Material Deformation
Combined damage from slow, time-dependent flow and cyclic stress characterizes the primary failure mode of common lead-free solder joints. Sac305 creep fatigue occurs when the tin-silver-copper alloy is held at a constant load during a temperature cycle. The material slowly deforms to relieve the internal stress caused by the mismatched expansion of the board and the component.
Dwell Impacts
Time spent at the high and low temperatures of a cycle determines how much the solder will creep. Longer dwell times allow for more grain boundary sliding and atomic diffusion which weakens the joint. Because electronics often operate at temperatures that are a high fraction of the solder’s melting point, creep is a constant factor.
The interaction between the cyclic fatigue and the constant creep leads to the formation of micro-voids at the grain boundaries. These voids eventually join together to form a crack that travels across the entire joint. High silver content in the alloy helps to slow this process but does not stop it.
Cycle Prediction
Engineers use specialized models to account for the creep rate when estimating the life of an assembly. Accelerated testing must be designed to include enough dwell time to capture this behavior. The resulting data helps in selecting the right alloy for long-life applications.
