Development of a microstructurally adaptive unified primary-cumsecondary creep model for SAC387 solder joints

Sn-Ag-Cu (SAC) solders are susceptible to appreciable microstructural coarsening due to the combined effect of thermal and mechanical stimuli during service and storage. This results in evolution of joint properties over time, and thereby influences the long-term reliability of microelectronic packages. Accurate prediction of this aging behavior is therefore critical for joint reliability predictions. Here, we study the coarsening behavior of ball grid array (BGA) joints of Sn-3.8Ag-0.7Cu (SAC387) solder attached to ENIG bond-pads with four different thermo-mechanical histories. Both Ag₃Sn and Cu₆Sn₅ precipitates underwent rapid coarsening due to the combined effects of the temperature and superimposed inelastic strain. However, following aging, the Cu₆Sn₅ particles were much coarser than the Ag₃Sn precipitates, and numbered about ~100 times less. Therefore the mechanical behavior evolution during aging is dominated by the coarsening kinetics of Ag₃Sn. Based on these observations, a length parameter describing Ag diffusion in Sn (L_eff) was defined, and this is shown to capture the thermomechanical history dependence of Ag₃Sn particle size and inter-particle spacing. The creep behavior of SAC387 joints was characterized and a unified primary-cum-secondary creep model incorporating the parameter L_eff was developed, with the objective of enabling the model to adapt to in situ microstructural changes during thermo-mechanically induced coarsening. This microstructurally adaptive creep model was used to predict the creep behavior of SAC387 subjected to various thermo-mechanical histories and the predictions were compared with experimental results.