Issue link: https://iconnect007.uberflip.com/i/960594
52 SMT007 MAGAZINE I APRIL 2018 with cracks less than 50% of the pad length. Cracks greater than 90% were observed only on the SMD cells 1 and 4. However, these occurred under the die edge along row K which in principle should behave the same as Hybrid-A for these particular joints. This result also differs from the previous study [8] which found more advanced cracking in row B in the cross-sections. Overall the hybrid cells had less cracking than the SMD cells, matching the electrical test results in this regard. The solder joints with the most advanced cracking are presented in Figures 13 and 14. Failure locations within the solder joints are very typical: in the bulk solder near the inter- facial IMC (intermetallic compound), predom- inantly on the package side, but a few on the PCB side as well. Keep in mind that a solder joint with 100% linear crack length measured in cross-section would not necessarily fail electrically, since the sectioning plane may completely miss portions of the solder joint that are still intact. Dye-and-Pry after Cycling At 4,013 cycles, dye-and-pry was performed on an unmonitored sample from each of the cells, and degree of cracking calculated. Crack distribution maps are shown in Figure 15 using the same color-coding as above. The patterns here are consistent with the electrical test results. Cell 1—SMD with standard material— showed the most solder joints with significant cracking. Only one solder joint with greater than 50% cracking was observed in the low CTE cells versus 26 for the standard material. While cracking was observed in all portions of the array, the second-to-last ring of joints expe- rience the most. This ring experienced higher strain than the outer-most since the mold cap Figure 12: Distribution of degree of crack growth after 3,000 cycles, as determined by cross-section.