Failure Site Transition During Drop Testing of Printed Wiring Assemblies

Abstract

This paper reports on the effect of printed wiring assembly (PWA) flexural strain and strain rate and aging on the durability and failure site of Sn-Pb solders in area array packages. Two different plastic ball grid array (PBGA) package styles, one with a single die and the other with stacked die, are used for this study. We consider the effect of intermetallic growth on the failure site transition in the interconnects. Results are compared to similar studies in literature. This study quantifies durability in terms of the local PWA response (PWA flexural strain, PWA flexural strain rate, cyclic history) instead of the conventional approaches that typically use loading parameters (total impact energy, orientation and number of drops. This makes the results more generic and easier to extrapolate to different assemblies and different loading/orientation conditions. Four point bend tests are conducted on the PWAs assembled with PBGA components. A high speed data acquisition system with in-situ resistance monitoring is used to track the PWA response and the number of cycles to failure. The strain rate is varied over three orders of magnitude while the peak strain is studied over the range of 25 to 75% of the overstress limit. The durability of the specimens decreases with increasing PWA flexural strain and varies non-monotonically with flexural strain rate. As-reflowed samples (with a thin intermetallic layer), undergo a transition in the failure site from bulk solder to FR-4 board and copper trace with increasing PWA flexure. Aged samples (with a thick intermetallic layer), have lower durability and fail in the intermetallic layer for all values of PWA strain. This study identifies the PWA flexural strain and strain rate limits for failure site transition in aged and as-reflowed samples of a PBGA package.