Reliability of unencapsulated SMD plastic filmcapacitors

Anne SeppaÈlaÈTampere University of Technology, Electronics Laboratory, Tampere, FinlandKimmo SaarinenEvox Rifa Group, Virkkala, FinlandEero RistolainenTampere University of Technology, Electronics Laboratory, Tampere, Finland

Introduction

Resistance to heat, moisture and most common solvents isan important aspect of an electronic component. Plasticencapsulated components usually satisfy theserequirements. However, encapsulation can account formore than 50 percent of the component's volume. Smallsize and high volume efficiency are essential in today'strend towards miniaturization, such as in portableelectronics. By leaving out the encapsulation process, theprice of the component can also be reduced due to savingsin cost of labour and materials.

The dielectric film in an unencapsulated SMD plasticfilm capacitor has to resist temperatures up to 2608C andhas to have resistance to the most common chemicals usedin soldering processes, since there is no protective shield asa thermal or chemical barrier. Furthermore, terminal metalshave to be suitable for new lead-free soldering processes. Inaddition to solderability, the terminal metals have to havegood corrosion resistance to avoid degradation in electricalconductivity.

Solderability of the components is a complex parameter.There are three main aspects to solderability (Lea, 1988):1 thermal demand;2 wettability; and3 resistance to soldering heat.

The thermal characteristics of the component must enablethe solder joint areas to be heated to the desired temperaturefor soldering. A clean metallic surface on the substrates isrequired for good wetting by molten solder to occur. On theother hand, many solderability problems are due to choiceof terminal or lead materials, which, in air, rapidly formtough oxides that do not wet well. The wettability can bedetermined by several factors, such as the surface energy ofthe material surface, with metals with low surface energiesbeing more difficult to solder (Strauss, 1998). Thewettability of common substrates can be ranked in the orderof Sn > Sn/Pb > Cu > Ag/Pd > Ni (Hwang, 1996). Surfaceroughening and texturing control the wetting of thesubstrate (Lea, 1988). In addition, the solderability can beaffected by the porosity of the base material and themetallurgical affinity between the metal to be soldered andthe constituents of the solder (Hwang, 1996; Strauss, 1998).

The most commonly used solder is eutectic tin-leadalloy. Two factors are, however, driving the change awayfrom lead. One is the environmental issue of lead as it is anenvironmental pollutant. The other factor is the health andsafety in the workplace as lead is a toxic metal. Virtually allof the lead-free solder alternatives utilize tin as one of theprimary constituents. The other elements which could beincorporated in the alloy systems are, for example, Ag, Au,Bi, Cd, Cu, Ga, Hg, In, Sb, Tl, and Zn (Hwang, 1996; Lee,1997).

Experimental

Production of the test capacitorsUnencapsulated plastic film capacitors were produced usinga winding method. Polyethylene naphthalate (PEN) filmwas used as a dielectric. The manufacturing process startedwith vacuum metallization, which was used to coat the PENfilm with aluminum. During metallization, the strips ormargins which were not to be coated were protected by anoil masking system or by tapes which preventedmetallization. After metallization, films were slitted andtwo polymeric tapes, which had metallized electrodes onone surface and a non-metallized margin on one side, wereoffset in opposite directions so that the non-metallizedmargins lined up with the opposite sides of the tapes. Thenthe two tapes together were wound into cylindrical rolls.Flat windings were produced by pressing the cylindricalrolls. The opposite sides of the windings, or terminals, wereelectroded by using the Schoop process for flame sprayingof metal. During spraying capacitors were masked so thatthe sides of the windings were protected from metallization.Appropriateness of the selective plating process was alsotested in metallizing the terminals with nickel. Theproduction of the test capacitors is schematically presentedin Figure 1.

Flame spraying is an industrial coating process firstinvented in 1910 by Schoop in Switzerland (Ingham andShepard, 1969). Pure or alloyed metal in wire form is fedcontinually into a fuel gas-oxygen flame where it is melted.Compressed air surrounds the flame and atomizes themolten tip of the wire. This accelerates the spray of moltenparticles toward the surface to be coated. The surface is atlow temperature, which is an advantage of this method.

MaterialsThe capacitance value of the specimens was approximately1�F. The spacing between the terminals of the capacitorwas 14mm and the rated voltage was 100V. The terminalsof the capacitors consisted of three different metal layers.The choice for the base layer was aluminum, due to its goodadhesion to the dielectric film and its high melting point.Since the base layer exhibits poor solderability, a solderableintermediate layer must be added, particularly when the toplayer dissolves in the solder during the soldering process.Alternatives for the intermediate metal layer were brass andcopper. The top layer was a sprayed tin-based solder suchas tin-zinc, tin-copper or pure tin. Tin-lead solder was notused due to the harmfulness of lead. For a few specimens, abarrier layer of nickel was used between the top layer andthe intermediate layer.

Copper is widely used in electrical contacts because ofits high electrical and thermal conductivity, low cost, andease of fabrication. High purity copper is used for the

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Soldering & Surface MountTechnology12/1 [2000] 15±22

# MCB University Press[ISSN 0954-0911]

KeywordsSurface mount technology,

Lead-free soldering, Reliability

AbstractSmall and low cost

unencapsulated SMD plastic film

capacitors were manufactured

with different terminal metal

compositions and dielectric

materials. Capacitors made with a

polyethylene naphthalate film

dielectric were produced using a

winding method. The terminals

were metallized using the flame

spraying process. The terminals of

the test capacitors consisted of

three different metal layers. The

base metal layer, which was

aluminum, was coated with brass

or copper. The top layer was a

sprayed lead-free, tin-based solder

to ensure the solderability of the

terminals. The reliability of the

unencapsulated test capacitors

was evaluated using standard

temperature cycling, humidity

storage, and high temperature

environmental tests. Solderability

and resistance to soldering heat

were tested by mounting the test

capacitors using the reflow

soldering technique. The electrical

properties including capacitance,

insulation resistance, and

dissipation factors at 1kHz and

100kHz were verified.

Received: April 1999Revised: October 1999

conductors on printed circuit boards. Copper and its alloysare also used as materials for electrical contacts, forterminals and in electromechanical components. However,the melting temperature of pure copper is 1,0838C.

Tin is non-toxic, ductile, and corrosion resistant metal usedin electronic applications. Tin's ability to protect, forexample, copper from oxidation preserves the solderability ofthe basis metal. The melting temperature of pure tin is 2328C.

Tin-zinc alloys are used as solders. An alloy containing91 percent tin and 9 percent zinc is a eutectic compositionand is a candidate for lead free solder. It has good strength,but poor wetting and corrosion properties (Lee, 1997). Thetin-zinc alloy tested contained 20 percent zinc and 80percent tin for productional reasons. The liquidus of thealloy is approximately 2358C.

Solder with a 99.3 percent Sn and 0.7 percent Cu eutecticcomposition has shown good potential as a replacement forSn-Pb solder in lead-free soldering processes. An advantageof the alloy is good fatigue resistance (Lee, 1997). Thetested alloy of 3 percent copper and 97 percent tin has aliquidus of 3308C.

Brass is a copper-zinc alloy containing up to 40 percentzinc. The tested brass wire contained 27 percent zinc and 73percent oxygen-free copper. The liquidus of the alloy wasapproximately 9758C. The properties of brasses varywidely depending on the specific alloy. As the zinc contentincreases in the alloy, the melting point, density, electricand thermal conductivity, and modulus of elasticitydecrease while the coefficient of expansion, strength, andhardness increase (Brady et al., 1997).

Brass can be coated with nickel to avoid diffusion of zinc.A problem arises when a diffusion barrier, such as nickel, istoo thin and cannot stop the migration of zinc from a brassbase material into the solderable layer. Zinc can thenaccumulate on the surface of the solderable layer and formzinc oxide, which is not solderable (MacLeod Ross, 1996).

The appropriateness of selective plating, which is anelectrochemical deposition method, was tested inmetallizing the terminals of the capacitors with nickel andtin, since there were some difficulties in spraying nickel-based alloy due to the high melting temperature of thealloy. However, the electrical properties of the capacitorsdecreased markedly during the selective plating process. Asa consequence of that, no further testing was performedwith the nickel-plated capacitors. The materialcombinations that were tested are presented in Table I.

Reliability testsAfter initial measurements the reliability of the capacitorswas tested by several aging tests. The measured electricalproperties were capacitance, insulation resistance, anddissipation factor. For capacitance and dissipation factormeasurements an HP 4284A precision LCR meter was usedand a DB601 insulation resistance meter with DB640scanner was used when the insulation resistance wasmeasured. After the tests, and a period of at least two hoursof stabilisation at room temperature, the samemeasurements were carried out to determine the changes inthe properties during the tests. Solderability and resistanceto soldering heat were tested by mounting the capacitorsusing the reflow soldering technique. Reflow wasperformed in a convection reflow oven. The solderingprocess was a no-clean process performed by using SnPbsolder. The number of specimens was ten. After soldering,the capacitors were visually examined to see the quality ofthe solder joint and final measurements were performed.The endurance of the same specimens was tested at atemperature of 1258C for a duration of 2,000 hours.

Resistance to thermal shock was tested by subjecting thespecimens, which were mounted on the printed circuitboard, to a rapid change of temperature. The test was a two-chamber test with transition time less than ten seconds. Theduration of exposure at the temperature limits was 15minutes and the number of cycles was 100. The testtemperatures were ±558C and 1258C and the number ofspecimens in the test was 20. After the thermal shock test,the same specimens were subjected to a damp heat test,where the capacitors were exposed to a relative humidity of93 percent at a temperature of 408C for 56 days.

Results and discussion

An important property of the terminal metallisation of thecapacitor is its solderability. The solderability of thecapacitors was tested by mounting the capacitors using thereflow soldering technique. The solderability of thespecimens with a sprayed copper or brass intermediatelayer was good while the sprayed nickel-based alloy turnedout to be non-solderable, probably due to tough oxides thatformed during the spraying process. Poor solderability issaid to be a disadvantage of nickel even with activatedfluxes (Harper and Sampson, 1994). In addition, the hightemperature used in the flame spraying process of nickelcaused degradation of capacitors. In consequence of that,no testing was performed for these specimens. Solderableterminals were achieved by protecting the plated, cleannickel layer with a selectively plated thin tin layer.However, the electrical properties such as capacitance,dissipation factor, and insulation resistance of thecapacitors were degraded markedly during the plating andsubsequent soldering processes. As a result, it is notappropriate to replace the flame spray process with theselective plating process.

The results of the reliability tests are shown in Figures2-17. The average values are indicated by asterisks in theFigures. The maximum and minimum values of the testresults are also indicated. The change of capacitance isexpressed in percent. The change of dissipation factor isexpressed as absolute value of increase or decrease. Theinsulation resistance after the test is compared to thespecification insulation resistance, which is 3,000M, andthe ratio of these values is indicated in the Figures. Thechange in the insulation resistance value during the testingwas insignificant for all the specimens.

There is no international test specification forpolyethylene naphthalate SMD capacitors, but the testresults should meet the requirements generally used in theindustry. These requirements are also the draft limits of IECspecification under development. The requirements arepresented in Table II.

Figure 1Schematic presentation of the production of the test capacitors. Two metallized polymer films(a) were wound into cylindrical rolls (b). The rolls were pressed and opposite sides of thewindings were electroded (c)

Table IThe construction of the specimens

Specimen group Base layer Intermediate layer Top layer

Cu+Sn Aluminum Copper TinCu+SnCu Aluminum Copper Tin-copper (3 percent Cu)Cu+SnZn Aluminum Copper Tin-zinc (20 percent Zn)Brass+Sn Aluminum Brass (27 percent Zn) TinBrass+SnCu Aluminum Brass (27 percent Zn) Tin-copper (3 percent Cu)Brass+SnZn Aluminum Brass (27 percent Zn) Tin-zinc (20 percent Zn)

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The results of the resistance to soldering heat test showedthat the change of capacitance value of some specimenswith a sprayed Sn-Zn top layer exceeded the accepted limitof 2 percent, as shown in Figure 2. However, the averagevalue was within the recommended region for all thespecimens. On the other hand, the increase of dissipationfactor was always below the specified limit, as shown inFigures 3 and 4. In some cases, the dissipation factor hadeven decreased. Finally, the best results at 100kHz wereachieved with the specimens sprayed with Sn and Sn-Cualloys, as shown in Figure 4. The results of insulationresistance measurements in Figure 5 showed better resultsfor the specimens with Sn-Zn top layer.

All of the test results in the endurance test were withinthe acceptable range. The change of capacitance for allspecimens was very similar, as shown in Figure 6. Thedissipation factor at 1kHz was again decreased with thespecimens sprayed with Sn and Sn-Cu, as shown in Figure7, but at 100kHz the best results were achieved with thespecimens sprayed with Sn-Zn, as shown in Figure 8.Figure 9 shows that the ratio of insulation resistance andspecification insulation resistance is well above theminimum limit.

The results of capacitance measurements followingthermal shock testing are presented in Figure 10. Theresults were within the accepted region for all thespecimens. The change of dissipation factor at 1kHz wasalso acceptable, and showed similar behavior for all the

specimens, as shown in Figure 11. However, the increase ofdissipation factor at 100kHz for some specimens exceededthe specified limit of 0.003, as shown in Figure 12. Theaverage values were still below the specified limit. InFigure 13 it can be seen that the ratio of insulationresistance and specification insulation resistance was againwell above the minimum limit.

The results of the damp heat test show that the change ofcapacitance, which was within the accepted region for allthe specimens, is less for the specimens sprayed with brass,as shown in Figure 14. A similar trend is shown in Figure15, which presents the results for change of dissipationfactor at 1kHz. The increase of dissipation factor at 100kHzfor some specimens with Sn-Cu top layer exceeded thespecified limit in damp heat test also, but the average valuewas again below the limit, as shown in Figure 16. The ratioof insulation resistance and specification insulationresistance is still well above the minimum accepted limit, asshown in Figure 17.

As a result of the reliability tests it can be seen that thereare no significant differences between specimens sprayedwith copper and specimens sprayed with brass. The testresults for specimens with different top layers were alsoquite similar. However, brass may cause some problems inthe long term because of out-diffusion of zinc. In addition,the tin-zinc alloy may have poor wetting and corrosionproperties. The use of tin-copper may be advisable, becausethe strength of tin-copper is greater than that of pure tin.

Figure 3Change of dissipation factor at 1kHz in resistance to soldering heat test

Figure 2Change of capacitance in resistance to soldering heat test

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Anne SeppaÈlaÈ, Kimmo Saarinenand Eero RistolainenReliability of unencapsulated SMDplastic film capacitors

Soldering & Surface MountTechnology12/1 [2000] 15±22

Figure 4Change of dissipation factor at 100kHz in resistance to soldering heat test

Figure 5Ratio of insulation resistance and specification insulation resistance determined after resistance to soldering heat test

Figure 6Change of capacitance in endurance test

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Anne SeppaÈlaÈ, Kimmo Saarinenand Eero RistolainenReliability of unencapsulated SMDplastic film capacitors

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Figure 7Change of dissipation factor at 1kHz in endurance test

Figure 8Change of dissipation factor at 100kHz in endurance test

Figure 9Ratio of insulation resistance and specification insulation resistance determined after endurance test

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Anne SeppaÈlaÈ, Kimmo Saarinenand Eero RistolainenReliability of unencapsulated SMDplastic film capacitors

Soldering & Surface MountTechnology12/1 [2000] 15±22

Figure 10Change of capacitance in rapid change of temperature test

Figure 11Change of dissipation factor at 1kHz in rapid change of temperature test

Figure 12Change of dissipation factor at 100kHz in rapid change of temperature test

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Anne SeppaÈlaÈ, Kimmo Saarinenand Eero RistolainenReliability of unencapsulated SMDplastic film capacitors

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Figure 13Ratio of insulation resistance and specification insulation resistance determined after rapid change of temperature test

Figure 14Change of capacitance in damp heat test

Figure 15Change of dissipation factor at 1kHz in damp heat test

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Anne SeppaÈlaÈ, Kimmo Saarinenand Eero RistolainenReliability of unencapsulated SMDplastic film capacitors

Soldering & Surface MountTechnology12/1 [2000] 15±22

Conclusion

Different materials for terminal metal layers of theunencapsulated capacitors were tested. The terminals of theunencapsulated capacitors consisted of three different metallayers. Aluminum was used as a base layer to make a goodadhesion bond with aluminum metallizing on the dielectricpolymer of polyethylene naphthalate. The alternatives forthe solderable intermediate layer were copper and brass.Tin-based lead-free solders were used as a top layer.

Resistance to soldering heat was tested and it yieldedgood results. Although there was no case protecting thedielectric polymer, the change of the electrical properties ofthe capacitors was acceptable. The reliability of thecapacitors was studied by subjecting the specimens to therapid change of temperature test, the damp heat test, and theendurance test. In some cases, the increase of dissipationfactor at 100kHz exceeded the specified limit, but the other

test results were within the acceptable range. It should benoticed that the test limits used are the draft-limits of IECspecification for an encapsulated polyethylene naphthalateSMD capacitor, and thus may be too strict forunencapsulated capacitors. Finally, the results werepromising and the development of smaller and cheaperlead-free SMD plastic film capacitor families for the highertemperature soldering process can be finalized in duecourse to satisfy the market demands.

ReferencesBrady, G.S., Clauser, H.R. and Vaceari, J.A. (1997), Materials

Handbook, 14th ed., McGraw-Hill, New York, NY.Harper, C.A. and Sampson, R.M. (1994), Electronic Materials &

Process Handbook, 2nd ed., McGraw-Hill, New York, NY.Hwang, J.S. (1996), Modern Solder Technology for Competitive

Electronics Manufacturing, McGraw-Hill, New York, NY.Ingham, H.S. and Shepard, A.P. (1969), Metco Flame Spray

Handbook, Vol. I, Wire Process, Metco Inc, Westbury.Lea, C. (1988), A Scientific Guide to Surface Mount Technology,

Electrochemical Publications, Scotland.Lee, N.C. (1997), `̀ Getting ready for lead-free solders'', Soldering

& Surface Mount Technology, Vol. 9 No. 26, pp. 65-9.MacLeod Ross, W. (1996), A Comprehensive Guide to the Design

and Manufacture of Printed Board Assemblies, Vol. 1,Components and Assembly, Electrochemical Publications,UK.

Strauss, R. (1998), SMT Soldering Handbook, 2nd ed., Newnes,Oxford.

Figure 16Change of dissipation factor at 100kHz in damp heat test

Figure 17Ratio of insulation resistance and specification insulation resistance determined after damp heat test

Table IIRequirements for test results

Requirements

Change of capacitance ��2 percentIncrease of dissipation factor at 1kHz �0.002Increase of dissipation factor at 100kHz �0.003Insulation resistance/spec. insulation resistance �0.5 (50 percent)

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