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38 SMT Magazine • January 2017 ponents on ImmAg PCB surface finish uniform- ly display slightly better drop reliability than those on the Cu-OSP finish. Microstructure and Failure Analysis of BGA Joints Microstructural analysis was performed for all alloys, both as-reflowed (before drop) and af- ter repetitive drop failure. The assemblies were cross-sectioned along the body diagonal of the board such that the traces of two input channels at the corner pad are visible on either side of the sectioned joint. Metallographic sections are prepared in the usu- al manner: sequential grinding using 80, 200, 800, 1200, 2000 and 4000 grit SiC papers fol- lowed by a final polish with 3µm and 1µm dia- mond compounds and 0.05µm Al2O3. BGA Failure Modes Solder failure is identified during repetitive drop using an event detector to capture excur- sions of electrical resistance beyond a threshold value. Failure is declared when three such events are observed within five consecutive drops. The drop test of each assembly is stopped after its first failure was confirmed. In 95% of samples, a corner joint was found to fail first. Metallo- graphic samples are oriented such the failed corner joint is viewed on the right side of the cross-section, the component on the top side of the image and the circuit board on the bottom. The opposite corner joint (not yet failed) will then be visible in the far-left side of the section. Figure 11 shows a representative image of one of the drop failure modes observed: pad cratering beneath the BGA pad. One can see in Figure 11a a complete crack traversing from one side of the joint to the other in the PCB lam- inate structure beneath the pad. There is also a shorter crack visible on the left, inside IMC layer. The inner side and outer side labeled in Figure 11a refer, respectively, to the side nearest the center of the package and side away from the package corner. The crack within the IMC layer initiates from the inner side of the corner joint (Fig. 11b), while the PCB laminate crack initiates from outer side of the corner joint. The EFFECT OF SOLDER COMPOSITION, PCB SURFACE FINISH AND SOLDER JOINT VOLUME Figure 9: Weibull plots of BGA joints for five alloys on (a) Cu-OSP PCB surface finish (b) ImmAg PCB surface finish. Figure 10: Comparison of characteristic drop life of BGA256 components between two surface finishes for the five solder alloys listed in order of decreasing Ag content.