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January 2017 • SMT Magazine 39 EFFECT OF SOLDER COMPOSITION, PCB SURFACE FINISH AND SOLDER JOINT VOLUME SEM image of Figure 11c reveals the connecting crack between the two competing crack paths producing the electrical failure. Figure 12 shows a different joint with a sim- ilar laminate crack emanating from the outer sider the joint and propagating under the pad. It has not yet propagated through the joint to produce a failure. A second BGA failure mode also observed: a combination IMC/solder fail- ure. A complete through crack is seen to propa- gate in the solder as well as along the interfacial IMC. Since this IMC/solder crack has fully tran- sected the joint, the direction of propagation is not obvious simply from this image. The characteristic starting location for this failure mode, however, can be determined from the symmetrical opposite corner joint where the failure process is not yet complete. This joint is shown in Figure 13. Here, the outer side will be to the left of the joint. This joint has not yet failed and none of the visible cracks are complete. Again, a laminate crack can be ob- served starting from outer side of the corner joint and propagating under the pad. There is also a crack propagating along the interface sol- der joint IMC similar to that observed in the op- posite corner joint. This IMC crack is not com- plete and thus can be clearly seen to emanate from the right (inner) side of the joint (detail shown in Figure 13b). The left side detail image in Figure 13c confirms the IMC/solder cracking process is yet incomplete. From the above analysis, we see that two distinct cracking mechanisms are competing to produce interconnect failures in this BGA256 component. These alternate crack paths, shown schematically for a corner joint in Figure 14, arise from the oscillating flexure of the board af- ter initial impact. Board flexure from the initial Figure 11: Pad cratering failure mode: (a) full BGA view, (b) the local region of IMC cracking, and (c) SEM image of the same local area showing PCB laminate cracking. Figure 12: BGA failure mode: IMC/solder failure. Figure 13: Crack in a non-failed BGA corner joint (a) whole cross-section (b) local area of high magnitude on the left (c) local area of high magnitude on the right.