Effects of NEMI Sn / Ag / Cu Alloy Assembly ReflowOn Plated Through Hole PerformanceJohn J Davignon and Randy ReedMerix CorporationForest Grove , OregonAbstractAt the present time there is a major thrust to evaluate which of the many Lead Free solder alloys to use for assemblingpackages . Many of the newer alloys have higher eutectic temperatures and require higher reflow temperatures than theexisting Sn / Pb reflow temperature . This paper evaluates the changes in PTH performance and laminate material propertiesfor several epoxy - based laminate systems caused by the thermal reflow requirements of the NEMI higher temperature alloy . Although the epoxy systems evaluated will discolor at the higher temperatures , the Glass Transition temperatures ( Tg ) andthe Z - Axis Coefficient of Thermal Expansion ( CTE ) are not greatly affected . Time to Delamination ( T - 260 ) was lowered in 2 of the 3 systems but not to a level to cause concern . The PTH performance for the standard FR - 4 epoxy and the High Tgepoxy was significantly degraded , while the PPO / epoxy blend did not significantly change . Keywords : This paper will discuss the effects on the Plated - Through - Hole ( PTH ) performance and epoxy - based laminatematerial properties caused by the higher temperature assembly reflow profiles required for the new NEMI 95 . 5 Sn / 3 . 8 Ag / 0 . 7 Cu alloy . PTH performance was evaluated by the Interconnect Stress Test ( IST ) method . BACKGROUND : There are many OEM and consortia programs underway toevaluate the repercussion of moving to a Lead Free solderalloy . These programs appear to be divided into two areas ; projects evaluating the type of alloy composition requiredfor optimum solder joint reliability and projects evaluatingthe optimal surface finish metallurgy . Data on the responseof the existing FR - 4 epoxy laminate systems to these higherreflow temperatures is limited . The authors were concernedthat the higher temperature reflow process will degrade theperformance of the epoxy - based systems . A program was undertaken to evaluate the response of threeepoxy - based systems to the higher temperature reflowprofile , which will be required by the NEMI 95 . 5 Sn / 3 . 8 Ag / 0 . 7 Cu alloy . The authors chose the NEMIalloy because its use is supported by a large group ofOEMs , and it will allow for a phased transition into acomplete Lead Free product . This alloy will allow a smallpercentage of Lead ( from the packaged component leads ) toenter the solder joint without concern for a low meltingtertiary phase . Several other alloy choices are available withslightly lower temperature requirements ( lower eutectictemperatures ) . Plated - Through - Hole ( PTH ) performance and materialproperties were chosen as the beginning point of theinvestigation . EXPERIMENT : The test vehicle used in this project was a 18 layer , 0 . 125 ” ( 3 . 12 mm ) thick construction with a series of IST couponswhich modeled a 20 X 20 . 050 ” ( 1 . 25 mm ) pitch BGA with a . 0135 ” ( . 34 mm ) diameter drill hole ( 400 holes ) . The finalfinish was bare copper . The IST coupons were shipped forassembly conditioning in a 3 x 6 multipak with a 0 . 100 ” ( 2 . 5 mm ) border . The coupons were scored for easy removal fortesting . The lay - up construction is shown in Fig 1 . Please note thatthis figure shows only one half of the PWB . The completestack - up can be obtained by mirror imaging layers 1 - 9 around the centerline . Three laminate materials were chosen for evaluation duringthis project . Laminate # 1 was a standard FR - 4 epoxylaminate with a supplier listed Tg of 140 ° C ( DSC ) . Laminate # 2 was a High Tg epoxy laminate with a supplierlisted Tg of 170 ° C ( DSC ) . The third laminate was a highLyr 1 : SignalPrepregLyr 2 : SignalCore laminateLyr 3 : Plane ( 2 ) PrepregLyr 4 : SignalCore laminateLyr 5 : Plane ( 2 ) PrepregLyr 6 : SignalCore laminateLyr 7 : Plane ( 2 ) PrepregLyr 8 : PlaneCore laminateLyr 9 : SignalMidpoint SymetryFig 1 : Test Vehicle ConstructionMerix Corporationperformance epoxy / PPO blend laminate with a supplierlisted Tg of 180 ° C ( DSC ) . The boards of each laminate type were fabricated at thesame time and then divided into three sets . The IST sampleplan for the three laminate materials is in Table 1 . Table 1 : Sample PlanMaterial Type PreconditioningNcoupons / holesFR 4 Baseline ( B ) 12 / 4800 STD Sn / Pb ( SR ) 10 / 4000 NEMI ( NR ) 12 / 4800 High Tg FR 4 Baseline ( B ) 10 / 4000 STD Sn / Pb ( SR ) 10 / 4000 NEMI ( NR ) 9 / 3600 Epoxy / PPO Baseline ( B ) 10 / 4000 STD Sn / Pb ( SR ) 8 / 3200 NEMI ( NR ) 12 / 4800 Set # 1 ( designated “ B ” ) received no thermal conditioning orsimulated reflow cycles and was used as the fabricationbaseline . Although all circuit boards need some level ofreflow assembly , the authors were evaluating the full effectof the assembly reflow profiles , both the standard Sn / Pb andthe NEMI alloy , on the unpopulated boards delivered byMerix . Set # 2 ( designated “ SR ” ) went through 5 X simulated 63 Sn / 37 Pb Standard Reflow cycles using a Forced AirConvection reflow oven . The reflow profile in Fig 2 showsthat the boards were held above 183 ° C ( the Sn / Pb eutectic ) for approximately 55 seconds and saw a peak temperatureof 215 ° C . Set # 3 ( designated “ NR ” ) went through 5 X simulatedreflow cycles using a Forced Air Convection reflow ovenwhich had the thermal profile modified for the new NEMI 95 . 5 Sn / 3 . 8 Ag / 0 . 7 Cu alloy . The reflow profile in Fig 3 shows that the boards were held above 217 ° C ( the 95 . 5 Sn / 3 . 8 Ag / 0 . 7 Cu eutectic ) for approximately 69 secondsand saw a peak temperature of 259 ° C . eflowNEMI Sn / Ag / Cu reflow profile 0501001502002503003504000 00 50 00 250 300 ime ( sec ) TemperatureFig 3 : NEMI Sn / Ag / Cu R Profile 0 5 1 1 2 TSets # 2 & # 3 saw no pre - conditioning or pre - bake prior toreflow . Five reflow cycles were chosen to model thethermal budget that is being required by most OEM ’ s and tohighly stress the boards . No solder was put in the holesduring the reflow cycles to assure accurate cycle to failureresults in the performance testing . After the reflow cycles were completed the boards wereplaced on the IST tester and cycled until a 10 % resistancechange was measured . The failed boards weremicrosectioned and analyzed for the cause of failure . STD Sn / Pb re flow profile 0501001502002503003500 50 100 150 200 250 Time ( sec ) TemperatuThe boards were also evaluated for changes in thermalproperties , Coefficient of Thermal expansion ( CTE ) andGlass Transition temperature ( Tg ) and Time toDelamination ( T - 260 & T - 288 ) using a Perkins Elmer DMAin the TMA mode . INTERCONNECT STRESS TESTINGMETHODOLOGY : The Interconnect Stress Test ( IST ) method heats thecoupon internally by applying DC current . The current isapplied to a heater circuit that daisy chains from layer 2 tolayer 3 and from layer n - 1 to n - 2 . Switching the current onfor 3 minutes heats the coupon from room temperature to 150 ° C . Switching the current off and using forced aircooling fans for 2 minutes returns the coupon to roomtemperature . This thermal cycling induces cyclic strain inthe PTH barrels and internal layer interconnectsaccelerating any infant mortality or latent defects . Thenumber of cycles achieved is an assessment of the PTHperformance . Fig 2 : Std SnPb Reflow ProfileThe IST method uses a coupon design with 2 independentcircuits . The Power circuit heats the coupon by resistiveheating as described above and monitors the interconnectintegrity for separations ( post separation and smear ) andinternal layer foil cracks . The Sense circuit runs parallel tothe Power circuit and monitors the temperature of thecoupon by resistance measurements and monitors for PTHbarrel integrity ( barrel cracks ) . The IST software uses analgorithm to calculate the temperature of the coupon basedon resistance to an accuracy of ± 5 ° C . The IST coupons were fabricated and tested at Merix ’ Analytical Services Laboratory . Figure 4 provides thesequence for IST testing . The resistance prescreen assuredthe coupons were functional and the coupons were chosenwith the lowest resistance variation within the multipak andbetween the multipaks . R ESISTANCEPRESCREENMeasure bulk resistance ofcoupon ( s ) Determine resistancevariation of test couponpopulation andSELECT SAMPLE SIZELOAD and PROGRAMthe selected coupons on theIST TEST EQUIPMENTPRECYCLE SEQUENCEEquipment optimizes thecurrent to heat the couponsto the selected hottemperature in 3 minutesTheTEST SEQUENCE isstarted . Post and PTHcircuit resistancesimultaneously monitoredevery 6 secondsPRECYCLE wasable to optimize thecurrent for thecouponANALYZE resistance dataIDENTIFY FAILURELOCATION ( S ) on couponsusing thermal imageequipmentFAILURE ANALYSISby m i crosectionLOADRELIABILITY DATABASEwith reliability results , microsection results , lay - up attributes , and design attributesCoupon AcceptedCoupon RejectedFigure 4 : Typical IST Method Test FlowRESULTS : Cosmetic changes : The first obvious result of the NEMIthermal profile is a change in laminate coloration . Figures 5 - 7 shows pictures of the Baseline , Standard Reflow andNEMI Reflow cycled boards for the three laminate systems . The darker coloration of the NEMI coupon shows that thehigher thermal profile caused some oxidation of the epoxymatrix , which was not observed in the Standard Sn / PbReflow . All laminate systems evaluated showed thisdiscoloration . Fig 5 : FR - 4 EpoxyFig 6 : High Tg EpoxyFig 7 : PPO / epoxyPrior to this evaluation it was believed that thisdiscoloration would correlate with the finding of the ISTand thermal properties results . This belief was proven to beerroneous . Changes in the Glass Transition temperature ( Tg ) and theCoefficient of Thermal Expansion ( CTE ) are often a goodindicator of changes in the polymer structure . Glass Transition changes : Table 2 shows that the FR - 4 and PPO / epoxy laminates had very little change in Tgcaused by the higher reflow temperature . only the High Tglaminate showed a substantial change in Tg . Table 2 : Tg results in ° C ( TMA ) Baseline STD Sn / Pb NEMIFR - 4 134 138 135 High Tg 167 166 156 PPO / epoxy 151 150 149 Coefficient of Thermal Expansion : Table 3 contains thebelow Tg Z - Axis CTE results and shows that there was verylittle change in CTE due to the higher reflow temperature . Z - Axis CTE is one of the largest contributors to PTHfailure . The stress placed on the PTH is directly related tothe expansion of the laminate surrounding it . This CTEinduced stress imparts a strain on the hole barrel that cancause cyclic fatigue and eventual failure . Table 3 : Z - Axis CTE < Tg ( results in ppm ) Baseline STD Sn / Pb NEMIFR - 4 63 64 66 High Tg 64 63 62 PPO / epoxy 68 68 68 Time to Delamination ( T - 260 ) : Table 4 contains the T - 260 results . This test ramps the sample from 20 ° C to 260 ° C at 10 degrees / minute and holds the sample at 260 ° C untilcatastrophic failure occurs . The time to delamination at 260 ° C is a measure of the ability of the dielectric bond line toabsorb stresses . A degradation in T - 260 results shows thatthe higher reflow temperatures have affected the bond line . Although a reduction in time to delaminate does not meanthat the boards will fail in service , it does indicate a loss inmechanical strength . There are many factors that relate to T - 260 failures , but polymer embrittlement and oxide layerdamage are two of the major causes . Although thePPO / epoxy laminate had the greatest drop in T - 260 results , prior industry results suggest that any value greater than 5 minutes is acceptable . Table 4 : Time to Delamination ( T - 260 ) in minutesBaseline STD Sn / Pb NEMIFR - 4 13 . 5 12 . 7 10 . 2 High Tg 4 . 9 5 . 0 6 . 8 PPO / epoxy 26 . 7 25 . 8 19 . 8 Table 5 contains the T - 288 results . Time to Delamination at 288 ° C is not the standard temperature for this test and wasdone to evaluate the stress on the bond line at the highertesting temperature . Our customers have indicated aninterested in testing at this higher temperature , especiallysince the new assembly reflow temperatures will be close tothe standard T - 260 temperature . The problem with this testwas that the epoxy laminates carbonized at the highertemperature . The T - 288 testing was stopped after the initialHigh Tg sample charred . Table 5 : Time to Delamination ( T - 288 ) in minBaseline STD Sn / Pb NEMIFR - 4 * * * High Tg * * * PPO / epoxy 3 . 8 1 . 0 3 . 0 The time to delamination of all three sets of the PPO / epoxyblend degraded very strongly when held at the higher 288 ° C temperature as compared to the T - 260 results . The first part of the project , the material propertiesevaluation , did not show any significant change in materialsperformance due to the NEMI Reflow , except for the Tgreduction in the High Tg laminate . The second part of theproject , the IST method evaluation , did show a performancedrop due to the NEMI Reflow for two of the laminate types . PTH performance testing : The IST test equipmentprovides resistance data for the 2 independent circuits onthe coupon . The resistance is monitored every 2 secondsduring the heating and cooling cycles . The resistance datawas reviewed for cause of failure . There was no evidenceof any infant mortality failures or latent defects . Failuresduring the first 50 cycles are considered latent defects . The number of cycles achieved is an assessment of the PTHperformance . To judge the significance of the cycle tofailure , the results are compared against industrybenchmarks . The FR - 4 benchmark represents 1 , 500 coupons ( 1 . 05 million PTHs ) for a 0 . 093 ” ( 2 . 32 mm ) thickFR 4 board with a drill hole diameter of 0 . 020 ” ( 0 . 5 mm ) ona 0 . 050 ” ( 1 . 25 mm ) pitch tested on IST equipment . NOTE : The test vehicle used in this evaluation was 0 . 125 ” ( 3 . 12 mm ) thick with 0 . 0135 ” ( 0 . 34 mm ) diameter holes on 0 . 050 ” ( 1 . 25 mm ) pitch . Prior IST studies have shown thatthe performance in all categories would be degraded forthicker boards with smaller holes . Since there are no FR - 4 industry benchmarks for a standard Sn / Pb reflowedconditioned board , the industry benchmark used in thisstudy had no assembly conditioning . There are no industrybenchmarks available for the PPO / epoxy blend and theHigh Tg FR 4 . For comparison purposes , these materialswere compared to the FR 4 industry benchmark . Figures 8 - 10 contains the normalized performance resultsfor the three laminate materials under the various reflowconditions . Each graph contains information on the IST coupon design ( Select Criteria ) and normalized statistical data . Reliability Databases Report Benchmark Performance GraphBenchmark Value for 1 . 0 : FR 4 Industry Standard ( 0 . 093 " Thick , DHS : 0 . 020 ", Pitch : 0 . 050 ") Select Criteria Statistics +/- Limit = 98 % ConfidenceChemistry = H - ELB Type Mean 1 Sigma +/- Limit N # HolesMaterial = FR 4 Baseline 1 . 1 0 . 34 0 . 26 12 4800 Board Thickness = 125 Std Reflow 1 0 . 24 0 . 21 10 4000 Number of Layers = 18 NEMI Reflow 0 . 7 0 . 18 0 . 14 12 4800 PTH Diameter =. 0135 " Grid Size =. 050 " Figure 8 : Normalized Results : FR 4 MaterialNormalized Performance for Assembly Preconditioning 00 . 511 . 522 . 533 . 5 Baseline Std Reflow NEMI ReflowAssembly Preconditioning TypeNormalized IST Cycles To Crack FailureThe Baseline FR 4 material performance is 10 % better thanthe FR 4 industry benchmark with no assembly conditioning . The Standard Reflow ( 5 X above 183 ° C ) reduced theperformance of the FR 4 product by 10 % as compared to theFR 4 baseline . The NEMI Reflow ( 5 X above 217 ° C ) significantly reduced the performance by 36 % as comparedto the FR 4 baseline . All failures were due to barrel crackswith no evidence of interconnect failures ( separations and / or foil cracks ) . A common recommendation to overcome this degradationin performance is to use a High Tg laminate . The followinggraph proved this logic to be erroneous for the NEMIReflow cycle . Reliability Databases Report Benchmark Performance GraphBenchmark Value for 1 . 0 : FR 4 Industry Standard ( 0 . 093 " Thick , DHS : 0 . 020 ", Pitch : 0 . 050 ") Select Criteria Statistics +/- Limit = 98 % ConfidenceChemistry = H - ELB Type Mean 1 Sigma +/- Limit N # HolesMaterial = High Tg FR 4 Baseline 3 . 4 0 . 7 0 . 6 10 4000 Board Thickness = 125 Std Reflow 3 . 4 0 . 9 0 . 8 10 4000 Number of Layers = 18 NEMI Reflow 0 . 7 0 . 2 0 . 2 9 3600 PTH Diameter =. 0135 " Grid Size =. 050 " Figure 9 : Normalized Results : High Tg FR 4 MaterialNormalized Performance for Assembly Preconditioning 00 . 511 . 522 . 533 . 5 Baseline Std Reflow NEMI ReflowAssembly Preconditioning TypeNormalized IST Cycles To Crack FailureThe High Tg material is 240 % better than the FR 4 industrybenchmark with no assembly conditioning . Thisperformance improvement was attributed to the Tg beinghigher than the IST test temperature ( 150 ° C ) . The testtemperature was not increased to measure the advantage ofusing High Tg laminates . The Standard Reflow ( 5 X above 183 ° C ) performance is equal to the baseline performance . This indicates that the Standard Reflow cycle had littleaffect on the PTH performance . The NEMI Reflow ( 5 Xabove 217 ° C ) significantly reduced the performance by 80 % as compared to the High Tg baseline . An interestingpreliminary result is that the performance ( after the NEMIReflow ) of the High Tg FR 4 and FR 4 laminate are thesame . All failures were due to barrel cracks with noevidence of interconnect failures ( separations and / or foilcracks ) . Reliability Databases Report Benchmark Performance GraphBenchmark Value for 1 . 0 : FR 4 Industry Standard ( 0 . 093 " Thick , DHS : 0 . 020 ", Pitch : 0 . 050 ") Select CSelect Criteria Statistics +/- Limit = 98 % ConfidenceChemistry = H - ELB Type Mean 1 Sigma +/- Limit N # HolesMaterial = Epoxy / PPO blend Baseline 1 . 5 0 . 42 0 . 36 10 4000 Board Thickness = 125 Std Reflow 1 . 1 0 . 17 0 . 17 8 3200 Number of Layers = 18 NEMI Reflow 1 0 . 13 0 . 11 12 4800 PTH Diameter =. 0135 " Grid Size =. 050 " Figure 10 : Normalized Results : Epoxy / PPO MaterialNormalized Performance for Assembly Preconditioning 00 . 511 . 522 . 533 . 5 Baseline Std Reflow NEMI ReflowAssembly Preconditioning TypeNormalized IST Cycles To Crack FailureThe PPO / epoxy blend material performance is 50 % betterthan the FR 4 industry benchmark with no assemblyconditioning . The Standard Reflow ( 5 X above 183 ° C ) reduced the performance of the PPO / epoxy blend materialby 26 % as compared to the PPO / epoxy baseline . The NEMIReflow ( 5 X above 217 ° C ) performance was reduced by 33 % as compared to the PPO / epoxy baseline . ThePPO / epoxy blend has similar performance losses for bothreflow processes . All failures were due to barrel crackswith no evidence of interconnect failures ( separations and / or foil cracks ) . The resistance data was summarized and evaluated forstatistical significance in Table 6 . The purpose of the F test ( variance ) and t test ( mean ) is to detect statisticaldifferences between the sample populations . Table 6 : Population C o m p a r i s o n M a t e r i a l TypePopulationComparisonF test t test ( 2 tail ) FR 4 B vs . SR Note 1 Note 1 SR vs . NR Note 1 Meansnot equalHigh TgFR 4 B vs . SR Note 1 Note 1 SR vs . NR Variancenot equalMeansnot equalEpoxy / PPOB vs . SR Note 1 Meansnot equalSR vs . NR Note 1 Note 1 Note 1 : There is not enough evidence to state that thevariance and means of the performance data are not equal ata 98 % significance level . • F test at 99 % significance • Student t test at 95 % significanceThe statistical evidence confirms the results in Fig 8 - 10 . The NEMI Reflow cycle significantly reduced theperformance of the FR - 4 and the High Tg epoxy laminate ( means not equal ) . The NEMI Reflow cycle had little effecton the PPO / epoxy laminate ( Note 1 ) . Metallurgical evaluations confirmed the IST results that allthe coupons failed for barrel cracks near the center of theboard . Figure 11 shows a typical example of the barrelcracks observed in the samples . This is the expectedresponse for reliable product that fails due to wearout . Theresistance data and microsections did not identify anyfailures due to interconnect failures ( separations and / or foilcracks ) . CONCLUSIONS : The higher temperature NEMI Reflow cycle using ForcedAir Convection caused discoloration of all three of thelaminate systems evaluated . It is unknown if using anitrogen purge would alleviate this situation . Colorationchange is not the only indicator of performance reduction . There was little coloration change seen in the comparison ofthe Baseline versus the Standard Reflow boards yet theStandard Reflow boards show a slight drop in PTHperformance for the FR - 4 laminate and a significant dropfor the PPO / epoxy . Merix has seen this performancereduction in other unrelated IST studies . The Tg ( TMA ) and Z - Axis CTE ( TMA ) values were notaffected by the NEMI alloy reflow temperature , except for adrop in Tg for the High Tg laminate . A general rule ofthumb used by industry is a 5 ° C degree change to certify atrue shift in Tg . The T - 260 results showed some decrease inthe PPO / epoxy laminate and only a slight change in the FR - 4 laminate . A general rule of thumb used by industry foracceptable results is a value that exceeds 5 minutes . Bothpure epoxy laminates evaluated were not able to withstandthe higher T - 288 test . The epoxy laminates carbonizedduring the heat up cycle . The IST method proved to be the best discriminator for theeffects of the NEMI Reflow on the performance of theboards . The IST method allows for evaluations to be madewith performance data rather than cosmetic attributes . Eventhough the PPO / epoxy blend had equal discoloration to theother two epoxy laminates , its ’ performance after the NEMIreflow cycle was not statistically different than the standardSn / Pb reflow cycle . The FR - 4 epoxy and High Tg laminatesshowed a statistically significant performance degradationafter the NEMI Reflow cycle . RECOMMENDATIONS : Since this evaluation showed that thermal properties are apoor indicator of post reflow performance , it isrecommended that Thermal Cycle testing be used in allLead - Free evaluations . Since the laminates discolored after the NEMI reflow cycle , it is recommended that PWB laminates and especiallysoldermasks be included in the Lead - Free evaluationprograms . Figure 11 : Barrel Cracks in PTH Since product lifetimes are shrinking , it is recommendedthat the industry reviews what is an acceptable postassembly Thermal Cycle PTH performance level . ACKNOWLEDGEMENTS : The authors would like to acknowledge ; Doug Lamond andHenry Garcia of Compaq Computers for providing all theassembly reflow activities and profiles , Steve York of PWBInterconnect Solutions for assisting in the IST results , andJuan Araneda of Merix for all the materials propertiesevaluation . Originally distributed at the SMTA Boston Lead - Free Symposium , Boston , MA , June 13 - 14 , 2000 저작 자 표시