36th International Electronic Manufacturing Technology Conference, 2014

Comprehensive Study in Mold Compound, Thermal Residual Stress, Package & LeadframeGeometry Influence towards Convex Cavity Package Warpage

Ng S.P, Wang H.T, Goo F.TInfineon Technologies Sdn Bhd,

Batu Berendam, Melaka 73500, Malaysia.SengPau.Ng@infineon.com ,HuiTeng.Wang@infineon.com, FuTat.Goo@infineon.com

Abstract

Higher demand of Rds(on) in power MOSFET packageintroduces the copper clip is used as interconnect material,this package construction increase CTE mismatch with moldcompound. Warpage can be categorised as convex warpage(leadframe warpage in cavity molding) and concave warpage(mold compound warpage in map molding). This paperfocus on convex leadframe warpage, where leadframejamming & damage are observed at molding offloadingtracks which subsequently effecting yield. Mold parameter,mold compound formulation, package outline, leadframedesign, thermal residual stress characterizations are essentialto minimize convex warpage.[1]

Four TDSON packages single heatsink, dual heatsink,singles heatsink fused lead and single heatsink fused leadLTI has been studied. Copper material within packageoutline increase > 2.34% in fused lead packages, increasingCTE mismatch between copper-die-mold compoundwarpage start to be seen. To maintain current fused leadpackages outline, optimization of leadframe design focusingat non-functional area is studied and the correlation ofwarpage versus copper material is established. Result showsreduction of warpage of the single heatsink fused lead from 5mm to minimal of 1.4mm.

Selection of basic resin, filler loading, warpage additivein mold compound plays an important role in thermal &chemical shrinkage. Lower Tg improves convex warpage,addition of short molecular chain biphenyl & LMW intoMulti-Aromatic is needed. Increment of CTE to 14ppm/dCalone reduce warpage from 5mm to minimal 3mm, additionof Warpage Relief Additive (abbreviated as WRA) reducesconvex warpage to 1.5mm. Correlation of WRA with moldshrinkage & convex warpage are established, result revealsaddition of WRA to max 4.6% reduces warpage by 3.5mm.Addition of unreactive WRA reduces mold compoundcurability.

Our studies also show thermal residual stress reliefidentified can improve convex warpage, this paper focus onof leadframe cooling time and design of degating coolingstation. Study show cooling station tubing position, blowingdirection and cooling mechanism contribute for differentequipment contribute to 20% warpage magnitude.

Introduction

Since the birth of MOSFET concept by 1920s, powersemiconductor has seen radical evolution towards higherpower demand with lower Rds(on) requirement. To meet thischallenge in power MOSFET, copper clip design is beingintroduced as a medium of interconnect material into powerpackages (illustrated in Figure 1) as well as the maximizationof contact between package heat-sink & lead with the contact

pad on the PCB. However, this comes with the price ofincreasing the CTE mismatch between the mold compoundwith the copper clip and the lead-frame surface due to theincrease of copper content in the package, causing thewarping of lead-frame during the molding stages leading toserious jamming of the machine causing yield losses andproblems to subsequent process.

Traditional plastic packages is comprised of lead-frame,mold compound, interconnect, die and the die-attachmedium solder paste. Each of these components is made upof different materials that behave differently under theinfluence of temperature change. One such importantbehaviour is the expansion of materials under changes oftemperature, termed the Coefficient of Thermal Expansion(abbreviated as CTE in the following articles) which causesthe entire assembled package to have different rate ofexpansion that resulted in change of shape. The CTE figureis listed in Table 1. This phenomenon could be particularlydetrimental during the molding process in which everycomponent of the semiconductor is assembled and constantchange of temperature is required as part of the moldingprocess nature.

Figure 1: Package construction of TDSON depictingcopper clip as interconnect between die and leads

At molding temperature of 175C, the package isconsidered stress free. When cooling down to roomtemperature, various material experiences different rate ofcontraction due to difference of CTE. The silicon die issubjected to compressive stress while that for moldcompound is tensile stress. Warpage after molding isclassified as concave (smiling) and convex (crying) warpage.Convex warpage is a phenomenon where copper leadframeshrink more than mold compound which is illustrated inFigure 1 part A. When mold compound experiences higherdegree of shrinkage (both thermal & cure shrinkage) thancopper material, concave warpage will be observed asdepicted in Figure 1 part B. Silicon chip has much lowerCTE compare to mold compound and copper, largerwarpage is associated with chip dimension increment as it issubjected to higher compressive stress. [1,2,3] Depending onthe magnitude of the warpage, the molded lead-frameassembly may jam upon offloading from the machine to theoutlet magazine resulting in unnecessary yield losses andmachine stoppages.

36th International Electronic Manufacturing Technology Conference, 2014

Table 1: Summary of Package Assembly MaterialCoefficient of Thermal Expansion

Selection of mold compound is crucial to resolve thewarpage issue. From Figure 1, glass transition temperature,coefficient of thermal expansion, thermal and cure shrinkageare identified as key factor to determine the degree ofwarpage. These thermal mechanical properties are associatedwith selection of basic resin and hardener which determinethe degree of cross-linking of the molded compound. Silicais widely used as filler in mold compound as it has lowerCTE compare to basic resin. Its content greatly affects theoverall mold compound CTE. Recent development of newfiller system focuses on developing organic filler which canimprove CTE, storage modulus and shrinkage. Addition ofspecial WRA is another option to enhance mold compoundshrinkage which subsequently improves warpage.

Figure 2: The convex and concave warpage phenomenalwhen different materials experince difference degree ofcontraction along temperature changes. Part A: Convexwarpage. Part B: Concave warpage.

Alternatively, the effect of CTE contributing to thewarpage can be further mitigated by reducing the coppermaterial presence in the assembly since copper has a linearCTE relationship. Other aspects of our investigation are theprocess parameter of the molding, i.e. cooling time, as wellas the cooling system geometry of the molding machine.

From the above, to reduce the impact of the CTE duringthe molding process, several methodologies could beimplemented in order to mitigate its effect through changesof the molding compound material and the geometry of thelead-frame. As our work presented below, we seek toestablish a comprehensive studies of the correlation ofwarpage with these factors. The target of this studies is toreduce the warpage level of lead-frame to less than 1.5mm.The reason for this target is to prevent the leadframejamming in the machine and be damaged at moldingoffloader system, casuing heavy yield loses in molding.

Methodology

a.) Mold Compound material formulation

Severe convex warpage could often be mitigated bycustomization of mold compound formulation. Thisformulation alteration focuses on the below points which issummarized in Table 2.I.) Increasing CTE of mold compound to match that of

copper leadframe

II.) Addition of stress relief additive to lower storage

modulus and enhance shrinkage

III.) Addition of shorter chain of basic resin and hardener to

reduce Glass Transition Temperature, Tg.

Epoxy mold compound consists of complicated mixtureof epoxy resin, hardener, filler, flame retardants, additive,coupling agents and mold release agents. The selection ofbasic resin determines the degree of polymerization. Thelonger chain and higher molecular weight imparts a highercrosslink density which increases the glass transitiontemperature, where the mobility of the backbone polymerchain is reduced.

Fillers are employed to lower the CTE of epoxy resin.Basic resin and hardener are the component determining thedegree of curability for mold compound, the selection ofbasic resin towards Tg has been studied. ThermalMechanical Analyzer is employed to measure moldcompound dimension change due to activation of molecularwhen it is subjected to a temperature range.

In our study, filler is reduced from 86wt% to 82wt% inorder to improve CTE1. WRA has been added into moldcompound to lower the thermal shrinkage stresses and elasticmodulus as well as flexibility.

MouldCompound

Formulation

Basic Resin Fillerwt %

WarpageAdditive

MC A LMW-1/OCN/Biphenyl

86 0.3

MC B MA/Biphenyl 86 2.7

MC C 3.6

MC D 4.2

MC E MA/Biphenyl 84 1.5

MC F 3.0

MC G 4.6

MC H MA/Biphenyl 82 3.0

MC I MA 85 NA

Table 2: Summary of mold compound formulation, fillerloading and additive

Hot hardness measures the stiffness and curability of themold compound where it has sufficient resilience andhardness to withstand the forces of ejection withoutsignificant deformation at the end of molding. Addition ofunreactive WRA may reduce the cross-linking of moldcompound. Hot hardness test is applied to test the hardnesswithin 10s of opening the mold. Hardness value ofapproximately 70 on the shore D at molding condition175C for 60s is considered normal. [1]

If the wax content of mold compound is insufficient orcurability of mold compound is low, the part can stick in themold chase and deform the molded part. Thus, release test isrequired to ensure the content of wax is sufficient to avoidthe sticking problem.

36th International Electronic Manufacturing Technology Conference, 2014

b.) Package and lead-frame design

Four similar TDSON packages with same packageoutline but different foot print have been chosen togetherwith the mold compound of MC A and MC G outlined inTable 2. The packages chosen are dual die pad package,single heat sink with standard lead, single heat sink withfused leads and single heat sink with fused leads of Lead-TipInspection feature (abbreviated as LTI). The package outlineis detailed in Table 3 together with the comparison of theassessment of warpage using mold compound of MC A andMC G.

It can be seen from the result of MC A, warpage resultfor all packages is 1mm. This is in contrast with the MC Gwhich shows a higher warpage level of 4 to 5mm for singleheat sink with fused leads and that of fused leads of LTI. It isworth noting:I.) The difference of warpage behaviour for MC A and MC

G is attributed to the differences of filler content as well

as the warpage additive as already explained in previous

methodology

II.) Warpage level for both single heat sink with standard

lead and dual die pad package still remain the same for

MC A and MC G.

Table 3: TDSON packages of different variant beingutilized in the studies

From Point I.) And II.), it can be noted that there areother factor influencing the warpage level than the moldcompound alone. Upon further study, this phenomenoncould be further explained by the lead-frame copper contentdifference between all the packages. The level of warpage inMC G is in the increasing trend with the copper content areaas tabulated in Table 3. By taking the single heat sink withstandard lead as a reference, it can be deduced that the singleheat sink with fused leads and that of LTI has the highestcopper content area while dual die pad has the least. This isdue to the fused leads as appear in the foot print.

Hence it is deduced that by reduction of copper contentin the lead-frame, the warpage level could be reduced andmitigated. The method is to reduce the copper of non-functional area such as lead-frame expansion slot and dam-bar of the lead-frame to avoid disturbing the foot print of thepackage. The comparison of the copper content is listed inTable 4.

The verification of warpage is then run on MC A andMC G with the modified frame.

Table 4: TDSON packages with the modified frame ascomparison

c.) Molding equipment

The molding equipments being studies shared similarfeature. There will be a cooling station at the in theequipment to cool down the leadframe after molding beforeproceeding to the degating station. Cooling is required forthese TDSON package in order to cool down the packagesstraight out of the molding station before transporting to de-gating station to avoid warpage or dented frame afterdegating. The degating station is normally maintainedbetween the temperature of 60oC and 90oC.

In our investigation, two cooling station system geometryin the molding equipments are being studied which isillustrated in Figure 3. Cooling station of Machine A wasblowing from the side of the system at a distance of around40 mm from the frames. Machine B has the cooling stationblowing at centre position of the frame at a distance ofaround 20 mm from the frames. Evaluation was carried outwith the MC H to assess the impact of the cooling systemgeometry towards the leadframe warpage.

d.) Figure 3: Design of the cooling station of the 2different machines

e.) Molding Processing Parameter

Molding process cooling parameters setting also act asimportant factor in leadframe warpage performance. Herethe assessment is on cooling time and the air blow flow rate.As a reference, the cooling time was set at 15s with the airblow being set at a range between 3 to 4 bar. Evaluation wasdone as per the following matrix in Table 5. Run A and Bwere focusing on the air blow flow rate setting to determinethe air blow impact towards the leadframe warpagebehaviour. Run C, D and E were focusing on the cooling

36th International Electronic Manufacturing Technology Conference, 2014

time setting to determine the effect of the cooling timesetting towards the leadframe warpage behaviour.

Table 5: Process parameter being studied

f.) Report of warpage measurement

The measurement of warpage leadframe was done usingthe Olympus measuring scope. As the frame having a cryingwarpage in a live bug condition, measurement was done indead bug position. The centre point of the frame was thelowest, therefore, at the side rail of the frame, zero point wasset at the centre of the frame and move to the both end of theframe (left and right). This was repeated at another side railof the frames. All the frames were using this method forwarpage measurement.

Result & Discussion

a.) Molding Compound

The problem reported in this paper is convex warpagewhere copper leadframe contraction is higher than moldcompound when cool down, thus the enhancement of moldcompound shrinkage is the primary focus to overcome thecopper material contraction. The study focuses on 5 areaswhere glass transition temperature Tg, CTE, content ofwarpage relief additive towards warpage, characterization ofWRA towards hot hardness and releasibility of moldcompound, and experimental verification of continuousmolding. The result is tabulated in Table 6.

Table 6: Summary of mold compound Tg, CTE,shrinkage and warpage result.

MC I with multi-aromatic basic resin has higher cross-linking density with the highest Tg of 113.2C. This canexcessively reduce the chain mobility and increase thebrittleness of the mold compound, therefore MC I resultingthe highest convex warpage of 10mm. This result indicateslower Tg is required to reduce the convex warpage. This canbe accomplished by addition of shorter chain and lowermolecular weight of resin which is biphenyl. Results shownby MC B to H successfully reduce the convex warpage toless than 5mm.

CTE of mold compound closer to copper leadframe CTE17ppm/C is another consideration in reduction of convexwarpage. Fused silica is widely used as filler in moldcompound and with 0.55ppm/C it has lower CTE thanepoxy resin. Result illustrates filler loading optimization inMC H is capable to improve CTE to 14ppm/C, howeverthis effect alone only improves warpage from 5mm to 3mm.Despite the reduction of filler loading to improve the convexwarpage, the risk of mold compound unable to fulfilflammability test UL94-V0 and higher moisture uptake arerising. As a result, the filler reduction is limited to 82wt%which still pass flammability test and reliability assessment.

Warpage relief additive is introduced in an attempt tomeet the target of warpage level reduction to less than1.5mm. Epoxy materials are inherently brittle due to theirhigh cross link density which confers the high temperatureperformance which is desirable; however this decreasespolymer chain mobility and relaxation response.Flexibilizing with warpage relief agents are added to epoxymold compounds to inhibit crack propagation as well as tolower the thermomechanical shrinkage stresses. In general,these materials are used to lower the elastic modulus andimprove flexibility. They may also lower the coefficient ofthermal expansion in some cases.

Figure 4 shows correlation of WRA additive effectivelyimproves mold shrinkage. Figure 5 establishes thecorrelation of warpage relief additive versus final convexwarpage for various mold compound filler loading. Thisenable mold compound designer to establish the targetdesign value of WRA in mold compound formulation.Experimental result in Table 6 illustrates MC B, C, D (withfiller 86%), increment of WRA from 2.7% to 4.2% improvewarpage by 3.5mm. MC E, F, G (with filler 84%), incrementof WRA from 1.5% to 4.6% improve warpage by 3.5mm.The result indicates addition of WRA is the most effectiveway to improve the final convex warpage to 1.5mm

Figure 4: Correlation of warpage relief additive contentversus mold shrinkage

36th International Electronic Manufacturing Technology Conference, 2014

Figure 5: Correlation of warpage relief additive versusfinal convex warpage for various mold compound fillerloading

Warpage relief agent can be unreative or reactive.Unreactive WRA include thermoplastic material which isintroduced in this study. If the added thermoplastic remainsin the epoxy matrix as a single phase material, then itscontent may affect the cross linking density. Sticking ofmold gate and package may occur if cross linkage isincomplete and wax content is insufficient. Longer cure timeor higher molding temperature is able to improve crosslinking of mold compound if wax selection and content areproperly selected. In this study, shortlisted mold compoundMC A, MC D and MC G are further subjected to hothardness test and releasibility test, all mold compoundsobserved promising result at molding condition of 175oC for45s. These result is further validated in experimentalcontinuous molding where no sticking are observed at moldgate and package for 8hours auto molding with short curetime of 45s.

Mold Compound MC A MC D MC G

Hot hardness,175dC(45s)

59 58 57

Releasability Test(175oC@45s)

Pass Pass Pass

Continuous 8hmolding(175C@45s)

Pass Pass Pass

Table 7: Hot hardness test, releasibility test andcontinuously molding test for shortlisted mold compoundwhich has pass the convex warpage criteria

b.) Package and lead-frame design modification

The result demonstrated that the modified leadframe ofthe TDSON fused lead is capable of reducing the warpage to1.4 mm from 5.0mm with MC G. This was significantimprovement by 3.6 mm or 65% from that of single heat sinkwith fused leads which is having leadframe warpage of 4.0mm. This prove the theory that copper content has directcorrelation towards the impact of the leadframe warpageperformance due to the high shrinkage value of the coppercomparing to the mold compound. This result is tabulated inTable 8 and illustrated in Figure 6.

Table 8: TDSON packages with the modified frame ascomparision

Figure 6: Copper content of leadframe versus thewarpage level (mm) with MC G and MC A

c.) Molding equipment

From Table 9, it shown that cooling station design for

Machine A having a better warpage performance compared

to Machine B. 40% of the frames from Machine A having

warpage lower than 2.4 mm compared to only 20% of the

frame from Machine B. This shown that the differences

between the cooling station can impact the warpage level of

the frames. The Machine A was proven to has better

warpage performance due to more uniform cooling with the

cooling station design. Whereas for Machine B cooling

station design, cooling for the leadframe was faster at centre

area only which cause the leadframes to be tilting one sided

due to the non-uniform cooling at the leadframes.

Table 9: Warpage level studies with Machine A andMachine B based on MC H

d.) Process parameter

From Table 10, it shown that the impact of the coolingair blow flow rate was low. This was because when the flowrate varies from 1 to 4 bar with constant cooling time, theresult of warpage was comparable for all groups (all frames

36th International Electronic Manufacturing Technology Conference, 2014

were measured below 2 mm). But when the cooling timevaries from 7s to 12s with constant of air blow flow rate, itshowed that with 7s cooling time the warpage was more than10mm which was not processabe. When the cooling timewas set at 10s, warpage was around 2mm, but dented wasobserved at both end of the frame randomly. Around 25% ofthe frames were facing this dented problem at both ends ofthe frames. This was mainly due to insufficient coolingcausing the frames to be warping when degating was done.When the frames were warping, there was high risk of thedegating punchers rubbing with the side of the framescausing this to happen. As for cooling time 12s, all framescan achieve the warpage of <2 mm with no dented issue. Sowith this result, cooling time must be set minimum at 12s toget the frame warpage below 2 mm and with no dented at theboth end of the frames.

Table 10: Warpage level result of the Process Parameterassessment

Conclusion:

1. Mold compound plays an important roles in warpage

management. Selection of shorter chain basic resin to

reduce glass transition temperature is one of most

effective way to improve warpage.

2. Increment of CTE effect alone only reduces warpage to

3mm. Filler reduction need to take into consideration of

flammability test and moisture uptake.

3. Addition of warpage relief additive is the most

effectively way to reduce convex warpage to 1.5mm. For

unreactive WRA such as thermoplastic, hot hardness and

continuous molding verification are essential to ensure

mold compound curability is sufficient.

4. Reduction of copper material of lead-frame in non-

essential area could effectively mitigate lead-frame

warpage during molding

5. Optimization of cooling station design is needed to

ensure uniform cooling to the frame to mitigate the frame

warpage.

6. Cooling time is of much greater in effect than air blow

but impact is not as significant as other factors as

mentioned above

7. The degree of influence on convex warpage by the

factors mentioned above could be ranked in the order

below:

Filler content & WRA = Leadframe copper content

reduction > Machine cooling station design > Cooling

parameter adjustment

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Microelectronic Devices, Van Nostrand Reinhold, pp 84-

92, 119

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proceedings of IEEE Transaction On Electronics

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3. Wei Tan, Fang Zhou, Xingming Cheng, Dong Ding, JuanWu, “The Warpage Control Method in Epoxy MoldingCompound”, 2009.

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