Issue link: https://iconnect007.uberflip.com/i/960594
56 SMT007 MAGAZINE I APRIL 2018 during assembly of the packages. Ball geom- etry and coplanarity, while slightly different, were within tolerance. These features can be further fine-tuned with the appropriate choice of relative pad sizes. With package coplanarity well within spec, the change was transparent to board assembly. One factor not studied was comparison of the hybrid design to a pure NSMD. Perhaps there were facets of this 512TEPBGA package that made it less susceptible to the trace cracking observed on the other packages [1] . The simi- larity in performance between the two differ- ent hybrid designs leaves this an open ques- tion. Since Hybrid-A had SMD along the die edge, while Hybrid-B had NSMD, we expected some difference in behavior. Other aspects of the NSMD pads also need exploring, such as the ability to withstand shock from handling and shipping, and performance in drop, shock and vibration testing. Lowering the substrate dielectric CTE from 16ppm/°C to 11ppm/°C changes the over- all mechanics of the package. As noted above, package warpage is driven primarily by the CTE mismatch between the mold compound and substrate. Given the mold compound expan - sion of 9ppm/°C, the low CTE core is only Figure 15: Distribution of degree of crack growth after 4013 cycles, as determined by dye-and-pry. Figure 16: Example dye-and-pry results after 4,013 cycles from cell 1. Images are PCB view, on top of BGA ball attached to PCB pad after pry. Fracture surfaces were between solder ball and package BGA pad. Red area was fractured during cycling. Shiny area was still intact.