Lead Free Solder for Flip Chip

Zhenwei Hou & R. Wayne JohnsonLaboratory for Electronics Assembly &Packaging – Auburn University162 Broun Hall, ECE Dept.Auburn, AL 36489 USA334-844-1880johnson@eng.auburn.edu

Erin Yaeger, Mark Konarski & Larry CraneLoctite Corporation1001 Trout Brook CrossingRocky Hill, CT 06067 USA860-571-2599Larry_Crane@loctite.com

Abstract

The electronics industry is moving to replace lead in electronic assemblies. The drivingfactors are potential legislation, primarily in Europe, and global market pressures for moreenvironmentally friendly products. The search for a ‘drop-in’ solder alloy replacement has beenongoing for more than 10 years and none have been found. The current industry trend is to usethe eutectic (or near eutectic) Sn-Ag-Cu alloy. The melting point of this alloy (~217oC) issignificantly higher (34oC) than eutectic Sn-Pb (183oC). This will impact the assembly process. Inaddition, the lead-free alloys have a higher modulus than eutectic Sn-Pb. This may change thestress distribution during thermal cycle testing, affecting reliability and failure modes. This paperexamines the assembly process for flip chip die with Sn-Ag-Cu solder bumps and initial reliabilitytesting.

Key Words: Flip Chip, Lead-free, Assembly, Reliability

Introduction

The elimination of lead in electronicsassembly has been discussed since 1990.Initially, the driving force was a proposedlegislative ban in the U.S. At the time nosolder alloy replacement alloys wereidentified and the legislation was droppedunder strong pressure from the electronicsindustry. However, increasing restrictions onhazardous materials in landfills, recyclingrequirements and manufacturerresponsibility for products from ‘cradle-to-grave’ have kept the topic of lead in themind of manufacturer’s. Today, withproposed legislation in Europe and globalcompetitive market pressures, particularly inJapan, the elimination of lead in many, if notall, electronic products appears imminent.The successful introduction of electronicproducts assembled with lead-free soldersbelies the arguments that it can not be done.However, just because some assemblytypes and reliability requirements can besatisfied with lead-free solder, does nottranslate to all products and all reliabilityrequirements. Much work remains to bedone.

One of the first challenges to theindustry was the selection of a replacement

solder alloy. The National Center forManufacturing Sciences (NCMS) concludedin 1997, after a major four year researcheffort that there were no ‘drop-in’replacements for eutectic Sn-Pb [1]. TheInternational Tin Research Institute (ITRI) [2]and the National Electronics ManufacturingInitiative [3] are both recommending the Sn-Ag-Cu eutectic (or near eutectic) alloy forreflow solder applications. Momentum doesappear to be building for this alloy selection.The Sn-Ag-Cu eutectic has a melting pointof ~217oC, significantly higher than eutecticSn-Pb (m.p. 183oC). This will require higherpeak reflow temperatures that may in turnimpact fluxes and flux residue. For flip chipapplications, the interaction between theunderfill and the flux residue may degradethermal cycle and thermal shock reliability.The higher Young’s modulus of the Sn-Ag-Cu alloy (46 GPa versus 33 GPa for eutecticSn-Pb) will also alter the stress distributionon the assembly, potentially impactingreliability and failure mechanisms.

This paper examines the assemblyprocess for flip chip die with Sn-Ag-Cu alloybumps and initial thermal shock testing. Diewith eutectic Sn-Pb solder were used forcontrols in the experiments.

Assembly

Two test vehicles were used todevelop the assembly process. The first testvehicle was the FB250 test die from DelphiDelco and test board from Flip ChipTechnology. The FB 250 die is 6.35mm x6.35mm with bumps on 0.457mm pitch. Thesecond test vehicle was the PB8 test diefrom Delphi Delco and test board from FlipChip Technology. The PB8 die is 5.1mm x5.1mm with solder bumps on a 204µm pitch.Both test boards are a four-layerconstruction with ten die sites. The designhas a trench in the solder mask to define thesolderable pads for flip chip attachment. ThePWB surface finish was electrolessnickel/immersion gold.

Two fluxes were used in theassembly evaluation, a tacky, no-clean fluxand a low residue, no-clean flux. The tackyflux was applied using the rotating fluxdipping station on the Siemens F5 pick andplace system. The depth of the flux film onthe rotating drum and the dip time controlledthe tacky flux volume transferred to thesolder bumps. The low residue flux wasapplied by spraying prior to die placement.All die were placed with the Siemens F5.

The reflow profile used for theeutectic Sn-Ag-Cu is shown in Figure 1.Thermocouples were placed on the boardsurface and under a die between the flipchip die and the board surface. Thetemperature variation between locations wasslight (<2oC). The peak temperature was240oC. The test vehicles were reflowed in anitrogen atmosphere (<25ppm O2) using aHeller 1700 oven.

Figure 1. Reflow profile for Sn-Ag-Cu SolderAlloy.

After reflow, die were sheared forinitial evaluation of solder wetting. The

solder sites were visually inspected.Samples were also mounted and cross-sectioned. Figures 2-4 show a series ofcross sections with FB250 die, Sn-Ag-Cusolder alloy and flux dip depths from 45µmto 65µm using the tacky, no-clean flux. Thehold time was 3 seconds in all cases. Forconsistent solder wetting, the 65µm depthwas required. For comparison, a flux depthof 55µm was used with eutectic Sn-Pbsolder.

Figure 2. Cross-Section of Sn-Ag-Cu FB250Solder Ball with 45µm Flux Depth Using theTacky, No-clean Flux.

Figure 3. Cross-Section of Sn-Ag-Cu FB250Solder Ball with 55µm Flux Depth Using theTacky, No-clean Flux.

The low residue, no-clean spray fluxalso achieved good wetting with the Sn-Ag-Cu solder alloy (Figure 5).

For thermal shock testing, PB8 testdie and boards were assembled. Again, diewere cross-sectioned after reflow toexamine the solder joints. Typical exampleswith each flux are shown in Figures 6 and 7.

Figure 4. Cross-Section of Sn-Ag-Cu FB250Solder Ball with 65µm Flux Depth Using theTacky, No-clean Flux.

Figure 5. Cross-Section of Sn-Ag-Cu FB250Solder Ball Using the Low Residue, No-clean Flux.

Figure 6. Cross-Section of Sn-Ag-Cu PB8Solder Ball with 65µm Flux Depth Using theTacky, No-clean Flux.

When examining Figure 7, therewas initial concern that the Sn-Ag-Cu solderdid not completely wet the sides of thecopper pad.

Figure 7. Cross-Section of Sn-Ag-Cu PB8Solder Ball Using the Low Residue, No-clean Flux.

Closer examination revealed defectiveplating on some of the PB8 test boards. Theelectroless nickel was thin and did not plateonto the sides of the copper pad on someboards. The Sn-Ag-Cu completely wet to theelectroless nickel, but not to the exposed,unprotected copper on the sides of the padsFigure 8). The FB 250 boards had theproper nickel plating and this problem wasnot observed. The problem does not appearto be related to the solder alloy, flux orassembly process.

Figure 8. Close-up of Left Side of Figure 8Showing Nickel and Solder Wetting on theTop of the Pad, but No Nickel or Solder onthe Side.

Some cross-sections revealed voidsin the solder joints (Figure 9). Unassembleddie were cross-sectioned and voids werefound in the solder balls as-received (Figure10). Thus, void introduction during the reflowprocess is not suspected. The effect ofthese voids on thermal shock reliability will

Solder

Nickel

Copper

be investigated during failure analysis aftercompletion of the thermal shock tests.

Figure 9. Cross-Section Showing a Void inthe Solder Joint after Assembly.

Figure 10. Cross-Section Showing a Void ina Sn-Ag-Cu Solder Ball 'As-Received'.

After reflow assembly, the die wereunderfilled with Loctite 3563, a fast flow,snap cure underfill. The die were underfilledusing a Camalot 3700 dispense system. Thesubstrate temperature was 95oC. Afterdispensing, the underfill was cured in aconveyor oven with a peak temperature of165oC for 5 minutes. A Sonix scanningacoustic microscope was used in C-mode toinspect the underfill. (Figure 11). No voidswere observed.

Figure 11. Example C-SAM Image.

Thermal Shock Testing

Liquid-to-liquid thermal shock wasused to evaluate the reliability of theunderfilled test vehicles. Two boards (20die) with each flux (tacky and low-residue)and solder alloy (Sn-Ag-Cu and Sn-Pb) weretested. The cycle was from -40oC to +125oCwith 5 minutes at each extreme and a 1-minute transition time. In-situ resistancemonitoring is required for accurate thermalshock or thermal cycle testing of underfilledflip chip die. The authors have observedfailures (high resistance/open circuits) withdaisy chain test die in the hot bath hundredsof cycles before the failure is observed atroom temperature or at the cold extreme.This can be explained using the illustrationin Figure 12. From finite element modeling,the maximum stress on the solder jointoccurs at the low temperature extreme ofthe cycle since the ‘stress-free” temperatureshould be near the cure temperature of theunderfill. However, at the cold temperatureextreme the contraction of the underfill (theCTE of the solder is lower than that of theunderfill) will hold the cracked solder jointtogether maintaining electrical continuity.

Upon heating to the hightemperature extreme, the expansion of theunderfill will open the crack and a failure canbe measured. Thus periodic measurementsfor continuity at room temperature will notdiscover high temperature intermittent opensand will over estimate the reliability of theconnections.

Room Temperature(a)

High Temperature(b)

Figure 12. Illustration of the Need for In-situMonitoring.

The thermal shock test is ongoing.At the time of this writing, 1445 cycles havebeen completed with no failures. The testwill be continued until >90% of the die havefailed and failure analysis will be performed.

Summary

An initial assessment of the flip chipassembly process with Sn-Ag-Cu solderdoes not indicate any significant change inprocessing will be required with capillaryflow underfills. A higher reflow temperaturemust be used, but appears to be compatiblewith existing PWBs and fluxes. The effect ofthe new alloy and higher processingtemperature on reliability must still bedetermined. Thermal shock tests areunderway.

References1. “National Center for Manufacturing

Sciences Lead-Free Solder Project FinalReport”, National Center for ManufacturingSciences, Ann Arbor, MI, August, 1997.

2. Kay Nimmo, “Worldwide EnvironmentalIssues In Electronics And The TransitionTo Lead-free,” Proceedings of the IPCLead-Free Summit, Minneapolis, October1999

3. NEMI News Release, January 24, 2000,www.nemi.org/PbFreePUBLIC/index.html