Mounting Guideline of Golden DRAGON With Lens

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February, 2008 Page 1 of 13

Mounting Guideline for Golden DRAGONwith Lens

Application Note

Abstract

The following application note represents ageneral guideline for mounting LEDs of theGolden DRAGONwith Lens product family.A basic overview of the construction of LEDsand important design rules for thedevelopment of LED systems are provided.In addition, the principal processing stepsare illustrated for the Golden DRAGONARGUS LED, and general informationabout mounting is summarized.In the following, a recommended selectivesoldering method by means of laser diodesis also introduced, because it is not possibleto process these LED type with aconventional standard soldering process.Thereby the important process parametersand steps for processing of the GoldenDRAGONARGUSLED are described.In a final manner alternative soldertechniques are presented.

Introduction

With the availability of new high powerLEDs, new application areas have beenopened which were previously reserved toconventional lamps. These high power LEDsare based on new highly efficient chiptechnologies in combination with thermallyoptimized device packages.In many cases LEDs must be fitted with

additional optics attached to the LED, and/orwithin the system to be able to fulfill therequirements of the specific applications.In that connection the main task of lenses isto adapt the nearly point-formed LED lightsource to the application relatedrequirements.Typically, light emitting diodes (LEDs) aremounted to the circuit board using standard

soldering methods such as IR reflow orwave soldering. Due to their low meltingpoint, however, optical plastics are not ableto withstand the thermal stresses whicharise during soldering. The lens bodybecomes deformed, ultimately leading tofailure of the LED.

In the following, an example is provided forthe Golden DRAGONARGUSLED, a highpower LED with lens, in which important

design rules for the construction of LEDsystems as well as basic tips aboutmounting for processing are presented.

Furthermore, the recommended selectivelaser soldering method is introduced andimportant process parameters and steps forthe Golden DRAGON ARGUS LED aredescribed.

Construction and Design Rules

Golden DRAGONARGUS

The Golden DRAGON ARGUS is basedon the high power package of the GoldenDRAGON LED combined with a specialwide-radiating ARGUSlens (Figure 1).

Figure 1: Golden DRAGONARGUSLED


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The LED is particularly well suited foruniform illumination of large areas. For directbacklighting of large screen TV displays orto serve as the basis for LCD panels orbillboards it represents the ideal LEDbacklight product.

The basic model - the housing of the GoldenDRAGON (Figure 2) - is suited for standardSMT soldering methods such as reflow orvapor phase soldering.

Figure 2: Basic model of the GoldenDRAGONLED family

With the Golden DRAGON ARGUS LED,however, the maximum processingtemperature is limited to 125 C.

Standard soldering processes in which theentire LED is exposed to thermal stresscannot be used for further processing, as thelens could get damaged.

Instead, a selective soldering method suchas the use of a laser beam is recommended,in which only the solder connections at theleads experience the maximum soldertemperature; the thermal stress for the entireLED remains low.

Printed Circuit Board (PCB)

When designing an appropriate thermalmanagement for high power LEDs (see alsoapplication note "Thermal Management ofGolden DRAGON LED), the circuit boardplays a key role. As a direct interface withcontact to the LED, it significantlycontributes to effective thermal transfer andheat dissipation. By selecting an appropriate

material with sufficient surface area, thecircuit board itself can provide the cooling ifrequired.By examining the internal construction ofGolden DRAGON LED package (Figure 3),it can be seen that the semiconductor chip isdirectly mounted to an integrated metal heatslug, in order to provide better heatdissipation.

Figure 3: Internal construction of theGolden DRAGONLED basic model

Since LEDs of the Golden DRAGONproduct family can dissipate up to 2 Watts ofpower depending on the chosen operatingparameters, additional heat transfer anddissipation by the circuit board is essential.

The choice of an appropriate material for thecircuit board is therefore particularlyimportant. Materials with insufficient heattransfer capability lead to a degradation inreliability and prevent operation at optimalperformance levels, since the heat whicharises cannot be sufficiently dissipated.When constructing light sources with GoldenDRAGON LEDs, insulated metal substrateprinted circuit boards (IMS-PCBs) aretypically used.Usually, these consist of an aluminummounting plate, a thin dielectric insulator anda conductive layer of copper for electricalcontact.

Flexible circuit board material can also beused in conjunction with a metal substrate.The combination with flexible circuit boardmaterial offers the advantage that a three-


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dimensional light source design can berealized, for example.

Standard substrates such as FR4, forexample, are normally not suitable for usewith high power LEDs like those of theGolden DRAGON family, due to their lowheat transfer capability.

Tests at OSRAM OS with thin FR4 materialin combination with thermal vias andadditional cooling (e.g. an aluminum plate)show that this construction can also be used(Figure 4), if a good thermal connection isprovided between the FR4 material and thecooling unit through the use of a thermalinterface material (see also the application

note "Thermal Management of GoldenDRAGONLED").

Compared to IMS-PCBs, a cost advantageof around a factor of 3 can be achieved.

Figure 4: Cross section of FR4 withthermal vias

Solder Pad Design and Surface

When designing solder pads as a directcontact between the circuit board and the

component, differing requirements should betaken into account.The design goal is to achieve an optimalbalance between processability, lowpositional tolerance and a reliable solderconnection. In addition, the requirements fora good thermal management should also befulfilled.

Figure 5 shows the general optimized solderpad design for the Golden DRAGONARGUSLED.

Figure 5: General solder pad design forthe Golden DRAGONARGUS

The specified design refers to the openings

in the solder or adhesive mask, since formany applications the copper pads areusually larger in size.

To fulfill the requirements of a good thermalmanagement with the high power GoldenDRAGON LEDs, the copper area aroundthe integrated slug should be kept as largeas possible. This serves to dissipate orspread the accumulated heat and is typicallycovered with a solder resist layer.

Additionally, care should be taken that thecopper surface for the heat sink is insulatedfrom other solder pads or heat sink surfaces,since the anode and heat sink of the GoldenDRAGON housing are electricallyconnected.When the integrated heat sink of the GoldenDRAGON ARGUS is attached with athermally conductive adhesive, this designaspect is less critical. It is recommended,however, that the separation be maintainedin order to prevent undesirable short circuitsfrom occurring.Figure 6 shows the recommended copperpad design of the Golden DRAGONARGUSLED.


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Figure 6: Recommended copper paddesign for the Golden DRAGONARGUSLED

The detail view (Figure 7) additionally showsthe heat barrier between the solder pads

and the electrical connections. Such a heatbarrier is recommended in order to keep theheat introduced by soldering almostcompletely within the area of the solder pad.Larger connection paths would dissipate theheat away from the solder pad, making thesoldering process more complicated.

It would be more difficult to achieve thenecessary soldering temperature, and wouldincrease the time required to form the solderconnection.

Figure 7: Heat Barrier between solder padand electrical connection

Plating of the solder pad surfaces can becarried out with various types of materials:

HAL lead free (Hot Air Leveling)

OSP (Organic SolderabilityPreservative)

Chemical Tin


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Principal Assembly Steps

The general assembly process for theGolden DRAGON ARGUS LED differsonly slightly from the conventional SMD

soldering method.In the following the individual processingsteps, shown in Figure 8, are specified.

Figure 8: General assembly process flowfor Golden DRAGONARGUSLED

Compared to a standard soldering method,the Golden DRAGON ARGUS LEDrequires two additional processing steps.First of all, this includes the application of athermally conductive adhesive, and

secondly, the curing of this adhesive at theend of the entire process. Both steps arenecessary in order to achieve a thermalconnection between the slug for the housingand the circuit board.

In general, however, all individual processsteps are based on standard methods usedfor mounting semiconductor components.

Solder Paste Stencil Print

The application of solder paste has asignificant influence on the solder quality ofthe component. Since it is the cause ofaround 60 - 70% of all failures which arise

during the assembly process, this generallyrepresents the most critical process in theentire chain.To apply the solder paste, various methodsare available:

Silkscreen/Stencil Printing Dispensing

For large scale production, stencil printinghas established itself as a standard due tothe long operating lifetime of the stencil and

its high processing speed.

Figure 9: Stencil Printing - FunctionalDiagram

With stencil printing, the solder paste istransferred to the circuit board through the


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laser-cut or etched stencil in a fashionsimilar to screen printing.To apply the solder paste, a spreadertraveling at a constant speed and a specificcontact pressure is moved across thestencil, such that the rolling paste passesthrough the openings in the stencil and isprinted onto the circuit board.In order to achieve a high quality soldering,the paste must be applied in the exactposition, geometry and volume required,without so called edge tears or smearing ofthe paste after the stencil is removed.

The volume of the printed solder paste isdetermined by the stencil aperture (openingof the stencil) and the stencil thickness.

For the Golden DRAGONARGUS LED, itis recommended to use stencils with athickness of 150 m (in the range of 120 180 m). Furthermore, laser cut (usuallymade from stainless steel (CrNi)) orelectroformed stencils (Ni) are preferable.

A general recommendation for dimensioningthe stencil openings cannot be provided,since the optimal geometry is dependent onadditional parameters such as the

composition and reflow characteristics of thesolder paste, the wetting characteristics ofthe circuit board plating and componentleads and the soldering method employed.The geometry of the solder pads and stencilopenings should always be adapted to therespective customer requirements andspecific process parameters.

In practice however, the stencil openings areoften dimensioned to be 50 m smaller thanthose for the solder pad, or a general 10

percent reduction in size.

Dispensing of Thermal Conductive Glue

Nowadays, the available dispensingmethods can be subdivided into two maincategories:

board contact dispensing non-contact dispensing

The primary difference between the twocategories is whether or not the dispensingunit is in contact with the circuit board duringthe process.With the board contact method, thedispensing unit must be lowered to the levelof the circuit board and raised again.This movement in the z direction betweenthe individual gluing steps is not present in

the non-contact method. In this case, theadhesive is sprayed (jetted) onto the circuitboard from a particular distance so that apoint of glue (dot) is formed.

Although board contact dispensing methods(e.g. time/pressure dispensing, auger valvedispensing or positive displacementdispensing) are most commonly used, thetrend is going towards non-contactdispensing (e.g. jet dispensing) methods.These offer several advantages over board

contact procedures, such as:

minimal stringing faster dispense cycling time due to

elimination of Z-axis movement better dot reliability

When selecting a suitable thermal adhesive,there are several thermally conductivecompounds available on the market with oneor more components. In general however, itis strongly recommended to test the

suitability of an adhesive before its use.

As a thermal adhesive for the mounting andthermal connection to the internal heat sinkof the Golden DRAGON ARGUS LED, theadhesive AMICON E3503-01 from Emerson& Cuming is suitable, for example.


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Since the performance of the LED isinfluenced by the adhesive, care should betaken that:

the thermal conductivity of theadhesive is greater than 1 W/mK;

the entire contact surface of the heatsink is moistened with adhesive;

the film thickness is kept as low aspossible (typically 50 m, max.70 m);

sufficient mechanical force is applied;

Pick & Place LED

For selecting and inserting the components,

automatic placement devices with variousmethods such as Pick & Place orCollect & Place can be used.

The handling of the components within themachines is similar for the Pick & Place orCollect & Place methods.The SMD components are delivered in rolledtapes and are automatically fed to theplacement head. The placement head picksthe component from the tape with a vacuumpipette and places it at the prescribedposition on the circuit board.

Nozzle Design

During processing, mechanical stress to thesurface of the encapsulant or lens should beminimized to the extent possible.Essentially, a vacuum pipette which meetsthe requirements of each LED type shouldbe used.For population of the Golden DRAGON

ARGUS

LED, the use of specialcommercially available pipettes with arubber coating on the collar isrecommended.The rubber coating provides an easy way toprevent scratching or damage to the primaryoptics.

Figure 10 shows an example of aPick & Place nozzle for the GoldenDRAGONARGUSLED.

Figure 10: Sample for a Pick & Placenozzle for Golden DRAGON ARGUSLED


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SelectiveLaser Soldering

Due to the material properties of the lens, itis not possible to process the GoldenDRAGON ARGUS LED with a

conventional standard soldering process.Instead, a selective soldering process bymeans of a laser is recommended forproduction.In comparison to other selective solderingmethods, laser technology offers severaladvantages:

Precise positioning of the laser beam

The energy required for soldering isselectively applied

The solder temperature is very

quickly achieved No overheating of the LED and lens

Lower maintenance costs - no wearof soldering tips / dies

No mechanical damage to the leadsduring soldering

Controllability (temporal and spatial)

Reliability

Ease of automation

Thereby especially laser systems based on

high power laser diodes are suitable formanufacturing process. Compared to otherlaser systems, these offer the advantagesthat they are small in size, efficiently convertelectrical energy into photons, are more costeffective and are easier to operate.

Depending on the dimensions of the solderconnections and the speed required, theaverage laser power needed lies in therange of up to 80 Watts, with a wavelengthin the near IR range (800 - 970 nm).

The following Table 1 shows a few importantparameters for laser soldering the GoldenDRAGON ARGUS LED. The listedparameters are meant to serve as a basis,however, and should be verified with regardto the selected laser system, PCB material,layout, solder material, etc.

High Power Diode Laser Soldering

Wavelength ~ 940 nm

Power ~20 - 30 W per solder pad

PulseDuration

>1 sec.

BeamGeometry

~ Rectangular

Dimension ~ 2.3 x 2.3 mm

Pulse Form

Table 1: Basic information for lasersoldering of the Golden DRAGONARGUSLED

For the soldering process itself, a fewadditional points, both product specific aswell as material related, need to be takeninto account.Depending on the material used for thecircuit board, preheating of the board isnecessary, in order to achieve the required

melting temperature of the solder during thesoldering process.

For processing of Golden DRAGONARGUS LEDs on IMS-PCBs, the boardmust be preheated to 80 to 90 C, due to thehigh thermal capacity of the board. If FR4with thermal vias is used, preheating canhave a very advantageous effect on thesoldering process.

Furthermore, the laser power must be

individually adapted to the anode andcathode of the component. That is, whensoldering the lead on the anode side, moreenergy is required, due to the internalconnection to the slug, than for the cathodelead (see Figure 3, Internal Construction).If this is not taken into account and bothsides are soldered with the same high or lowpower, this can either lead to a melting ofthe LED housing at the cathode side (Figure11, top) or lead to an insufficient solder


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connection at the anode side, due to the lowpower used (Figure 11, bottom).

Figure 11: Possible failures duringsoldering process

Furthermore, during the soldering process,care should be taken that both leads, anodeand cathode, are soldered in the same step,

and thus simultaneously fixed to the board(Figure 12). This permits the self-centeringeffect to positively influence the alignment ofthe LED.

Figure 12: Principal Laser SolderingProcess

If the leads are soldered individually, thiscan cause the LED to tip or stand upright("tombstone" effect). In the worst case, thesecond lead cannot be soldered because ithas risen too far from the solder pad.

In comparison, Figure 13 shows a goodsolder connection. As can be seen in theimage, the entire solder meltssimultaneously, and exhibits a good wettingof the contacts.

Figure 13: Good solder connection / lasersoldering

When one compares the laser solderingprocess with a conventional standard IR-Reflow process, tests with the GoldenDRAGON device show equally goodresults. All tests listed in Table 2 carried outwith the laser process resulted in nocomponent errors or failures.Also, with respect to mechanical rigidity, nodifference was seen between completelysoldered components and those which wereadhesively soldered. The pull and shearingpower show comparable values.


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Mechanical Test

Test Performed Condition Duration

Shock

IEC 60068-2-27

500 g >4 ms

VibrationIEC 60068-2-6

Sin 20 -1kHz> 20 g, 3 axes / 1.5 h each /500 g >4 ms

Temperature Cycle(TC) Temp cycle 2 chambers

T = -40 C/125 CtTextreme = 15 min

1000 cycle

High Temperature Storage(HTS)

T = 125 C1000 h

Operating TestTest Performed Condition Duration

Steady state life test(SSLT) JESD22-A108

TS = 85CIF = 700 mA(overstress)

1000 h


TS = 85CIF = 400 mA(overstress)

1000 h


TS = 55CIF = 400 mA

1000 h

Temperature & Humiditybias(T&HB) JESD22-A101

TA = 60C, r.H. = 90%IF = 400 mA

1000 h

Table 2: Overview of Mechanical and Operating Tests

Glue Curing

In order to achieve final mechanical strengthand a reliable thermal connection to theinternal heat sink, a final process must be

carried out so that the thermally conductiveadhesive between the PCB and the GoldenDRAGONARGUSLED can cure.

Depending on availability, several systems(e.g. continuous furnace, furnace unit,furnace with an IR module, convection etc.)can be used for the curing process.The curing process specified by theadhesive manufacturer should be followed;in any case, however, the maximum

processing temperature of 110C for theGolden DRAGONARGUSLED should notbe exceeded.

The adhesive Amicon E3503-01 from

Emerson & Cuming should be cured at110C for a period of 30 min, for example.The color of the adhesive changes fromcream color (off white) to beige during theprocess.

Since the LED system reaches its maximalrigidity after curing, reworking the board tofix soldering errors at this point becomesextremely difficult, often causing damage tothe board or LED.


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It is therefore recommended that the LEDsystem be visually checked and theelectrical function tested before the curingprocess.

Alternative Solder Technology

In addition to the previously mentioned lasersoldering technique, there are otherselective soldering methods such as roboticor manual hand soldering. In general, theGolden DRAGON ARGUS LED can alsobe processed with these methods.

Robotic Soldering Technique

In principle, the robotic soldering techniqueis an automated soldering with an automaticfeed of the solder material (soldering wire).The mounting procedure corresponds to thatof the manual hand soldering process(Figure 16). That is, soldering first occursafter application and curing of the thermaladhesive for the internal heat sink of theLED.For the soldering process itself, the roboticsoldering iron is moved to the solderlocation, lowered into position and the

required amount of solder is simultaneouslyapplied (Figure 14).

Figure 14: Robotic Soldering

Compared to other soldering methods, theautomated hand soldering method requiresthe highest time expenditure. There is also acertain wear of the soldering tip associatedwith this method, which must be taken intoaccount.

Due to the preferred use of an adhesive,possible rework of the board is very difficult,and most often can damage the board orLED.

As process parameters for the solderingprocedure, the employment of manual handsoldering methods can be applied here. Inaddition, the basic recommendations forlead free soldering should also be observed.

Manual Hand Soldering for Prototyping

In addition to automated soldering methods,the Golden DRAGON ARGUS LEDs canalso be hand soldered - for the constructionof prototypes, for example.

The processing steps for manual solderingare basically the same as those for theautomated process. The application of asolder paste is not required here, since thesolder is applied manually during soldering.In general, it is recommended that thethermal adhesive be allowed to cure beforemanually soldering the LED.This considerably facilitates the handling ofthe soldering iron. This also helps to preventdifficulties due to tipping, sliding or standing

of the component.

Figure 15: Manual soldering

Depending on the type of PCB (InsulatedMetal Substrate-PCB or FR4 with thermalvias), preheating of the board by means of aheating plate may also be required.


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Figure 16: Recommended Hand Soldering Process for Golden DRAGONARGUS LEDs

Care should also be taken not to damage

the insulation layer of the PCB since this canlead to shunts or short circuits in the board.

In order to achieve good results when handsoldering with lead free solder, severalfundamental points should be observed:

Use soldering wire with a minimaldiameter in order to obtain better flowcharacteristics;

Keep the operating temperature aslow as possible in order to prevent

overheating of the LED; If possible, the shape and size of the

soldering tip should be adapted tothe geometry of the leads;

The soldering tip should be welltinned in order to provide a good,large heat surface between thesoldering tip and the solder location;

The use of an intelligent soldering

station with precise temperature andpower control is stronglyrecommended;

Furthermore, all contact between thesoldering iron and the optics of the LEDshould be avoided, since this can damagethe plastic lens and ultimately lead to failureof the device.Of the numerous types of soldering tipsavailable, the use of a conical tip with asolder repository (gull wing) is

recommended; alternatively, a chisel-shapedtip can be used.Ideally, the width of the tip shouldapproximately cover the width of the lead(1.8 mm).


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Summary

For the development and construction oflight elements with the Golden DRAGONARGUS LED, there are basically several

points to keep in mind:

Due to the high power dissipation, anappropriate thermal management isabsolutely necessary in order toguarantee optimal performance andlifetime of the LED (see also "ThermalManagement of Golden DRAGONLED);

Solder pads have to be designed in sucha way that the internal slug has no

electrical contact to the neighboringLEDs;

The heat sink cannot be soldered;instead, a thermal adhesive with lowthermal resistance should be used;

More energy is required when solderingthe anode, due to its connection with theslug;

Damage to the lens during the assemblyprocess must be prevented;

The maximum processing temperatureof the primary optics is limited to 110 C;

No degradation in performance of theGolden DRAGON ARGUS LED due to theextended assembly process could beobserved.

The quality and reliability of laser solderedGolden DRAGON ARGUS LEDs areabsolutely comparable to reflow solderedstate of the art SMT LEDs

Authors: Andreas Stich, Kurt-Jrgen Lang, Michael Sailer

About Osram Opto SemiconductorsOsram Opto Semiconductors GmbH, Regensburg, is a wholly owned subsidiary of Osram GmbH,one of the worlds three largest lamp manufacturers, and offers its customers a range of solutionsbased on semiconductor technology for lighting, sensor and visualisation applications. Thecompany operates facilities in Regensburg (Germany), San Jos (USA) and Penang (Malaysia).Further information is available at www.osram-os.com.

All information contained in this document has been checked with the greatest care. OSRAM Opto

Semiconductors GmbH can however, not be made liable for any damage that occurs in connectionwith the use of these contents.

Mounting Guideline of Golden DRAGON With Lens

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