Shear testing and failure mode analysis for evaluation of BGA ball attachment

With increasing focus on BGA technology as a high performance package, there is an urgent need to develop industry-wide standards for package quality and reliability. A significant factor in BGA package quality is the robustness of solder ball attachment to the package. Currently, ball shear testing is used to assess solder joint integrity. The solder ball failure mechanism can not be determined from shear force data alone, but is critical to understanding solder joint quality. A failure mode analysis, along with a fundamental understanding of the shear test process, is needed to assess solder joint quality. A common pad finish on BGA packages and PWB substrates is Ni/Au, using either electrolytic or electroless deposition. There are reports of brittle fracture in BGA and PWB interconnects with both types of Ni/Au surface finishes. During post-assembly test and manufacturing, the solder to pad interconnection has been shown to separate under certain conditions. In this study, ball shear testing is performed in conjunction with thermal preconditioning to evaluate brittle fractures on a thermally enhanced BGA with electroless Ni/Au bond pads and on a plastic BGA with electrolytic Ni/Au bond pads. Cross sectioning and fractography reveal interfacial failure in both package types. However, metallography and SEM with EDX confirm two distinct interfacial failure mechanisms. The failure analysis is interpreted in terms of the implications for package quality and performance. Although this evaluation is performed on BGA packages, the failure analysis results are applicable to brittle fractures on PWBs using a Ni/Au surface finish.