Issue link: https://iconnect007.uberflip.com/i/960594
54 SMT007 MAGAZINE I APRIL 2018 did not extend to the package edge. This result is consistent with the prior published study, which more fully explains the mechanism [8] . Example images of the dye-and-pry from cell 1 are shown in Figure 16. The fracture surfaces were between solder ball and package BGA pad, consistent with cross-section. Discussion The original hypotheses were (1) a hybrid design with SMD pads under the die and NSMD pads outside the die have longer solder joint lifetime than a pure SMD design, and (2) low CTE substrate dielectric material would perform better than the standard material. Let's examine each in turn. It's been long established that NSMD pads reduce strain in the solder joint [1, 2] , offer- ing potential to improve solder joint lifetime. Our recent investigations on 292MAPBGA and 416PBGA packages also demonstrated slower crack growth on NSMD BGA pads. However, these packages failed prematurely due to pack- age side substrate trace cracks [3] . Once it was recognized that this alternate failure mode occurred only in the die shadow, the hybrid designs were created which placed NSMD only on the outer BGA rings. The hybrid designs outperformed the pure SMD designs in all metrics studied in the current investigation. They had on average approximately 1,300 cycles longer characteris- tic life, and 1,200 more cycles to first failure. Less cracking was noted using both dye-and- pry and cross-section. The manufacturability of mixing pad types on the same package was proven feasible. There were no abnormalities or yield issues reported Figure 15: Distribution of degree of crack growth after 4013 cycles, as determined by dye-and-pry.