Study On The Reliability Of SMT Solder Joint In Electronic Packaging

Focused on the reliability issues of the SMT solder joint. the following aspects were studied 1) Theapplication of lead-free solders (Sn96.5Sn3.5 and Sn95Sb5) in automotive electronic assemblies 2)Investigation of geometry of duplex SnPb solder joint and its influence on solder joint reliability Themain results are summarized as follows1 The microstructure evolution and the change of shear strength of SnAg/Cu and SnSb/Cu solderjoints during thermal aging were investigated It was found that, compared With the very popularsolder of Sn62Pb36Ag2, the microstructure of SnAg/Cu and SnSb/Cu solder joints is more stablethan that of SnPbAg/Cu during thermal aging, and that the growth rate of IMC at the solder/Cuinterface and the coarsening rate of bulk solder were lower than those of SnPbAg Meanwhile, theshear strength of SnAg/Cu and SnSblCu solder joints is higher than that of SnPbAg/Cu (especiallywhen measured at high temperature), and the decreasing rate of shear strength of SnAg and SnSbsolder joint with the increase of aging time was smaller than tha of SnPbAg Therefore, SnAg and- SnSb solder are more subole for the application of high temperature device2 The influence of Pb-contomination on microstructure and shear strength of SnAg/Cu and SnSb/Cusolder joints prepared by SMT were staied by mixing eutectic SnPb solder paste into leadfreesolder pastes Pb-rich phase was found at the grain boundaries of the solder joint with thecontarmination of Pb, but there was little influence on the growth of IMC at solder/Cu interface Atroom temperature, there was no difference in shear strength of solder joint for those with or withoutPb contamlnatlon Howeveq at l25'C, the shear strength of the solder Jomt wlth Pb contaInmatlonwas lower than that wlthout Pb conanlnatlon as a result of the contaInmat1on of Pb3 The rellablllty of SnSb solder Jomt was exammed dur1ng thermal agmg, thermal cycllng andstorage at 85'C/85% an according to JESD-22 and compared wlth SnPbAg solder Jomt The Pb-rlch phase, whlch ls orlginally from the dlssolutlon of eutectlc SnPb w1thln the PCB finlshes or thetermlnatlons of LCCC devlces dunng reflow process and takes the form of preclpltatlon at gralnboundarles of the solder JomL wlll become coarsened wlth the 1ncrease of aging tlme The shearstrength of SnSb solder Jomt ls hlgher than that of SnPbAg, and wlll decrease wlth the lncrease ofaglng t1me, but the decreaslng rate ls lower than that of SnPbAg When the thermal cycles ls lessthan l000~l500, the shear strength of SnSb solder Jomt ls larger than that of SnPbAg, however,when the thermal cycles exceeds l000~l500, the shear strength of SnSb soIder Jolnt becomessmaller than that of SnPbAg These phenomena may be thebuted to that Ktandall volds werefOrmed at the lnterface of SnSb/Cu due to dlfferent dlffeslon rate of Cu and Sn atom along oppos1tedirectlon, and these volds may condensed and grew wlth the help of thermal stress Then wlthfurther thermal cycles, the adheslon betWeen IMC and Cu wlll be degraded due to the extendedvolds or the cracks along the IMC/Cu mterface, and finally the shear strength of SnSb wlll decreasequlckly Whlle dunng stOrage at 85'C/85% RH, there was llttle change ln the mlcrostructUre andthe shear strength for SnSb solder Jond. but for SnPbAg solder Joot there was certam coarsenmgof bulk solder and the shear strength decreased wlth the increase of storage tlme4 The potentlal energy cOntrolled equatlon for duPlex SnPb solder Joint was establlshed Themfluence of the sixe of pad and solder bump, the volume of eutectlc solder and reflow temperatUron the geometry of duPlex SnPb solderJomt with hlgh Pb solder bUInP and eutectlc SnPb fillet wassimulated by finlte element method w1th the software of SURFACE EVOLVER The regresslonmodel for predwtmp the stand-Off hegh of duPlex solder Jomt was presented in a dlmenslonlessform5 Based on the unlfied vlscoplamc Anand model wnd the single mtemal vanablede defOrmatlonr...