note that tin-lead have the highest wetting force

The wetting time and wetting force indicate the relative wetting performance among the lead-free alloys. More pronouncedly, they correlate to the spread and fillet formation under a given soldering process. As expected, the process temperature affects the wetting results and an alloy's wetting capability rises with increasing temperature until approaching a relatively stable state.

Normally lead-free solder has wetting force less than tin-lead solder and lead-free solder has wetting time higher than tin-lead solder.

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2.5 Soldering in oxygen and soldering in moisture

Oxygen is an amazing gas. We would all be dead without it. Oxygen combines with all manner of things and the world is a better place for it

Sometimes oxygen gets in the way. Oxygen and many metals have a great affinity for each other. Soldering involves metals: the wires are metal; the bits on the board and of course the solder itself is metal. We don't really want a lot of oxygen around when soldering. Oxygen and metals form metallic oxides; oxides form much more rapidly in a hot; these oxides shield the conductors from each other, keep the solder from adhering to whatever metal is left, and are of no use.

Oxidation of tin/lead solder in contact with air at the temperature 240oC for 100 seconds. The rate of formation of dross over the molten solder drops with reduced oxygen concentration is shown in the figure below: [14]

Figure 9 Dross formation in the solder pot as a function of oxygen concentration [14]

Oxygen has a negative effect on the fatigue life of the solder interconnections because it attacks grain boundaries and, because of oxide formation, weakens the fatigue resistance of the joints at the fracture tip as the crack propagates during cyclic stress. Under low-cycle fatigue conditions and

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reasonable strains, which prevail in most applications in microelectronics. The fracture mode is generally intergranular, and the existence of oxygen greatly boosts the failure mechanism, such that the lifetime of the joint is considerably shortened.[14]

Moisture induced soldering failures

Moisture evaporates and tries to expand when the package is exposed to rapid high temperatures during soldering. The internal pressure can result in the separation of the plastic encapsulate from the semiconductor chip, internal and external cracks, and damage to thin films and wire bonds. In severe cases, soldering may cause an integrated circuit to bulge and then explode with an audible pop. In order to decrease the effects of moisture-induced stress during soldering, it is recommended that some of its moisture is baked out before reflow soldering and soldering without moisture. My goal therefore is to design a soldering machine to solder in the inert zone and also protect solder joints from oxygen and moisture. [55]

2.6. Process Conditions of Electronic Assembly

Current processing equipment and conditions for electronic assembly are optimized for Sn-Pb solders. Any new conditions for lead-free alloys must make sure both productivity and reliability are at least equivalent to present level of Sn-Pb solders. One of the most sensitive parameters for the quality of solder joints is soldering temperature. The melting temperature of Sn-Pb eutectic alloy is 183oC, and the typical soldering temperatures are 230oC and 250oC for reflow and wave soldering respectively. The temperature margin between the melting point of the solder and processing temperature is around 50oC for reflow soldering. In contrast, melting temperature for typical lead-free solders are higher than Sn-Pb eutectic solders by about 30oC, which makes the process window narrower. Since some electronic components cannot withstand an increase in reflow temperature, processing requirements need to be developed to incorporateheat-resistant components.[]

2.7 Method for constructing electronic circuits

There are a number of soldering practices employed in the electronic packaging industry. These techniques are specific to particular need and application. Variations in soldering methods arise due to different schemes in applying heat, flux and solder material to the joining components. Nevertheless, all the soldering methods can broadly be categorized into two basic methods. Currently, these methods are predominantly used by the electronic packaging industry.

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There are two basic methods for mounting a component to a printed circuits board, the first is known as through hole technology and the second method is called surface mount assembly

2.7.1 Through-hole technology

It refers to the mounting scheme used for electronic components that involves the use of leads on the components that are inserted into holes drilled in printed circuit boards and soldered to pads on the opposite side either by manual assembly by hand placement or by the use of automated insertion mount machines.

Through-hole technology almost completely replaced earlier electronics assembly techniques, for example point-to-point construction. From the second generation of computers in the 1950s until surface-mount technology became well known in the late 1980s, every component on a typical PCB was a through-hole component.

Although through-hole mounting offers strong mechanical bonds in comparison to surface-mount technology techniques, the extra drilling required makes the boards more expensive to produce. They also limit the available routing area for signal traces on layers immediately below the top layer on multilayer boards since the holes must pass through all layers to the opposite side. Due to the above stated reasons, through-hole mounting techniques are at present usually reserved for bulkier components such as electrolytic capacitors or semiconductors in larger packages [9]

Figure 10 Through-hole technology

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2.7.2 Surface-mount technology

Surface-mount technology is a technique for constructing electronic circuits in which the components are mounted directly onto the surface of printed circuit boards. Electronic devices made by this process are known as surface mount devices or SMDs. In the industry it has largely replaced the through-hole technology construction method of fitting components with wire leads into holes in the circuit board.

An SMT component is usually smaller than its through-hole counterpart because it has either smaller leads or no leads at all. It may have short pins or leads of various styles, flat contacts, a matrix of solder balls (BGAs), or terminations on the body of the component.

Figure 11 Surface-mount technology

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The key benefits of SMT over the older through-hole technique are:

Smaller components. Smallest is currently 0.4 x 0.2 mm. (.01" x .005" - 01005)

Much higher number of components and many more connections per component.

Fewer holes need to be drilled through abrasive boards. Simpler automated assembly. Small errors in component placement are corrected automatically

(the surface tension of the molten solder pulls the component into alignment with the solder pads).

Components can be placed on both sides of the circuit board. Lower resistance and inductance at the connection (leading to

better performance for high frequency parts). Better mechanical performance under shake and vibration

conditions. SMT parts generally cost less than through-hole parts. Fewer unwanted RF signal effects in SMT parts when compared to

leaded parts, yielding better predictability of component characteristics. Faster assembly. Some placement machines are capable of

placing more than 136,000 components per hour.

Main disadvantages of SMT over the older through-hole technique are:

The manufacturing processes for SMT are much more complicated than through-hole boards, raising the initial cost and time of setting up for production.

Manual prototype assembly or component-level repair is more difficult (more so without a steady hand and the right tools) given the very small sizes and lead spacing of many SMDs.

SMDs can't be used directly with breadboards (a quick snap-and-play prototyping tool), requiring either a custom PCB for every prototype or the mounting of the SMD upon a pin-leaded carrier. For prototyping around a specific SMD component, a less-expensive breakout board may be used. In addition, stripboard style protoboards can be used, some of which include pads for standard sized SMD comportments.

SMDs' solder connections may be damaged by potting compounds going through thermal cycling.

Solder joint dimensions in SMT rapidly become much smaller as progress is made toward ultra-fine pitch technology. The reliability of solder joints becomes more of a concern as less and less solder material is allowed for each joint. Voiding is the phenomenon that is usually related with solder joints especially when reflowing a solder paste in the SMT application. The presence of these voids can worsen the joint strength and eventually lead to joint failure [9]. Due to the above mentioned advantages of SMT over through hole technology, I focused my research on soldering applied to SMT.

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2.8 Soldering process method

2.8.1 Reflow Soldering

In reflow soldering, a mixture of solder and flux called the solder paste is applied to every joint to be made in the assembly. Heat is applied by meas of radiation, conduction or convection in a controlled environment. The whole assembly is heated and held at temperature above the melting temperature of solder followed by cooling. During the process the solder melts and fills the gaps around, forming the joint. An added benefit of this approach of soldering components to the printed circuit boards and cards is that there is no geometry dependence, nor is there any limitation to types of components on a board such as surface mount. The operational sequence of a reflow soldering process is:

1. Apply the solder paste (solder and flux) to the joint 2. Apply heat 3. Cool to the room temperature [9]

Figure 12 Schematic diagram showing the principle of reflow soldering

25

2.8.2 Wave soldering

In wave soldering, the printed circuit board populated with the components to be joined is passed across the crest of a molten, standing solder wave. Only the bottom of the board is exposed to the molten solder. The molten solder serves both as the source of heat to the board and components, as well as the solder source for the joints. The operational sequence of a wave soldering machine is:

1. Applying the flux 2. Applying heat 3. Applying solder and heat 4. Cooling to the room temperature

Figure 13 Schematic diagram showing the sequence of wave soldering [9]

2.8.3 Compare wave soldering and reflow soldering

In reflow soldering, the solder is applied as a 'solder paste' typically by using a stencil mask and a squeegee. The solder paste contains both a solder flux and the solder in the form of minute solder balls. The solder paste operation is done before adding the components to the board. The board is then passed through a reflow oven at which time, the solder paste melts to complete the soldering of the components to the circuit board. The reflow oven will have several heating zones designed to preheat, activate the flux, reflow the solder, and cool the board. The oven is setup so the components receive as little thermal shock as possible, and spend only a few seconds above the solder melting temperature.

26

In wave soldering, the components are placed on the board. Then the board is placed in the wave soldering machine. Typically the first step in the wave solder, the bottom of the board passes through a fluxing operation which either sprays flux on the bottom or passes the bottom of the board through flux foam. After the flux, the board will usually pass a pre-heater to activated a flux and pre-heat the board for the soldering. Then, the bottom of the board is passed through a molten solder wave (looks like a smooth waterfall only consisting of molten solder). Afterwards, there will be a stage of cool-down.

Reflow soldering, like wave soldering, is not a new manufacturing process. The hybrid industry has used and refined the art of reflow soldering for many years. However, with the advent of Surface Mount Technology (SMT), reflow soldering has expanded in the number of types and has been studied, refined and explored as never before.

Wave soldering is used for both through-hole printed circuit assemblies, and surface mount. In the latter case, the components are glued by the placement equipment onto the printed circuit board surface before being run through the molten solder wave.

As through-hole components have been largely replaced by surface mount components, wave soldering has been supplanted by reflow soldering methods in many large-scale electronics applications. [23]

27

2.9. Types of Soldering Method

Soldering methods are broadly divided into two types: the partial heating method and the total heating method.

Partial heating method: Heat is applied to the package leads and/or PWB in a localized manner.

There are four types of soldering methods:1. Soldering iron2. Hot air3. Laser4. Pulse heating Partial heating involves less heat stress on the device and printed wiring

board, but is unsuitable for large volume production. Therefore, this method is mainly used to correct soldering or for devices with a low heat resistance.

Total heating method: Heat is applied to the entire package and/or PCB. There are five types of soldering methods:1. Infrared reflow2. Convection reflow3. Infrared convection combined4. VPS (Vapor Phase Soldering)5. Flow (wave) soldering

Because of excellence in productivity and running cost, these types are widely used.

However, this method can place considerable heat stress on the semiconductor device and board.

Select the soldering method best suited to your application by taking into consideration the advantages and disadvantages of each soldering method, as well as the heat resistance of the SMD.[9]

A. Partial heating method

2.9.1 Soldering Iron Method

In this method, the package leads are soldered to the mounting pads on the printed wiring board using a soldering iron and wire solder.

The thermal capacity of the soldering iron used must be determined based on the size and shapes of the places to be soldered and the melting point of the solder.

Care is required, since increasing the temperature more than necessary can lead to degradation due to exceeding heat tolerances, for example peeling of the mounting pads from the printed wiring board.

Since the actual temperatures of the places soldered depend on the heating capacity of the soldering iron (the heat source) and the thermal

28

capacities of the package and mounting board, it is necessary to take these issues into account by, for example, measuring thermal characteristics before starting work. Also, soldering irons with temperature adjustments should be used if at all possible.[9]

Figure 14 Soldering Iron Method

This method applies for manual soldering or repairs in electronic assembly. It is not able to uses in SMT method and mass production.

2.9.2 Hot Air Soldering

This method solders the SMT by heating air or N2 gas with a heater and flowing hot gas from a nozzle onto the joint on the PCB. The temperature is adjusted by adjusting the heat source and/or the flow of gas.[9]

29

Figure 15 Hot air soldering method.

This method is able to uses in manual soldering and repair in electronic assembly. It is not able to uses in SMT method and mass production.

2.9.3 Laser Method

In this method, devices are soldered by heating with a laser beam. The temperature is adjusted by adjusting the intensity of the laser output and by changing the heating time.[9]

Figure 16 Laser soldering method

30

2.9.4 Pulse Heating Method

In this method, the Joule heating that occurs due to a current pulse in the tool is used for soldering. The temperature is adjusted by adjusting the amount of current and the time for which the current is applied. [9]

Figure 17 pulse soldering method

This method not able to uses to solder BGA components.

B. Total Heating Methods

Total heating methods include infrared methods, VPS (vapor phase soldering), and convection methods.

2.9.5. IR Method (IR Reflow)

In this method, components are heated by emitted IR radiation (radiative heating) using an IR heater as the heat source.

  Reflow soldering by infra-red heating, often called infra-red soldering, is used mainly for the soldering of substrates with surface mounted components. Usually, the substrates are conveyed through a machine having a series of heater elements, e.g. rod-shaped radiators positioned transversely to the transport direction. The elements may be placed above the substrates being conveyed, but in many cases there are also elements below the substrates to

31

increase the rate of heating and to improve the homogeneity of the temperature. A possible set-up of such a machine is shown in figure below.[9]

Figure 18 IR method . Sketch of an IR-soldering furnace.

The heating is chiefly characterized by the wavelength of the elements in the machine.

During infrared soldering, the energy for heating up the solder joint will be transmitted by long or short wave electromagnetic radiation.

Benefits

Easy setup No compressed air required No component-specific nozzles (low costs) Fast reaction of infrared source (depends on used system)

Disadvantages

Central areas will be heated more than peripheral areas Temperature can hardly be controlled, peaks cannot be ruled out Covering of the neighbored components is necessary to prevent

damage, which requires additional time for every board

32

Surface temperature depends on the component's reflection characteristics: dark surfaces will be heated more than lighter surfaces

The temperature additionally depends on the surface shape. Convective loss of energy will reduce the temperature of the component

No reflow atmosphere possible[9]

2.9.6 Convection Reflow Method (Air or N2 Reflow)

The basic principle of convection reflow soldering is that an atmosphere (air or N2) heated by a heater is circulated within a furnace and heat is transmitted to the work by convection heating to perform the soldering. The result of this process is that an even temperature distribution is achieved after a fixed time even if there are differences in thermal capacities between the board and components.[9]

Figure 19 Convection Reflow Method

  Here, the heat transfer between gas and the material to be heated is relatively low, the processes using hot gas are in most cases slow. The temperature of gas is set to about 4000 C. The high temperature of the gas is necessary to attain an acceptable heating rate, but this introduces the risk of damaging components in the vicinity of the place to be soldered. If these components are not heat resistant, they can be screened from the hot gas. Hot gas provides a suitable method of heating, especially in small scale production. If this method can be used, it is always a cheap solution. Alumina substrates are usually preheated to a temperature of 1000 C to 1500 C before hot-gas soldering is carried out. The bodies of the components positioned in the stream of gas  are often heated more rapidly than the terminations to be soldered, so that for this reason also overheating may be encountered. The advantage of hot gas soldering is that the components cannot be mechanically damaged. One of the

33

main disadvantages is that the energy transfer is limited and so consequently is the speed of heating. The size of the objects to be heated is also limited. Also, there is a risk of blowing components out of position. A particular field of application is repair or rework of soldered boards, especially surface mounted boards. Here the use of hot gas minimizes the risk of damaging the sensitive pad areas.  

During hot gas soldering, the energy for heating up the solder joint will be transmitted by a gaseous medium. This can be air or inert gas (nitrogen).[9]

Benefits

Simulating reflow oven atmosphere Switching between hot gas and nitrogen (economic use) Standard and component-specific nozzles allow high reliability and

reduced process time Allow reproducible soldering profiles Efficient heating, large heat amounts can be transmitted Even heating of the affected board area Temperature of the component will never exceed the adjusted gas

temperature Rapid cool down after reflow, resulting in small-grained solder joints

(depends on used system)

Disadvantages

Thermal capacity of the heat generator results in slow reaction whereby thermal profiles can be distorted (depends on used system)

A rework process usually undoes some type of error, either human or machine-generated, and includes the following steps:

Melt solder and component removal Residual solder removal Printing of solder paste on PCB, direct component printing or

dispensing Placement and reflow of new component.

34

2.9.7 Vapor phase soldering

Figure 20 Vapor phase soldering

VPS uses the latent heat of liquid vaporization to provide heat for soldering. This latent heat is released as the vapor of the inert liquid condenses on component leads and PCB lands. In VPS, the liquid produces a dense, saturated vapor that displaces air and moisture. The temperature of the saturated vapor zone is the same as the boiling point of the vapor phase liquid. The peak soldering temperature is the boiling temperature of the inert liquid at atmospheric pressure.

VPS does heats uniformly, and no part on the board (irrespective of its geometry) exceeds the fluid-boiling temperature. The process is suitable for soldering odd-shaped parts, flexible circuits, pins, and connectors, as well as for reflowing tin/lead and lead-free surface mount package leads.

The key advantages of vapor phase soldering: - soldering in oxygen free environment (eliminates the need to use nitrogen).

Lower peak temperature making possible to reduce the cost of PCB by 10-15% by using the substrates with lower thermal resistance and to avoid overheating the components. Guaranteed control of the max temperature in the oven due to the physical properties of the liquid used .

Better heat transfer to the PCB due to the use of liquid for that and not air and nitrogen. This reflects in considerable reduction of electrical power consumption.

Flexible and easy control of the PCB temperature until reaching the reflow values.

Faster and easier profiling. Smaller footprint in comparison with convection ovens.

35

HEATER

VAPOR

PCB

LIQUID

COOLING TUBE

Reduction of the voids in the solder joints. For further decreasing the amount of voids, a vacuum section can be added to the vapor phase systems.

Most complex and demanding PCB’s can be soldered with ovens that cost less compared to the forced convection ovens.

The number of soldering defects is usually smaller [9,12]

2.10 Heating/Cooling Rate of Assembly

Reflow soldering is a process in which a solder paste (a sticky mixture of powdered solder and flux) is used to temporarily attach one or several electrical components to their contact pads, after which the entire assembly is subjected to controlled heat, which melts the solder, permanently connecting the joint. Heating may be accomplished by passing the assembly through a reflow oven or under an infrared lamp or by soldering individual joints with a hot air pencil or vapor phase zone.

Reflow soldering is the most common method of attaching surface mount components to a circuit board. The goal of the reflow process is to melt the solder and heat the adjoining surfaces, without overheating and damaging the electrical components. In the conventional reflow soldering process, there are usually four stages, called "zones", each having a distinct thermal profile: preheat, thermal soak (often shortened to just soak), reflow, and cooling. [9]

36

Figure 21 Lead-free soldering profile

Preheat

Maximum slope is a temperature/time relationship that measures how fast the temperature on the printed circuit board changes. The preheat zone is often the lengthiest of the zones and often establishes the ramp-rate. The ramp–up rate is usually somewhere between 1.0 °C and 3.0 °C per second, often falling between 2.0 °C and 3.0 °C per second. If the rate exceeds the maximum slope, potential damage to components from thermal shock or cracking can occur. Solder paste can also have a spattering effect. The preheat section is where the solvent in the paste begins to evaporate, and if the rise rate (or temperature level) is too low, evaporation of flux volatiles is incomplete.[57]

Thermal soakThe second section, thermal soak, is typically a 60 to 120 second

exposure for removal of solder paste volatiles and activation of the fluxes, where the flux components begin oxide reduction on component leads and pads. Too high or too low a temperature can lead to solder spattering or balling as well as oxidation of the paste, the attachment pads and the component terminations. Similarly, fluxes may not fully activate if the temperature is too low. At the end of the soak zone a thermal equilibrium of the entire assembly is desired just before the reflow zone. A soak profile is suggested to decrease any delta T between components of varying sizes or if the PCB assembly is very large. A soak profile is also recommended to diminish voiding in area array type packages.[57]

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Time

Preheat

Thermal soak

Reflow

Cooling

ToC

Reflow ZoneThe third section, the reflow zone, is also referred to as the “time above

reflow” or “time above liquidus” (TAL), and is the part of the process where the maximum temperature is reached. An important consideration is peak temperature, which is the maximum allowable temperature of the entire process. A common peak temperature is 20-40°C above liquidus. This limit is determined by the component on the assembly with the lowest tolerance for high temperatures (The component most susceptible to thermal damage). A standard guideline is to subtract 5 °C from the maximum temperature that the most vulnerable component can sustain to arrive at the maximum temperature for process. It is important to monitor the process temperature to keep it from exceeding this limit. Additionally, high temperatures (beyond 260 °C) may cause damage to the internal dies of SMT components as well as foster intermetallic growth. Conversely, a temperature that isn’t hot enough may prevent the paste from reflowing adequately.

Time above liquidus (TAL), or time above reflow, measures how long the solder is a liquid. The flux reduces surface tension at the juncture of the metals to accomplish metallurgical bonding, allowing the individual solder powder spheres to combine. If the profile time exceeds the manufacturer’s specification, the result may be premature flux activation or consumption, effectively “drying” the paste before formation of the solder joint. An insufficient time/temperature relationship causes a decrease in the flux’s cleaning action, resulting in poor wetting, inadequate removal of the solvent and flux, and possibly defective solder joints. Experts usually recommend the shortest TAL possible, however, most pastes specify a minimum TAL of 30 seconds, although there appears to be no clear reason for that specific time. One possibility is that there are places on the PCB that are not measured during profiling, and therefore, setting the minimum allowable time to 30 seconds reduces the chances of an unmeasured area not reflowing. A high minimum reflow time also provides a margin of safety against oven temperature changes. The wetting time ideally stays below 60 seconds above liquidus. Additional time above liquidus may cause excessive intermetallic growth, which can lead to joint brittleness. The board and components may also be damaged at extended times over liquidus, and most components have a well-defined time limit for how long they may be exposed to temperatures over a given maximum. Too little time above liquidus may trap solvents and flux and create the potential for cold or dull joints as well as solder voids.[57]

Cooling zone

The last zone is a cooling zone to gradually cool the processed board and solidify the solder joints. Proper cooling inhibits excess intermetallic formation or thermal shock to the components. Typical temperatures in the cooling zone range from 30–100 °C (86–212 °F). A fast cooling rate is chosen to create a fine grain structure that is most mechanically sound. Unlike the maximum ramp-up

38

rate, the ramp–down rate is often ignored. It may be that the ramp rate is less critical above certain temperatures; however, the maximum allowable slope for any component should apply whether the component is heating up or cooling down. A cooling rate of 4°C/s is commonly suggested. It is a parameter to consider when analyzing process results. [57]

39

Chapter III

List of goals3.1 Global goal

There has been a relentless world-wide effort by the environmental movement to promote the use of ‘non-toxic’ products instead of lead by the industry. Other substances have taken the place of lead in casting alloys for toys, in addition to solders for certain plumbing applications. Lead-free ammunition is currently available and experiencing a considerable growth in demand, predominantly in the USA where the likelihood of litigation against environmental pollution or employee contacts with harmful materials is high. In addition to this, there are at present a series of proposals across the globe that outline targets for electronic equipment re-use and recycling. Within these initiatives, the use of harmful materials, for example lead, is often restricted so as to enhance the ease of recycling.

Legislation possibly directly affecting the solder and electronic assemblyindustries has been passed by the European Commission in the waste from electronic and electrical equipment (WEEE) and limitation of harmful substances (RoHS) directives outlining targets for electronic equipment re-use and recycling. This legislation also limits the use of hazardous materials to improve the ease of recycling. Europe’s transition took place on July 1st, 2006. Lead in solders for automotive purposes have a temporary exemption from the lead ban.

The Japanese Ministry of Trade (MITI) passed a recycling law for electrical appliances with effect as of April 2001. This suggests but does not include lead phase-out timetable. Original Equipment Manufacturers (OEM's) are getting rid of lead from electronics largely due to market pressures.Despite the fact that there is no federal legislation yet in the US, there are a number of State electronics recycling proposals to consider. The Environmental Protection Agency (EPA) has suggested a crack-down on lead emissions coming from plants that may affect the soldering industry. [2]

One more reason that pushes for the search of a different soldering material is the rising demand of miniaturization. An electronic interconnection is very small in size. A bonding wire is usually of the order of 0.001 inch (25 µm) in diameter [3]. A very high density of these interconnections is required on a substrate. In case of a solder interconnection between two copper pads, it is necessary that the pad is not completely dissolved by the tin in the solder.

Presently, aluminum or copper wiring on a very-large-scale-integration (VLSI) Si chip is 0.5 µm or less. Keeping in mind that the spacing between two

40

wires is also the same as diameter, the pitch becomes about 1.0 µm. For that reason, on a 1 cm2 area of chip, one can have approximately 104 wires. To supply the electrical leads to these wires on the chip, several thousands of input/output (I/O) pads are required on the chip. The only possible way to provide such a high density of I/O pads is to employ area array of tiny solder balls. Assuming the diameter of a solder ball as 50 µm with a 50 µm spacing in between, there will be 10, 000 solder balls on a 1 cm2 chip surface. A typical cross-section of a Plastic Ball Grid Array (PBGA) is presented in Fig 2.1. The presented copper is around 25-30 µm while each ball of the solder is around 100 µm.

Figure 22 Typical setup for Plastic Ball Grid Array (PBGA) [4]

For these miniature parts, properties like strength, reliability, thermal andmechanical fatigue of solder joints are vital for the overall reliability of the device. Traditional lead-base solders are not always able to meet the requirements of industries of today. In this regard, there is a quest to improve the lead-free solders also to improve upon the industry standards [4].

41

3.2 List of goals

Most soldering equipment that had been designed for tin-lead so the

objective of the study: Make a lead-free soldering machine satisfied some

condition below:

The machine can solder lead-free solder paste

Uniform solder joints.

Heating times are short to improve solder quality

Able to solder the small element because now electronic elements

are increasingly smaller

Soldering process in oxygen free

Soldering process without nitrogen to reduce high cost.

Reduce repairs and part replacements.

Maximum cleanliness of the completed assembly.

Flexibility to allow soldering a large number of circuits with

minimum changeover time.

Minimum time above the liquidous solder temperature to reduce

solder grain growth, resulting in a more durable solder joint.

Minimum stress and damage to the Printed Circuit Board (PCB).

Minimum damage and stress to the SMT parts.

Minimum “leaching” of part termination materials.

Reduce peak temperature to increasing life of electronic elements

and reduce PCB cost.

Reduce number of soldering defects

42

Chapter IV

Model heat transfer in vapor phase soldering

The focus of this study is to understand the process of heat transfer in

vapor soldering.

I used Comsol software to model the following processes: the heat transfer from

the heater to the liquid, from the liquid to the vapor zone, from the vapor zone to

the PCB, as well as the cooling process.

4.1 Choice of vapor phase soldering method

From many difficult of lead-free alloy I decided to chosen vapor phase

soldering method cause the reasons bellows

Figure 23 Heat transmission paths for difference methods [9]

As can clearly be seen from the transmission paths, for IR methods (IR

reflow), soldering sections that are in the package shadow are heated indirectly

by transmission heating. Since it is easy for uneven temperatures to occur,

convection methods (air or N2 reflow) are mostly used when soldering is

performed in the areas under packages such as BGA and LGA packages.

Vapor phase can satisfy some condition such as: cleanliness of the

completed assembly, soldering process in oxygen free, soldering process without

nitrogen,

43

Easy to limit peak temperature by choose the liquid boiling temperature

Compare with infrared. Vapor phase soldering solve the problem of

infrared soldering such as: not steady temperature and some components are

overheated temperature can hardly be controlled, peaks cannot be ruled out.

Covering of the neighbored components is necessary to prevent damage, which

requires additional time for every board. Surface temperature depends on the

component's reflection characteristics: dark surfaces will be heated more than

lighter surfaces. The temperature additionally depends on the surface shape.

Convective loss of energy will reduce the temperature of the component

Compare with hot air soldering. vapor phase soldering solve the problem

of hot air soldering such as: thermal profiles can be distorted, Melt solder and

component removal, residual solder removal, printing of solder paste on PCB,

direct component printing or dispensing, placement and reflow of new

component.

Vapor soldering can apply to solder surface mount method and BGA

components.[9]

4.2 Model heat transfer in preheat process

The preheat process involved increasing the temperature of the PCB from

25oC to 150oC. In order to model the preheat process, the assumption that the

vapor phase soldering device depicted in fig. 4.2 was at my disposal was made.

Fig 4.1 Structure of assumed vapor phase soldering device

44

VAPOR

PCB

LIQUIDHEATER

Figure 24 Vapor phase soldering device

The following initial conditions were set in the comsol software: temperature of every element set to 25oC, material of tank is ceramic, liquid is perfluoropolyether and has boiling point of 230oC, vapor is perfluoropolyether, PCB is FR4, solder paste is made up of 95.6 percent tin, 3.7 percent silver and 0.7 percent copper. The results shown in fig.25 to fig.31 were obtained after running the program.

Figure 25 Heat transfers in preheat process at 0 second

45

Figure 26 Heat transfers in preheat process at 40 seconds.

Figure 27 Heat transfers in preheat process at 80 seconds

46

Figure 28 Heat transfers in preheat process at 120 seconds

Figure 29 Heat transfers in preheat process at 160 seconds

47

Figure 30 Contour temperature at 160 seconds

Blue curve the temperature profile at the center top layer of BCB

Green curve the temperature profile at the center bottom layer of BCB

Figure 31 The point temperature graph at PCB in preheat process

48

The results of the preheat modeling shows how heat is transferred from

the liquid to the PCB. The temperature at the bottom layer of PCB is increasing

temperature little faster than the temperature at the top layer of PCB. But the

different temperature between the top layer and the bottom layer is less than

5OC. It satisfy requirement of lead-free soldering. When the temperature of the

PCB is around 150-170OC, the liquid starts boiling. So the soldering process

changes to the reflow process.

4.3 Heat transfer at reflow process

In the reflow process, when the temperature of PCB and electronic

elements is 150oC, the liquid starts boiling. The latent heat of liquid vaporization

provides heat for soldering. The condensed vapor releases heat unto the surface

of the PCB, the electronic elements and solder pastes. The assumption that we

have a PCB with dimensions 30x30 mm, an electronic element, and two solder

pastes comprising 95.6 percent tin, 3.7 percent silver and 0.7 percent copper

was made. Applying the initial conditions and the heat source to the Comsol

software and then running it, the results shown in fig. 32 to fig.39 were obtained.

49

Figure 32 PCB with electronic element and solder

Figure 33 Heat transfer in reflow process at 0 second

50

Figure 34 Heat transfer in reflow process at 1 second

Figure 35 Heat transfer in reflow process at 2 seconds

51

Figure 36 Heat transfer in reflow process at 5 seconds

Figure 37 Heat transfer in reflow process at 10 seconds

52

Figure 38 Temperature at the center of a slice of electronic element and a solder joint at 10 seconds

53

The red curve is temperature graph at corner of PCB

The Blue is temperature graph at center of the solder joint

Figure 39 The temperature profile of the solder and the PCB in reflow process

The results show that the temperature of the solder joint increases rapidly

over the melting point temperature of 217oC. After about 5 seconds the solder

joints temperature increases up to 217oC. The reflow time in peak temperature is

very important since it affects the quality of solder joints. Therefore, the device

must be designed to have a short peak time temperature.

The modeling also shows that the temperature in the PCB is steady. The

temperature at the center is a little bit lower than at the corner. The temperature

at top of the PCB is the same as at the bottom of the PCB (fig 38).

54

4.4 Modeling heat transfer in cooling process.At the start of the cooling process, the temperature of the device is 230OC.

Assuming that we use cooling by cold liquid which flows through a copper tube in

hot liquid, and the temperature of cold liquid is 20oC. After applying the initial

conditions, cooling source and then running the program, the results from fig.40

to fig.45 were obtained.

Figure 40 Heat transfers in cooling process at 0 second

55

Figure 41 Heat transfers in cooling process at 4 seconds

Figure 42 Heat transfers in cooling process at 8 seconds

56

Figure 43 Heat transfers in cooling process at 16 seconds

Figure 44 Heat transfers in cooling process at 30 seconds

57

Green curve is the temperature graph at center of the top layer of the PCB

Blue curve is the temperature graph at center of the bottom of the PCB

Figure 45 The point temperature in cooling process

The results of the cooling process show that the temperature at the top is

cooling down little slower than the temperature at the bottom layer. The PCB

temperature decreases rapidly below 150oC and can therefore be applied as a

cooling method in the vapor phase soldering device.

58

Chapter V

Vapor phase soldering device with peltier heater

5.1 Peltier

Peltier element is a thermoelectric element consisting of semiconductor

materials paired to accomplish heating or cooling processes as a result of peltier

effect. If a voltage is placed on a Peltier element, one side is cooled and the

opposite side simultaneously heats up. Simply by reversing the polarity of the

supply voltage, the hot and cold sites of the Peltier element can be swapped.[19]

Figure 46 Semiconductor combination[19]

59

Figure 47 Peltier Effect[19]

Peltier element can be used in single or cascade combination. The

element is arranged properly to fit the needs. In this project, the element will be

interconnected in series and parallel constructing a cascading layer of peltier

element. Each element and combination in any layer need to be controlled

precisely regarding the temperature curve.

Figure 48 Series of peltier[19]

60

5.2 Structure of soldering device with peltier heater

Figure 49 Structure soldering device with peltier heater

61

PELTIER HEATER

VAPOR

PCB

LIQUID

Figure 50 Real device in the lab

62

This soldering device use HB TEC1-12710

Table 2 Performance Specifications

Hot Side Temperature (ºC 25ºC 50ºC

Qmax (Watts) 85 96

Delta Tmax (ºC) 66 75

Imax (Amps) 10.5 10.5

Vmax (Volts) 15.2 17.4

Module Resistance (Ohms) 1.08 1.24

The difference in temperature between the hot and cold side can be a

maximum of 75ºC. Hence, in order to increase the temperature of the liquid to

230ºC, five layers of peltier are required as shown in fig 3.5

In keeping the temperature at the bottom layer equal to the environment,

the average delta T on 1 layer is:

(230-20)/5= 42.5o C

5.3 Supply energy calculation for heater

Within the vapor-phase soldering system, the liquid chosen for soldering

determines the peak temperature of the board [6]. The preferred peak

temperature for applications of Pb-Free/no-clean solder pastes is 220-240 °C.

From peak temperature we chose liquid low molecular weight

Perfluoropolyether (PFPE) fluids having the general chemical structure of:

CF3-{(O-CF-CF2 )m-(O-CF2 )n]O-CF3

CF3

PFPEs have exceptional thermal and oxidative stability, as well as

extreme chemical inertness. Their non-reactivity, high dielectric strength, low

toxicity, non-flammability and non-solvent features make PFPEs ideal for

electronic reliability testing including thermal shock and hermetic seal testing

[21].

63

Table 3 Properties of PEPE [21]

Typical Property D05

Boiling Point °C 230

Pour Point °C -77

Density ρ 25°Cg/Cm3 1.82

Density, -54° Cg/Cm3 1.98

Kinematic Viscosity 25°C cSt 4.4

Vapor Pressure, 25°C Torre <1

Specific Heat, 25°C cal/g°C 0.23

Heat of Vaporization C1 cal/g 0.15

Thermal Conductivity Watts/cm°C 0.0007

Coefficient of Expansion(°C) Cm 0.0011

Surface Tension, 25°C dynes/cm 20

Solubility of Water Ppm(wt.) 14

Solubility of Air Cm3 26

If we use volume v = 0.1L of liquid and preheat to temperature tsmin =

150°C with speed v(tsmin) =3°C/second the required power is:

(1)

Preheat time is T1= 43,3seconds

The required power to heat from 150°C to peak temperature tp = 230°C

with speed

v(tp) = 6°C/s is:

(2)

Time necessary to increase the temperature from 150°C to 230°C :

T2= (230-150) / 6 = 13,3 seconds (3)

Required power to keep the liquid at boiling point is

64

(4)

where C1 Heat of Vaporization of liquid

To cool down from tp = 230°C to 120°C with speed 6°C/s by reverse

current, The required power to cold from 230 to 120

(5)

Time to cool down from 230°C to 120°C is

T4= (230-120)/6=18.33 seconds

This is the process in theory. But practical results are somewhat different

because there is the influence of heat transfer to environment and latency of heat

transfer.

5.4 Running and result

To measure the real temperature profile, we used thermocouples type K

and used Agilent device to record temperature profile.

Running this device at different voltage and current we had the result

shows in the graph below

65

Figure 51 Temperature at different voltage and current

Experiment increase temperature to boiling temperature

Figure 52 Experiment increases temperature to boiling temperature

From the tests we had the result of soldering device with peltier heater

shown in the table.

66

Table 4 Parameter of soldering device with peltier heater Profile of soldering Device with peltier

heater Profile requirements

Average ram up rate 0 - 7 °C 3C0/ second max

Preheat    

Temperature Mn (tsmin) 1500C 1500C

Temperature Mmax (tsmax) 2000C 2000C

Preheat time (ts) 21-200 second 60-180 second

Reflow    

Peak temperature 160- 230 °C 220 – 260 °C

Time from preheat to peak 11-150 second 60-150 second

Time at peak temperature (tp) >5 second 10-40 second

Ram-Down rate 7 °C/second max 60C/ second max

Time from peak temperature

to 25°C

8 minute max 8 minute max

5.5 Conclusions of vapor phase soldering with peltier heaterFrom the parameters of the soldering device with a peltier heater, it is

shown that the soldering device with a peltier heater satisfies the requirements of

a lead-free solder such as: ram up average is 3 oC/ second. Reflow speed is 6oC/

second, time peak temperature is 10-40 oC , and cooling down speed is 6 oC/

second.

With respect to the results above, the following conclusions can be made:

Soldering device with peltier heater can solder lead-free solder

alloy.

Soldering device with peltier increases quickly liquid temperature to

the peak temperature.

It is easy to build the device since it basically involves the setting

up of some layers of peltier.

Easy temperature control.

Electrical energy is transferred immediately to thermal energy.

Cooling is done without a fan but with a cooling liquid by reversing

the current

67

Easy to control temperature profile similar with requirements of a

lead-free profile

In comparison with cooling by liquid, peltier cooling is a much more

efficient system at decreasing heat from the hot liquid. This allows for high speed

cooling, because the energy transfer from hot liquid to peltier is very fast and the

heat tranfers involves the entire surface of the peltier. On the other hand, liquid

cooling kits require a large amount of space within the tank to work effectively. In

order for the system to work properly, there must be space for items such as the

pump, the fluid reservoir, the tubing and power supplies. This has a tendency to

require larger space. Cooling by peiltier requires less space but is expensive .

The temperature of the peltier heater is higher than that of the liquid

so the peltier heater is easily broken by over heat.

Currently, high temperature peltiers are expensive.

We have applied a patent for Vapor phase soldering device with

peltier heater.

68

Chapter VI

Vapour phase soldering with resistance heater

6.1 Structure

Because vapor phase soldering with peltier heater have some problem

such as high temperature peltiers are expensive now, and peltiers are easy

break at high temperature. I therefore built another device which heats by a

resistance heater and provides cooling by liquid.

Figure 53 structure of soldering device with resistance heater

69

RESISTANCE HEATER

VAPOR

PCB

LIQUID

COOLING LIQUID IN

COOLING LIQUID OUT

COOLING TUBE

THERMOCOUPLE

Figure 54 Solder tank

70

Figure 55 Real device in the laboratory

Figure 56 Block diagram of the device

71

Controller

Heater

Cooler

The major parts of this oven are the Soldering Tank, resistance heater,

Temperature Sensor, Cooling tube, cooling pump, and Controller.

Working process of automated vapor phase soldering device with

resistance heater.

Step 1 Preheat: The controller supplies power to the resistance heater at a

rate of 1-3 oC till the temperature of the liquid reaches 150 oC. Which consists of

gradually ramping up the temperature to the preheat zone temperature at which

the solvents will be evaporated from the solder paste

Step 2: At the temperature 150 oC. The temperature sensor sends signal

to the controller which in turn increases the power. It is supplying to the

resistance heater to 3-6 oC/second till the temperature of the liquid reaches

boiling temperature (230oC). Flux activation, which consists of bringing the

dehydrated solder paste to a temperature at which it is chemically activated,

allowing it to react with and remove surface oxides and contaminants.

Step 3 Actual Reflow: the controller then adjusts the power so that the

temperature of the liquid remains at boiling temperature for about 10 to 40

seconds.

Actual Reflow, which consists of ramping up the temperature to the point

at which the solder alloy content of the solder paste melts, causing the solder to

sufficiently wet the interconnection surfaces of both the SMD’s and the board and

form the required solder fillet between the two; the peak reflow temperature

should be significantly higher than the solder alloy’s melting point to ensure good

wetting, but not so high that damage to the components is caused

Step 4 Cold down: After reflow, the controller stop supplying power to

resistance heater, and starts supplying power to the pump, which starts pumping

cold liquid to reduce the temperature of the heated liquid at maximum rate of 8 oC/second, till its temperature reach a value of 120oC.

Cold down, which consists of ramping down the temperature at optimum

speed (fast enough to form small grains that lead to higher fatigue resistance, but

slow enough to prevent thermo-mechanical damage to the components) until the

solder becomes solid again, forming good metallurgical bonds between the

components and the board.

72

6.2 Supply energy calculation for heater

Within the vapor-phase soldering system. The preferred peak temperature

for applications of Pb-Free/no-clean solder pastes is 220-240 °C. Similar with

calculation energy supply to peltier. I used the liquid perfluoropolyether, and the

solder paste has melting point temperature of 230oC.

Because vapor phase soldering with resistance heater cooling by liquid.

So we use volume v = 0.15L(10cmX10cmX1,2cm) of liquid thicker than peltier

and preheat to temperature tsmin = 150°C with speed v(tsmin) =2°C/second the

required energy is:

(1)

Preheat time is T1= 62,seconds

The required energy to heat from 150°C to peak temperature tp = 230°C

with speed

v(tp) = 4°C/s is:

(2)

Time necessary to increase the temperature from 150°C to 230°C :

T2= (230-150) / 4 = 20 seconds (3)

Required energy to keep the liquid at boiling point is

(4)

Where C1 Heat of Vaporization of liquid

To cood down from tp = 230°C to 120°C with speed 6°C/s by pumping cold

liquid,

This is the process in theory. But practical results are somewhat different

because there is the influence of heat transfer to environment and latency of heat

transfer.

73

6.3 Performance test

To measure the real temperature profile, we used thermocouples type T,

Agilent device and labview software to record the temperature profile.

Operating the device while varying the rate of heating, we obtained the

results shown in the graph below

0

50

100

150

200

250

0 50 100 150 200 250

Time (Second)

Tem

per

atu

re(C

elsi

us)

t1

t2

Series t1: preheat at the rate of 1oC/s, Flux activation at the rate of 3oC/s, reflow during 40

seconds.

Series t2: preheat at the rate of 3oC/s, Flux activation at the rate of 6oC/s, reflow during 6

seconds.

Figure 57 Experiments at different heating up rate.

The results of the soldering device with a resistance heater from the

experiment above are shown in the table below.

74

Table 5 . Parameters of soldering device with resistance heater

 Profile of soldering Device with

resistance heater

Profile

requirements

Average ram up rate 0 - 6 °C/second 3C0/ second max

Preheat    

Temperature Mn (tsmin) 1500C 1500C

Temperature Mmax 2000C 2000C

Preheat time (ts) 21-200 second 60-180 second

Reflow    

Peak temperature 160- 230 °C 220 – 260 °C

Time from preheat to peak 13-150 second 60-150 second

Time at peak temperature (tp) >6 second 10-40 second

Ram-Down rate 7 °C/second max 60C/ second max

Time from peak temperature to

25°C

8 minute max 8 minute max

From the parameters of soldering device with resistance heater, it is

shown that the soldering device with resistance heater satisfies the requirements

of lead-free solder such as: maximum ram up rate is 3oC/s, reflow speed is 6oC/s,

time peak temperature is 10-40oC and cooling down speed is 6oC/s.

Chapter VII

75

Measurements of heat transfer by thermal camera

I used the thermal camera FLIR systems 50 to measure the temperature

in PCB. The obtained results of the heat transfer in the vapor phase soldering

device with resistance heater are shown in fig.7.1 to fig.7.7

Figure 58 Heat transfer at 0 sencond

Figure 59 Heat transfer in PCB when center of PCB is 35oC and 39oC

76

Figure 60 Heat transfer in PCB when center of PCB are 51oC and 59oC

Figure 61 Heat transfer in PCB when center of PCB are 84.7oC and 99.6oC

Figure 62 Heat transfer in PCB when center of PCB are 120oC and 124oC

77

Figure 63 Heat transfer in PCB when center of PCB are 139oC

Figure 64 Heat transfer in PCB when center of PCB are 160oC and 170oC

The following conclusions can be made from the results above:

The temperature across the PCB surface is almost the same at all points.

The temperatures at the edge and corner of the PCB are slightly higher than at

the center. It is similar with the simulation model.

The temperature of the PBC measured by the thermal camera is similar

with the temperature of PBC in modelled by the Comsol software.

The vapor begins to cover the surface of the PCB at a temperture of

135oC. When the temperature of the vapor zone reaches over 160oC, the vapor

covers the PCB. The temperature of the PCB cannot be therefore measured by

78

the thermal camera since the measured result is the temperature of the vapor

zone.

The temperature of the vapor could only be increased to a maximum of

170oC because the cover of the automat vapor phase soldering device had to be

left open to be able to carry out measurements and to take pictures thereby

causing loss of heat.

79

Chapter IIX

Experiment with electronic elements and printed circuit board.

In my experiment I used lead-free solder paste  95.6 percent tin, 3.7

percent silver and 0.7 percent copper. It has properties shows in the table below:

Table 6 Properties of lead free solder paste 95.6 percent tin, 3.7 percent silver and 0.7 percent copper

PROPERTY UNITS

Density 8.5 G/cm3

Specific Heat 0.045 Cal/g-k

Thermal conductivity 0.13 Cal/cm-s-k

Thermal Expansion 26x10-6 1/K

Melting Point 451/217 K/ oC

Resistivity 14.7x10-6 Ohm-cm

TCR Ppm/K

Young’s Modulus 3330 ksi

Poisson’s Ratio ------

Yield Strength psi

Ult Tensile strength 7000 psi

Elongation at break 7 %

Hardness 33.4 Brinell

In infrared or hot air soldering process this solder paste need to increase

temperature to 260oC to has good solder joints.

80

8.1 Experiment soldering at good profile temperature

Good profile

0

50

100

150

200

250

0 20 40 60 80 100 120 140

Time(Second)

Tem

per

atu

re(C

elsi

us)

t1

t2

Figure 65 Soldering at good profile temperatureSoldering with preheat rate of 1-3 oC/second, Flux activation rate 3-6 oC,

and keep peak temperature at 230OC from 6 to 20 second. The results show in

the pictures bellows.

81

Figure 66 Solder joint as good profile temperature.

Qualities of solder joints are very good with a good profile temperature

process. Peak temperature time and preheat rate is very important. In this

process peak temperature is only 230oC. So it reduces peak temperature 30 OC

compare with hot air or infrared soldering process.

8.2 Experiment at slow rate and maintaining the peak temperature for a long time

0

50

100

150

200

250

300

350

0 100 200 300 400 500 600

Time(Second)

Tem

per

atu

re(C

elsi

us)

Figure 67 Long peak temperature profile

82

Figure 68 Solder joint as maintaining the peak temperature for a long time

Maintaining the peak temperature for a long time of about 40 seconds

causes the solder paste to over melt, spread out, and the melted solder paste is

sensitive with vibration hence, a displacement joint

8.3 Experiment at fast rate and maintaining the peak temperature for a short time

Comment

0

50

100

150

200

250

0 20 40 60 80 100 120

Time (second)

Tem

pera

ture

(cel

sius

)

Figure 69 Keep Peak temperature time less than 4 seconds

83

Figure 70 Soldering with short peak temperature

Maintaining the peak temperature for a short time of less than 4 seconds

does not allow the solder paste to melt and create the solder joint. Compare with

the modeling in chapter 4 is quiet similar. The temperature of solder over melten

temperature 217oC after the liquid boiling 5 seconds

8.4 Experiment soldering both sides.

In this experiment I used 2 thermocouples to measure difference between

top side and bottom side.

Figure 71 Position of thermocouple

84

Thermocouple at top side

Thermocouple at bottom side

PCB

The result shows in the graph below.

T1: Temperature at bottom side. T2: Temperature at top side.

Figure 72 Difference temperature at top side and bottom side.

This experiment illustrates that the temperature profile for the top and

bottom sides are similar with a difference of about 4 seconds in the heating up of

the upper part compared to the lower part. This means that, it is possible to

simultaneously solder the upper and lower parts of the PCB using the device.

Figure 73 Bottom side

85

Figure 74 Top side.

8.5 Experiment to measure the difference in temperature at the center and corner of printed cicuit board

In this experiment I uses 3 thermocouples to measure how difference

temperature between center, corner and liquid.

Figure 75 Position of thermocouple

86

Thermocouple at corner (t1)

Thermocouple at center (t2)

PCB

Difference temperature in center and corner of PCB

0

50

100

150

200

250

0 100 200 300 400 500

Time ( second)

Tem

pera

ture

(Cel

sius

)

t0

t2

t1

t0 is temperature in liquid. t1 temperature at corner. t2 temperature in center.

Figure 76 Difference temperature in center and corner of PCB and the liquid

Figure 77 soldering at difference posiion.The result shows that the temperature at the center and corner is steady.

Average temperature difference is less than 5 oC. Therefore the quality of the

solder joint at the center and corner is similar. So the device resolves the

87

problems of infrared soldering and hot air soldering since in both cases, the PCB

is hotter at the centre than at the corners, causing a deformation of the PCB.

8.6 Experiment measure temperaure at different distance from liquid

In this experiment I used 3 thermocouples to measure temperature difference at

1cm, 2cm, 3cm from liquid.

Figure 78 Position of thermocouple.

0

50

100

150

200

250

0 50 100 150 200 250 300 350

Time (second)

Tem

per

atu

re (

cels

ius)

t3

t2

t1

t0

t0 is temperature in liquid. t1 is temperature at 1cm high.

t2 is temperature at 2cm high. t3 is temperature at 3cm high.

Fig 8.15. Temperature at different distance from the liquid.

88

Heater

Cooling tube

Thermocouple

Thermocouple 2Thermocouple 1 Liquid

1cm

1cm

1cm

Thermocouple 0

This graph shows that the change in temperature profile depends on the

distance from the liquid. Profile at higher levels are similar. And the profile closer

to the liquid is similar to the profile of the liquid. Therefore if the goal is to solder a

single side, it is necessary to place the PCB close to the liquid to allow easy

control of the temperature profile. In order to solder both sides, place the PCB

farther away from the liquid to have a similar temperature profile at the top and

bottom sides of the PCB.

8.7 Soldering small electronic elements.

Figure 79 Soldering small electronic elements

The device is able to solder very small electronic elements. An example is

shown in the fig were an electronic element of SMD site 0402.

89

8.8 Experiment bridging, wetting force, de-wetting

In those experiment in the left solder paste poured across the PCB suface

as shown in fig (A). After the soldering process. The device corrects the problem

of brigging that normally exists after soldering with other devices fig (B)

A. Before soldering

B. After soldering

Figure 80 Experiment bridging joint.

90

This experiment shows that this device can resolve the bridging problem.

It automatic repair mistaken of filling solder paste.

De-wetting experiment

Figure 81 De-wetting experimentThis picture shown that vapor phase soldering device has good weting

force. Wetting angle less than 10o is very small. So it reduce de-wetting effect.

91

Chapter IX

Checking quality of solder jointMicroscope checking: There are many methods of checking solder joints.

For complex analysis I used the method of destructive checking by cutting

through the solder joint with a diamond cutting machine and then performing

measurements and finally taking pictures with the aid of the microscope, Neophot

32 Carlzeiss Jena. The results of the above mentioned experiment are shown in

fig.82 to fig.86

Figure 82 Left and right sections of the solder joint after cut, a step is

500µm

Figure 83 Left and right sections of the solder joint after cut, a step is

100µm

92

Figure 84 Solder joint bettwen solder and copper on the left and solder on

the right. A step is 50 µm

Figure 85 Solder joint bettwen solder and electronic element on the left and

solder on the right at scale 50 µm

Figure 86 Solder layer on the top of the electronic element on the left and

on the right. A step is 50 µmThe results show that the quality of the solder joint is very good. The

solder alloy is firmly in place and does not have metallization failure, such defects

93

like voids, cracks, separations, depressions, notches or tunnels or any

combination in the cross sectional reduction that is a basis for rejection. The

contact angle is very small (fig 9.1). The alloy layer between the solder, the

copper, and the electronic element is very thin. It is less than 10 µm which

satisfies the soldering standad (IPC-J-STD-001E,Requirements for Soldered

Electrical & Electronic Assemblies).

94

Chapter X

List of result and noveltyThe most important result is the vapor phase soldering device with

resistance heater reduce peak temperature more than 30oC. In comparison with

infrared or hot air heat is used. Reduce peak temperature from 260oC to 230oC.

Soldering device with resistance heater satisfies the

requirement lead-free solder paste. Such as ram up average is 3oC/ second.

Reflow speed is 6oC/ second, time pick temperature is 10-40oC, and cooling

down speed is 6oC/ second.

This device able to solder both sides at the same

time.

Able to solder any position of electronic elements on

PCB

Able solder any color, shape of electronic elements.

Able to solder the small element

Uniform solder joints. The result shows that solder join have good

wetting force, and the temperature in PCB and electronic elements is steady.

Therefore solder joints is uniform

Heating times are short to improve solder quality. The device able

to flexible controls speed heating up from 1 to 6oC/second, time at peak

temperature 6oC/second, and cooling speed. So it improves solder quality.

Soldering process in oxygen free atmosphere. Because the liquid

produces a dense, saturated vapor that displaces air and moisture.

Soldering process without nitrogen to reduce high cost. This device

does not need supplies nitrogen to protect solder joint.

Minimum repairs and part replacements. With good solder profile

will due the good solder joint. I did experiment more than 50 times. It does not

due any error.

Maximum cleanliness of the completed assembly. This device is

soldering without cleaning.

Maximum flexibility to allow soldering a large number of circuits with

minimum changeover time.

95

Minimum time above the liquidous solder temperature to reduce

solder grain growth, resulting in a more durable solder joint. Sort time above also

minimum “leaching” of part termination materials

Reduce peak temperature to increasing long-life of electronic

elements and reduce PCB cost. Compared with infrared soldering and hot air

soldering the peak temperature reduces about 30oC. with lead-free solder paste

62Sn/36Pb/2Ag the peak temperature reduce from 260 to 230oC

Minimum damage and stress to the SMT parts. Because the peak

temperature reduce 30oC, the temperature steady, and the temperature at the

same high level is the same. Therefore reduce damage and stress to the STM

parts and printed circuit board.

The Novelty of my work is reducing peak temperature about 30oC.

Reducing temperature has a lot of signification in lead-free soldering. Resolve

the difficult of chosen material for electronic elements and PCB. Reduce

soldering defect, stabilize electronic device and increasing life of electronic

devices. Reduce temperature also reduce cost of PCB, electronic element.

96

Chapter XI

Conclusion

The soldering device with a resistance heater satisfies the requirements of

lead-free solder paste, for example, ram up average is 3oC/s, reflow speed is

6oC/ s, time peak temperature is 10-40oC and cooling down speed is 6oC/s.

The vapor phase soldering device can be employed in both surface mount

and through-hole methods. It can solder both sides of the PCB simultaneously;

solder any color or shape of electronic elements, as well as any part of an

electronic element on a PCB. The solder joints are of the same quality.

With regard to vapor phase soldering, the peak soldering temperature is

the boiling temperature of the inert liquid at atmospheric pressure. In comparison

with when infrared or laser heat is used, variations in temperature are much less

because, even if power is increased, the rate of vapor production will rise but the

temperature stays the same. Primary fluids are available in a wide temperature

range, (155°C to 290°C).

When soldering with lead-free solder paste 62Sn/36Pb/2Ag, the peak

temperature in infrared soldering and hot air soldering is more than 260oC but

compared with the vapour phase soldering device, a maximum peak temperature

of 230oC is required to perform the same task. Therefore in using the vapour

phase soldering device, we reduce the peak temperature by at least 30OC. Due

to this reduction in peak temperature, in addition to the fact that temperatures at

all points of the PCB are similar, thermal stress, which lowers the life of lead-free

solder joints is reduced. Another implication is that damage to PCB and

electronic elements is reduced.

A reduction in peak temperature also leads to an increase in the life of

electronic elements and a reduction of PCB cost.

As a result of the high vapor density of most fluids, soldering is done in an

inert atmosphere, thus eliminating the added cost incurred from the use of inert

gases, for example nitrogen in other types of soldering systems. Soldering in an

97

oxygen free environment and without moisture reduces popcorn defect and

protects the solder joint.

Due to the fact that the heat transfer coefficient of vapor condensation is

faster than hot air and infrared heat, the process is very fast and efficient. The

heat transferred is independent of the surface type (whether dark or light),

therefore it resolves the problem associated with infrared soldering, since with

infrared soldering, dark surfaces are heated more than lighter surfaces.

Vapor Phase soldering device is independent of size, shape or geometry.

With maximum surface area exposed, every component part experiences exactly

the same temperature thereby eradicating the problem encountered with infrared

or hot air, since a shadowing element receives more heat than the shadowed

element and intricate shaped elements in comparison to normal elements are not

heated up fast enough.

The vapor Phase soldering process is clean as the components are

exposed to distilled vapors. The solder paste in this process has good wetting

and it reduces both non-wetting and solder ball defect. In addition, the inertness

of most fluid does not cause a compatibility problem with these components.

Also, most fluids evaporate quickly from the hot surface due to their low heat of

vaporization, leading to a rapid, residue free drying of the parts.

98

Chapter XII

Future Work

Future work will involve me researching deeper into vapor soldering

device with peltier, since peltier at the moment is very expensive and cannot

operate at very high temperatures. In my work, the lead-free alloy comprising

95.6 percent tin, 3.7 percent silver and 0.7 percent copper was used. Further

research would be done using other lead-free solder pastes with different

temperature profiles. Likewise, liquids other than perfluoropolyether could be

used for the purpose of cooling in the soldering device with the resistance heater.

Soldering of Ball Grid Array electronic elements would be investigated

since the usage of such elements is growing at a fast rate.

The quality of solder joints could be tested by other methods like x-ray or

by mechanical testing methods such as adhesion, compression, drop (shock),

and so on, although testing was carried in this work by the optical method.

I would as well investigate product and component testing by the

evaluation of finished products or components through performance in electrical,

life, environmental exposure or other specialized tests

Last but not least, further research into how vapor phase soldering could

be applied for mass production purposes.

99

LIST OF PUBCLICATION

Do, M 50%. - Novák, M.: Device for Vapour Phase Soldering with

Peltier Heaters. In Sborník odborného semináře Nové metody a postupy v oblasti

přístrojové techniky, automatického řízení a informatiky [CD-ROM]. Praha: Ústav

přístrojové a řídicí techniky FS ČVUT, 2009, p. 3-8. ISBN 978-80-01-04353-0.

Do, M. 70% - Novák, M. - Uhlíř, I.: Experience with Vapor Phase

Soldering Device. In ARTEP - Zborník príspevkov [CD-ROM]. Košice: Technical

University of Košice, 2010, p. 16-1-16-9. ISBN 978-80-553-0347-5.

Do, M. 60% - Novák, M. - Uhlíř, I.: VAPOR PHASE SOLDERING

DEVICE. In Proceedings of The 9th International Conference Process Control

2010 [CD-ROM]. Pardubice: Universita Pardubice, 2010, p. 1-9. ISBN 978-80-

7399-951-3.

Do, M. 50% - Novák, M. - Uhlíř, I.: Vapour Soldering System with

Peltier Heater. In Applied Electronics 2009. IEEE conference in Plzen. C Plzeň:

Západočeská univerzita v Plzni, 2009, p. 91-94. ISBN 978-80-7043-781-0.

Novák, M. - Uhlíř, I. - Do, M.L. 10% - Gumalay, R. -

Sigalingging, A.A.: Vapour phase soldering device. Užitný vzor Office of

Industrial Ownership, 19396. 2009-03-09. (in Czech).

PAP - Patentová přihláška

Autor: Novák Martin Ing 60%. Ph.D, Uhlíř Ivan prof. Ing. DrSc 10%,Ing.

Do Mai Lam 10%, Gumalay.R %10, Sigalingging 10%. A. A. Název angl: Vapour

phase soldering device, Název česky: Zařízení pro pájení v parách. Anotace

angl.: This inventions concerns a vapor phase soldering device with Peltier

thermoelectric elements. Anotace česky: Předkládané řešení se týká zařízení pro

pájení v parách. Zařízení je určeno pro pájení elektronických součástek pro

povrchovou montáž v parách teplovodivé inertní kapaliny s vysokou teplotní

vodivostí.

http://isdv.upv.cz/portal/pls/portal/portlets.pta.det?

pskup=1&propv=2008&pcipv=801. Datum 17.12.2008 12:25:11, Ident: 149330 ,

Místo vydání: Praha. Stat : CZ

100

REFERENCES

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http://www.rayprasad.com/home/rp1/page_69/smt-

vapor_phase_soldering_the_comeback_kid.html PRASAD RAY:

29.1.2010

[2] Source:www.lead-free.org http://www.rohs.eu/english/system/login/index.html

1.3.2011

[3] Principles of Electronic Packaging, Donald P. Seraphim, Ronald C. Lasky and

Che-Yu Li. 1999

[4] National Semiconductor Application Note, 1126,

http://www.national.com/an/AN/AN-1126.pdf August 2003

[5] Surface Mount Technology, Butterworth-Heinemann Ltd Rudolf Strauss, ISBN

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