The Role of Aging on Dynamic Failure Envelopes of OSP-Sn37Pb Interconnects in Plastic Ball Grid Array (PBGA) Packages

This paper quantifies the effect of aging on the durability of printed wiring assemblies (PWAs) subjected to dynamic loading conditions. The test specimen is a FR4 board with a single 256 I/O, full grid, PBGA component at the center. The pad finish on the board and component side of the Sn37Pb eutectic solder interconnects is organic solderability preservative (OSP) and Sn15Pb, respectively. The test matrix is designed to cover one order of magnitude of PWA flexural strain (1E-3 to 1E-2), three orders of magnitude of PWA flexural strain rate (1E-3 to 1E0 s^-1), and two aging conditions (as-reflowed and 125 ^degC for 100 h). Fatigue failure envelopes, based on a mechanics-inspired empirical rate-dependent model, are used to characterize the durability in terms of PWA flexural strain and strain rate. A failure site transition zone (FSTZ) is defined in terms of the damage parameters, beyond which the failure site changes from the solder to other parts of the PWA. The combined effect of load amplitude, loading rate, and thermal aging on the FSTZ and solder durability is quantified and presented.