CREEP FATIGUE MODELING FOR SOLDER JOINT RELIABILITY PREDICTIONS INCLUDING THE MICROSTRUCTURAL EVOLUTION OF THE SOLDER

In this paper we discuss the effect of microstructural evolution on the mechanical integrity of solder joints. To ensure reliability under service conditions, the possibility of failure by thermal cycling fatigue must be considered. For typical service temperatures (about half the absolute melting temperature) there is a significant interaction between creep and fatigue. The fatigue failure processes (crack nucleation and crack growth) involve time-dependent effects (e.g., stress relaxation by creep), and therefore, depend on cycle frequecy and hold times. The creep in turn depends on microstructure (e.g., the grain size and the size and spacing of second-phase particles). For several widely used solders, the microstructure may evolve during thermal cycling, changing the mechanical properties and creep relaxation responses. The time scale for these changes is often comparable to the cycle times for service applications or accelerated tests. Reliability predictions require extrapolations from laboratory time scales to service lifetimes. Such extrapolations are only accurate if the pattern of microstructural evolution is the same for the testing conditions as for the service conditions, or if the differences are understood and correctly modeled. We consider here the modeling of the microstructural evolution processes of precipitation, dissolution, and coarsening of the tin phase for low tin, lead-tin solders. © 1990 IEEE