WLCSP器件焊点可靠性

Rate-dependent properties of Sn–Ag–Cu based lead-free solder joints for WLCSPY.A.Su a ,L.B.Tan a ,T.Y.Tee b ,V.B.C.Tan a,*a National University of Singapore,Department of Mechanical Engineering,9Engineering Drive 1,Singapore 117576,Singapore bAmkor Technology,Inc.,2Science Park Drive,Singapore 118222,Singaporea r t i c l e i n f o Article history:Received 22July 2009Received in revised form 18January 2010Available online 24February 2010a b s t r a c tThe increasing demand for portable electronics has led to the shrinking in size of electronic components and solder joint dimensions.The industry also made a transition towards the adoption of lead-free solder alloys,commonly based around the Sn–Ag–Cu alloys.As knowledge of the processes and operational reli-ability of these lead-free solder joints (used especially in advanced packages)is limited,it has become a major concern to characterise the mechanical performance of these interconnects amid the greater push for greener electronics by the European Union.In this study,bulk solder tensile tests were performed to characterise the mechanical properties of SAC 105(Sn–1%wt Ag–0.5%wt Cu)and SAC 405(Sn–4%wt Ag–0.5%wt Cu)at strain rates ranging from 0.0088s À1to 57.0s À1.Solder joint array shear and tensile tests were also conducted on wafer-level chip scale package (WLCSP)specimens of different solder alloy materials under two test rates of 0.5mm/s (2.27s À1)and 5mm/s (22.73s À1).These WLCSP packages have an array of 12Â12solder bumps (300l m in diameter);and double redistribution layers with a Ti/Cu/Ni/Au under-bump metallurgy (UBM)as their silicon-based interface structure.The bulk solder tensile tests show that Sn–Ag–Cu alloys exhibit higher mechanical strength (yield stress and ultimate tensile strength)with increasing strain rate.A rate-dependent model of yield stress and ultimate tensile strength (UTS)was developed based on the test results.Good mechanical perfor-mance of package pull-tests at high strain rates is often correlated to a higher percentage of bulk solder failures than interface failures in solder joints.The solder joint array tests show that for higher test rates and Ag content,there are less bulk solder failures and more interface failures.Correspondingly,the aver-age solder joint strength,peak load and ductility also decrease under higher test rate and Ag content.The solder joint results relate closely to the higher rate sensitivity of SAC 405in gaining material strength which might prove detrimental to solder joint interfaces that are less rate sensitive.In addition,speci-mens under shear yielded more bulk solder failures,higher average solder joint strength and ductility than specimens under tension.Ó2010Elsevier Ltd.All rights reserved.1.IntroductionElectronic components are shrinking in size to meet demands for lightweight and feature filled portable electronic products.This leads to decreasing solder joint dimensions,where mechanical reli-ability has become an issue [1],especially under high strain rate conditions during testing,transport and handling,impact loading under automotive [2]and consumer portable applications.Tin lead alloy (SnPb)was commonly used as a solder material in microelectronic packaging,but it is also hazardous to the environ-ment and health.Therefore,the industry made a transition to lead-free solders,with the implementation a ban on lead (Pb)from elec-tronic products by the EU RoHS (restriction of the use of certain hazardous substances in electrical and electronic equipment)inJuly 2006.The transition to lead-free solders is led by the widely adopted Sn–Ag–Cu (SAC)eutectic [3].However,some studies have shown that standard SAC alloys such as SAC 405(Sn–4%wt Ag–0.5%wt Cu)have poorer mechanical performance than eutectic SnPb under high strain rate conditions [4].Moreover,with the increasing popularity of portable devices,the performance of Sn–Ag–Cu solder joints under high strain rate and large rate ranges typical of drop impact situations is a major concern.In this study,dogbone-shaped bulk material tensile tests were conducted to investigate the effect of strain rate and silver content on the material properties of Sn–Ag–Cu solders.Solder joint array shear and tensile experiments were conducted on WLCSP speci-mens of different alloy materials under different strain rates and loading orientations to investigate the effects of strain rate,silver content in Sn–Ag–Cu solder joints,and loading orientation on microelectronic packages.Failure analyses were also performed on the fractured dogbone-shaped bulk material test specimens and WLCSP solder joints.0026-2714/$-see front matter Ó2010Elsevier Ltd.All rights reserved.doi:10.1016/j.microrel.2010.01.043*Corresponding author.E-mail address:mpetanbc@.sg (V.B.C.Tan).Microelectronics Reliability 50(2010)564–576Contents lists available at ScienceDirectMicroelectronics Reliabilityjournal homepage:w w w.e l s e v i e r.c o m /l oc a t e /m i c r o r el2.Experimental details and methodology2.1.Test methodology for dogbone-shaped bulk solder tensile test Dogbone-shaped bulk solder specimens were used to perform tensile tests to characterise the mechanical properties of SAC 105and SAC 405.The bulk solder specimens had a gauge length of 19mm and diameter of 3mm as shown in Fig.1.The specimens were fabricated by machining from solder ingots and annealed at 70°C for 24h to reduce residual stresses.The bulk solder tensile tests were conducted on a universal tes-ter.Three to four samples were tested at various test rates from 10mm/min (0.0088s À1)to 65,000mm/min (57.0s À1).2.2.Test methodology for solder joint array shear and tensile test Package pull and shear tests were conducted on wafer-level chip scale packages (WLCSP)with SAC 105or SAC 405alloy solder joints at 0.5mm/s and 5mm/s.Solder joint strength and ductility data are collected and failure analysis (FA)of the WLCSP joints performed via optical microscopy and SEM.The FA findings are then statisti-cally tabulated and jointly analysed and correlated with test results.The WLCSP specimens were sawed out from board assemblies which were as-reflowed,unaged and non-solder mask defined,as shown in Fig.2.Each WLCSP specimen had an array of 12Â12sol-der joints sandwiched between a die substrate and a printed circuit board.These WLCSP packages have solder bumps that are 300l m in diameter and double redistribution layers with a Ti/Cu/Ni/Au under-bump metallurgy (UBM)as their silicon-based interface structure.The test samples were bonded onto fixtures using a cyanoacry-late base adhesive and tested on an Instron Microtester.Specimens of two solder alloy materials (SAC 105and SAC 405)were tested at room conditions.Two to three samples were tested for each test parameter.The tests were carried out at test rates of 0.5mm/s (2.27s À1)and 5.0mm/s (22.73s À1)and two loading orientations of shear and tension,as shown in Fig.3.3.Results and discussion3.1.Dogbone-shaped bulk solder tensile test resultsThe dogbone-shaped bulk solder specimens were tested un-der four test rates,with four samples per test rate.Nominal stress–strain data were derived from the load–displacementraw data and plotted as shown in Fig.4a ,where the mechanical properties such as yield stress and ultimate tensile strength (UTS)were obtained.Yield stress was obtained at the point of 0.5%offset.True stress–strain curves were also plotted for comparison.3.1.1.Effect of strain rate on bulk solder material propertiesFig.4b shows the true stress–strain curves of SAC 405,where the most representative sample curves of each of the four test rates were extracted and combined.It is observed that strain rate affects the bulk solder material properties,with increasing strength at higher rate of loading.Both Pang [5]and Che [6]had performed similar dogbone-shaped bulk solder tensile tests and also showed that material properties such as the yield stress and UTS of Sn–Ag–Cu solder alloys increase at higher strain rates.3.1.1.1.Rate-dependent model development.Mechanical properties such as yield stress and UTS have linear logarithmic/power relationship with respect to strain rate.As such,a rate-dependent model can be developed to express this relationship quantitatively.NomenclatureSn–Ag–Cu tin–silver–copper (Sn–Ag–Cu)alloy WLCSP wafer-level chip scale package r y yield stress,MPa r UTS ultimate tensile stress,MPa _e strain rate,s À1Std.dev.standard deviationDTBTSR ductile to brittle transition strain rateSEM scanning electron microscope UBM under-bump metallization IMC inter-metallic compound Cu RDL copper redistribution layerA T total failure cross sectional area,m 2r aveaverage solder joint array strength,MPaF maxpeak load from solder joint array shear and tensile tests,NFig.1.Dimensions of dogbone bulk solderspecimen.Fig.2.Printed circuit board assembly and sawed WLCSP testspecimen.Fig.3.Schematic of solder joint array shear and tensile tests.Y.A.Su et al./Microelectronics Reliability 50(2010)564–576565This is expressed in logarithmic scale as shown in Figs.5a and5b. Different solder alloy materials exhibit similar linear trend,with yield stress and UTS increasing at higher strain rate.The yield stress and UTS relationships with strain rate are obtained through curvefitting of the plots in Figs.5a and5b respectively to give:r yð_eÞSn—Ag—0:5Cu¼b1logð_eÞþb2ð1ÞrUTSð_eÞSn—Ag—0:5Cu¼c1logð_eÞþc2ð2ÞFor ease of comparison with existing literature,the rate depen-dence in yield stress and UTS can be further expressed in the fol-lowing power relationships:r yð_eÞSn—Ag—0:5Cu¼b3ð_eÞb4ð3ÞrUTSð_eÞSn—Ag—0:5Cu¼c3ð_eÞc4ð4ÞThe coefficients b1,b2,c1,c2,b3,b4,c3and c4are listed in Table1.The material parameters of SAC105and Amkor’s internal data for SAC305are compared with the results obtained by Che[6], which are presented in Table2.The development of such a rate-dependent model allows for better understanding of the relationships between mechanical properties and strain rate of applied loading.This enables the pre-diction of the mechanical properties at the strain rate of interest.It also enables the comparison of experimental results across similar literature.3.1.1.2.Strain rate sensitivity.Referring to Figs.5a and5b,the high-er gradient of the linear curves translate to larger increase in strength with a given increase in strain rate,which can also be ex-pressed as higher strain rate sensitivity of material strength.The coefficients b1and c1correspond to the gradient of the linear curves,which also directly relate to the strain rate sensitivity of yield stress and UTS respectively.Strain rate sensitivity can be use-ful in the study of the ductile to brittle transition strain rate (DTBTSR)in solder joints,which is affected by the sensitivity of bulk solder strength to strain rate,as depicted in Fig.12a.3.1.2.Effect of Ag content on mechanical properties of bulk solderFigs.5a,5b,6a and6b show the effect of silver(Ag)content on the yield stress and UTS of Sn–Ag–Cu solder alloy.Silver(Ag)con-tent of1%corresponds to SAC105and4%corresponds to SAC405. The yield stress and UTS of4%Ag content(SAC405)are consis-tently higher than1%Ag content(SAC105).Therefore,yield stress and UTS increase with higher Ag content in Sn–Ag–Cu alloys.Che[6]had also obtained results that show higher yield stress, UTS and lower elongation(equivalent to ductility)in Sn–Ag–Cu al-loys with higher Ag content.Sn–Ag–Cu alloys have three phases of primary Sn,Ag3Sn and Cu6Sn5[4].Suh[4]had presented the mechanical properties of the three phases of Sn–Ag–Cu micro-structure and primary Sn has the lowest elastic modulus and strength of the three phases.Sn–Ag–Cu alloys are strengthened by the internal stress accumulated due to the difference in elastic modulus and volume fraction between Ag3Sn and Sn matrix[7]. Higher Ag content will increase the amount of Ag3Sn phase[2] and also lower the amount of primary Sn phase in Sn–Ag–Cu solder alloys[4].SAC105,with lower Ag content,is expected to have less Ag3Sn and more primary Sn phase.This contributes to the lower566Y.A.Su et al./Microelectronics Reliability50(2010)564–576strength of SAC 105compared to Sn–Ag–Cu alloys with higher Ag content [4].3.1.3.Failure analysis of fractured dogbone-shaped bulk solder test specimensPhotographs of the fractured dogbone-shaped bulk solder test specimens were taken under an optical microscope.The sideviews/overviews and closeup views of the specimens are presented in Fig.7.It is generally observed that 45°shearing is the primary mode of failure at low test/strain rates while necking is mostly ob-served at higher test/strain rates.At low strain rates,the solder al-loy is able to undergo slip deformation across the whole cross section of the dogbone shaped specimen,resulting in 45°shearing.As Sn–Ag–Cu alloy materials have high homologous temperature,there is not much strain hardening under low strain rate loading.At higher strain rates,the material is not able to respond to the ap-plied loading by slip deformation and experiences greater strain hardening,therefore necking occurs.From Fig.7,it can be observed that the diameter of fracture generally decreases at higher test/strain rate due to necking.3.2.Solder joint array shear and tensile test results3.2.1.Failure analysis of WLCSP jointsOptical microscopy and scanning electron microscopy (SEM)were used to examine the fractured solder joints,which are around 0.3mm in diameter each.Three modes of failure were identified and categorised,namely:(1)bulk solder failure,(2)UBM IMC fail-ure and (3)pad matrix failure.Although there are other less pre-vailing failure modes observed (such as Cu RDL and Pad IMC),the three failure modes are chosen to be analysed due to their high frequency of occurrence (adding up to >90%of the failure modes of all the solder joints).They are also the most common failures iden-tified by researchers [8,9].Images of the respective failure modes at both the die/substrate side and board side were taken under the optical microscope and scanning electron microscope (SEM),as shown in Fig.8a–c .The photographs and SEM images are accompanied by cross sectional schematics to show the location of each failure mode.3.2.1.1.Bulk solder failure.Bulk solder failure occurs by fracturing through the solder sphere as shown in Fig.8a.These failures are usually detected near the die side interface and can be easily iden-tified by the silvery solder residue on the die substrate.From the board side,it can be recognised by the uneven fracture surface on the solder bump that is still attached to the board.3.2.1.2.UBM IMC (under-bump metallization intermetallic compound)failure.Brittle interface failure occurs at the interfacial regions be-tween the bulk solder and the die substrate.The failure crack usu-ally occurs in the intermetallic compounds (IMC)or at the interfaces with the substrate.These intermetallic compounds are usually more brittle than the bulk solder [1].UBM IMC failure refers to fracture at the intermetallic com-pound (IMC)interface layer between the UBM layer and the bulk solder.It can be identified by a smooth grey surface on the die sub-strate.From the board side,it can also be identified by its charac-teristic ring step and smooth surface at the top of the solder bump,shown in Fig.8b.3.2.1.3.Pad matrix failure.Pad matrix failure is failure in the pad re-sin at the printed circuit board.It occurs at the matrix of the com-posite that makes up the board.It is identified by the distinct redTable 1Material parameters of rate dependent yield stress and UTS of different solder alloys.Logarithmic Power Yield stress UTS Yield stress UTS b 1b 2c 1c 2b 3b 4c 3c 4SAC 105 2.92241.373 4.17454.71740.3780.077553.0930.081SAC 4052.97349.8645.29572.21348.7410.059669.6330.0779Table 2Comparison of material parameters of rate dependent yield stress and UTS of different solder alloys between Amkor’s data and Che’s data based on Eqs.(3)and (4).Yield stress UTS AmkorChe Std.dev.Amkor Che Std.dev.b 3b 4SAC 10540.3832.15 5.8190.07750.05130.01853SAC 30552.74643.32 6.6650.07150.04450.01909c 3c 4SAC10553.09353.990.6340.0810.09180.00764SAC30570.25860.187.1260.07770.06080.01195Y.A.Su et al./Microelectronics Reliability 50(2010)564–576567copper pad on the solder bump that is attached to the die.It can also be identified by ‘cratering’[3]in the board side,as shown in Fig.8c.Solder joints that were fractured during tests were examined to determine their modes of failure.Both Zhao [9]and Darveaux [8]had identified three major modes of solder joint failures forballFig.7.Photographs of fractured dogbone-shaped bulk solder tensile testspecimens.Fig.8.Schematic diagram of:(a)bulk solder failure,(b)UBM IMC failure and (c)pad matrix failure on die substrate,board,and solder joint cross section.568Y.A.Su et al./Microelectronics Reliability 50(2010)564–576grid array packages (BGAs),namely ductile bulk solder failure,brit-tle interface fracture at the intermetallic compound (IMC)layer and pad failure,shown in Fig.9.Bulk solder failure is commonly regarded as a desirable mode of failure.A ductile bulk solder will deform when loading is applied,and this will minimise stresses in the interfacial regions due to bet-ter package/PCB curvature compliance.As failure in interfacial re-gions can occur under relatively small strain displacements and under rapid crack propagation rate,it will be preferable to reduce their occurrence to avoid catastrophic failures [8].Wong et al.noted that bulk solder failures correspond to high drop test life of ball grid array packages (BGAs),which is more desirable than IMC failures that correspond to low drop test life [10].Therefore,the observation of bulk solder failure is often related to better reli-ability of solder joints under high strain rate conditions.3.2.2.Derivation of average solder joint array strengthThe package pull/shear tests yielded load–displacement raw data,where the peak load was recorded.Darveaux had calculated the average stress in the solder joint array by dividing load over the sum of joint pad area [2].In this study,the WLCSP solder joint has varying cross sectional dimensions at the different regions of failure as shown in Fig.10.The number of solder joints that failed under each mode (n i )were counted based on photographs taken using an optical microscope,shown in Figs.11a and 11b .The fail-ure mode count (n i )was subsequently multiplied by the cross sec-tional area of that failure region in a single solder bump (a i ),to obtain the total affected cross sectional area for that particular mode of failure (A i ).The total failure cross sectional area (A T )can be obtained by summing up the failure cross sectional area of each failure mode (A i ).Total failure cross sectional area,A T ¼XiA i ¼Xia i n i ð5Þwhere i =1corresponds to failure mode 1(e.g.bulk solder failure),i =2corresponds to failure mode 2(e.g.UBM IMC failure),etc.The average solder joint array strength (r ave )is calculated by dividing peak load (F max )over the total failure cross sectional area,A T .Average solder joint array strength,r ave ¼F max Tð6ÞThe peak loads obtained from the tests and the resulting peak stresses derived are given in Table 3.As shown in the preceding paragraphs,there is a distinction be-tween bulk solder strength and solder joint strength.Bulk solder strength refers to the strength of the solder alloy in resisting fail-ure,whereas solder joint strength refers to the strength of the en-tire solder joint that consists of a multitude of different materials making up the joint,such as that of the bulk solder,PCB Cu pad,UBM,RDL Polyimide layer dielectric,and silicon die.Further,fail-ure of the solder joint may not lie within a particular material,but at the interfaces,e.g.UBM/RDL,solder/IMC,etc.Hence,a strong solder alloy may not necessarily create a strong and reliable solder joint.In general,solder joint reliability depends on its components’mechanical properties and interaction,and the proper selection of solder alloys and their properties on a particular joint structure and make-up would improve solder jointlife.Fig.10.Typical cross sectional diameters of WLCSP solderjoint.Fig.11a.Fractured solder joint array specimen after shear test (dieside).Fig.11b.Fractured solder joint array specimen after shear test (padside).Fig.9.Schematic of bulk solder failure and interface failures.Y.A.Su et al./Microelectronics Reliability 50(2010)564–5765693.2.3.Effect of strain rate3.2.3.1.Ductile to brittle transition.Sn–Ag–Cu solder alloys gener-ally attain higher strength at higher strain rates [6,5].This is repre-sented in Fig.12a as an upward sloping line.At low strain rates,bulk solders have low strength,resulting in predominant failure in the bulk solders,depicted as ‘100%bulk solder failure’in Fig.12b .At higher strain rates,the bulk solders are stronger and more resistant to mechanical loading.The gain in strength also re-sulted in less deformation and compliance for the solder,and the applied loading stress will increasingly accumulate at the solder joint interfaces [8]due to their proximity to geometric discontinu-ities,resulting in more interfacial failures than bulk solder failures.As the test rates increases,there will eventually be no more bulk solder failures,being replaced entirely by failures occurring at the interface regions,depicted by ‘100%interface failure’in Fig.12b .As such,there is a transition from predominant ductile bulk solder failures to brittle interface failures with increasing strain rate,as shown in Fig.12b .Darveaux had investigated the ductile to brittle transition strain rate (DTBTSR)by performing solder joint array tensile tests [2].The ductile to brittle transition strain rate (DTBTSR)was defined as the strain rate at which,50%of the joints fail at the pad interface’[2].In this study,‘ductile to brittle transition strain rate’will simply be the strain rate where 50%of the joints fail at the bulk solder,‘inter-facial failure’will collectively refer to all the other non bulk solder modes of failure.It can be seen that the value of DTBTSR is a good gauge of what application strain rate the particular structure of solder joints can withstand before they start to fail abruptly at the joint interfaces.Knowledge of the DTBTSR values will allow researchers to design their packages to avoid excessive strain rate ranges that the joint would not be able to handle.As it would be desirable to register a relatively high peak load and observe bulk solder failures in solder joints under high strain rate loading,from Fig.12b ,the preferred outcome then would be to design solder joints to have a higher ductile to brittle transition strain rate (DTBTSR)[8]for improved reliability under high strain rate conditions.3.2.3.2.Effect of strain rate on solder joint array strength.Figs.13a and 13b show the average solder joint array strength of the tested specimens at two different test rates (0.5mm/s and 5.0mm/s)un-Fig.12a.Diagram of ductile to brittle transition in the relationship between solder joint strength and strain rate (adapted from [2]).Fig.12b.Diagram of ductile to brittle transition in the relationship between %bulk solder failures and strain rate (adapted from [2]).Table 3Solder joint array shear and tensile test results and derivation of average solder joint array strength.Solder alloyLoading orientationTest rate (mm/s)Average solder joint array strength,r ave (MPa)Failure mode count,n i Bulk solderUBM IMC Cu RDL Pad matrix SAC 405Shear 539.4401083330.543.663510107Tension58.960001440.556.77060139SAC 105Shear 536.890644770.554.79750070Tension517.110511390.542.36331111‘Pad IMC’mode of failure is not observed for SAC 105and SAC 405.570Y.A.Su et al./Microelectronics Reliability 50(2010)564–576der shear and tension.It is observed that the average solder joint array strength decreases at higher test/strain rates,regardless of solder alloy material and test orientation.Newman had observed higher strength at higher shear rates in solder ball impact shear tests [11],and Darveaux also obtained similar trend in solder joint array tensile tests [8].However,Darveaux also noted that solder joint strength declined at even higher strain rates when ‘interface failures start to occur’[8].From the experiments conducted in this study,interface failures had already started to occur in all the test specimens under the test rates of 0.5mm/s and 5mm/s.The test/strain rates employed in this study (0.5mm/s and 5mm/s)may al-ready be high enough to lead to the decline in average solder joint array strength.Referring to Fig.12a of the ductile to brittle transition strain rate model,it can be seen that beyond the DTBTSR,there is a fall in interface strength of solder joints.Coupled with the increase in the amount of interface failures (see Fig.12b or Fig.17),this will lead to a decline in combined solder joint array strength at higher strain rate.The solder joint array test results in Figs.13a and 13b have also shown that average solder joint strength declined with higher test/strain rate.3.2.3.3.Effect of strain rate on failure mode (under shear).(1)Bulk solder failure.Fig.14a shows the percentage of bulk solder failures observed for the two different solder alloy solder joints tested under shear.At the higher test rate of 5mm/s,there is almost no more failure at the bulk solder,implying that all failures occur at the interfa-cial regions.As bulk solder becomes more load resistant and experience less deformation,it is less susceptible to mechanical failure at higher strain rates.In the transition from ductile to brittle failure,there is a decrease in bulk solder failures to even-tually no more bulk solder failure at higher strain rates [2],as shown in Fig.12b .This is also depicted in Fig.14b ,in which the experimental result of SAC 105from Fig.14a is superimposed onto Fig.12b of the ductile to brittle transition strain rate model.Fig.14b shows that the experimental results of SAC 105follow a similar trend as the ductile to brittle transition strain rate model.(2)UBM IMC failure.Fig.14c shows the percentage of UBM IMC failures for the dif-ferent alloy solder joints under shear.Such failure modes are prominent at higher shear application rates due to the transition from ductile (cohesive)to brittle (interface)failures at increasingly higher strain rates.Fig.14b.Experimental result of SAC 105(under shear)superimposed onto the ductile to brittle transition strain ratemodel.Y.A.Su et al./Microelectronics Reliability 50(2010)564–576571(3)Pad matrix failure.In Fig.14d,pad matrix failure is observed to be increasingly dominant for SAC105as the test rates increases for shear.In contrary,SAC405has a reduction of pad matrix failures at the higher test rate of5mm/s,as a result of a corresponding predom-inance of UBM IMC failures(see Fig.14c),which is also an interfa-cial mode of failure.From thefigures for SAC405,the predominant failure mode changes from pad matrix failure at0.5mm/s to UBM IMC failure at5mm/s.(see Fig.14c).The observed failure modes transition may be a result of the UBM IMC layer being weaker than pad ma-trix layer at the higher test rate of5mm/s for SAC405.It is sur-mised that the higher rate sensitivity of SAC’s405mechanical strength may have contributed to the mode transition that is not yet observable for solder joints of the other two alloys.Essentially,interfacial failures such as UBM IMC and pad matrix failures become more dominant at higher strain rates,due to the strengthening of the bulk solder.Thefinal failure location depends on the components mechanical characteristics and is a competition among the different interfacial regions for failure as they share the applied load.Figs.14e and14f shows the percentage of interface failures (UBM IMC and pad matrix failures)of SAC405and SAC105respec-tively at the two test rates under shearing.There is a higher per-centage of interface failures at the higher test rate.Non bulk solder modes of failures such as UBM IMC and pad matrix failures are interfacial failures that become more dominant at higher strain rate,due to the strengthening of the bulk solder.This is similar to Figs.12b and14b,depicting the transition from ductile to interface failures in solder joints at higher strain rates.3.2.3.4.Effect of strain rate on failure mode(under tension).(1)Bulk solder failure.No bulk solder failure is observed for the two test rates.It is likely that the designated test rates of0.5mm/s and5mm/s have already surpassed the ductile to brittle transition strain rate (DTBTSR)for the three different alloyed joints and hence experi-ence100%interface failures shown in Fig.12b.In Fig.15a,the experimental result of SAC105is superimposed onto Fig.12b of the ductile to brittle transition strain rate model.Fig.15a also shows that the experimental results of SAC105correlates with the ductile to brittle transition strain rate model.Fig.15a.Experimental result of SAC105(under tension)superimposed into the ductile to brittle transition strain rate model.572Y.A.Su et al./Microelectronics Reliability50(2010)564–576。