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42 SMT Magazine • January 2017 alloy:finish combination, the failure modes were sampled at three different places in the failure rate distribution: early failure, character- istic life failure (N63) and late failure. The ear- ly and late failure cases examined samples that failed with the shortest and longest drop life- time, respectively. For N63, samples that failed nearest the characteristic life of the Weibull dis- tribution fit are selected. From Table 3, one can see that more pad cratering failures occur for BGA joints with the ImmAg PCB surface fin- ish than those with Cu-OSP finish. In general, the combined IMC/solder failures tend to pro- duce shorter drop lifetimes than pad cratering failures. It is interesting to note that the lowest variability case (SAC-M on Cu-OSP) produced only IMC/solder failures. Weibull Distribution Plots of LGA Drop Failures Weibull distributions fits of drop test life of LGA assemblies for five solder alloys on two PCB surface finishes are shown in Figure 16. LGA joints with the SAC305 alloy show the best per- formance on Cu-OSP PCB surface finish. On im- mersion Ag boards however, LGA joints of the SN99CN and SN100C alloys are seen to have the best drop performance. Figure 17 compares the characteristic drop lives of LGA interconnects for five solder al- loys on two finishes from the Weibull fits of Fig- ure 16. With the notable exception of SAC305, the drop life of LGA joints on ImmAg is great- er than that of LGA joints on Cu-OSP. Observed failure modes provide some insight into these relative behaviors. LGA Failure Modes The failure modes observed in LGA solder joints at different relative drop lifetimes are list- ed in Table 4. For LGA joints on Cu-OSP fin- ish, all drop failures exhibit bulk solder failure (see for example, Figure 18). This failure mode was not observed in the larger volume BGA sol- der joints for any alloy. For LGA joints on Im- mAg finish, IMC failure, IMC/solder mixed fail- ure, pad cratering and bulk solder failures are Figure 16: Weibull distribution plots of LGA joints failures in drop for five solder alloys on (a) Cu-OSP PCB surface finish and (b) ImmAg PCB surface finish. Figure 17: Characteristic drop life of LGA joints for five alloys and two PCB surface finishes. Alloys listed in order of decreasing Ag content. EFFECT OF SOLDER COMPOSITION, PCB SURFACE FINISH AND SOLDER JOINT VOLUME