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Thermal cycling reliability, microstructural characterization, and assembly challenges with backward compatible soldering of a large, high density ball grid array

Accelerated temperature cycling (ATC) was used to assess the thermal fatigue reliability of a Pb free, 37.5 mm fully populated, 1295 I/O ball grid array (BGA) package assembled with backward compatible or mixed alloy (Pb free BGA/SnPb paste) processing. The Pb-free BGA components were fabricated with Sn-4.0Ag-0.5Cu (SAC 405) solder balls. The surface mount assembly was done using several custom SnPb eutectic soldering profiles designed to produce different levels of Pb mixing in the BGA solder balls. The test program also included SAC405-SAC405 assemblies for reliability comparisons. The temperature cycling and metallographic data indicate that assembly parameters that produce the lowest level of Pb mixing degrade the solder joint fatigue reliability. Failure analysis demonstrates that the lower reliability is due to poorly formed solder joints resulting from incomplete reflow. The failed solder joints exhibit inadequate BGA ball collapse, minimal Pb mixing, and fatigue failure at the printed circuit board (PCB) side of the joints instead of the package-side fatigue failures that are typical for ATC testing. The test cells with more complete Pb mixing and the pure Pb-free test cell exhibited better fatigue reliability. However, failure analysis of these samples revealed that the data were compromised by the occurrence of a second failure mode in plated through hole vias that were incorporated into the PCB test vehicle. The presence of mixed mode failures (solder joint and via) precluded direct quantitative comparisons among all the test cells. The results are discussed within the context of previously published test results from the literature, including the impact of the contribution of via failures. Recommendations are outlined for additional testing to quantify the relationships between Pb mixing level and Pb-free reliability for this high density BGA package.