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January 2017 • SMT Magazine 43 all observed. Representative images of each are shown in Figure 19. It is noted that in all the failed LGA joints on ImmAg PCB surface finish, cracks were observed in the PCB laminate under the pad. Partial pad cratering always happened regardless of the crack path producing the ul- timate interconnect failure. These competitive cracking modes dissipate additional drop shock energy and may in some instances serve to pro- long the drop lifetime of LGA over BGA. For LGA joints failing through the bulk solder (pri- marily those on OSP), greater solder Ag content was seen to correlate with better drop perfor- mance in these SnAgCu based alloys, presum- ably due to the increased strength of solder (see Figure 20a). Solder Joint Volume Effects: BGA vs. LGA The characteristic drop lifetimes of LGA and BGA solder joints are compared in Figure 20 for Table 4: Failure Modes of LGA256 in Drop. Figure 18: Representative failure mode for bulk solder failure of LGA joints on Cu-OSP PCB surface finish. Figure 19: LGA failure modes on ImmAg PCB surface finish: (a) solder bulk failure, SN100C, late (b) IMC failure, SAC305, early (c) IMC/solder mixed failure, SAC105, early (d) pad cratering, SAC305, late. (a) (b) (c) (d) EFFECT OF SOLDER COMPOSITION, PCB SURFACE FINISH AND SOLDER JOINT VOLUME