Microstructurally Adaptive Model for Primary and Secondary Creep of Sn-Ag-Based Solders

Sn-Ag-based 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 the creep properties of the solder over time, necessitating the development of a thermo-mechanical history-dependent creep model for accurate prediction of the long-term reliability of microelectronic solder joints. In this paper, the coarsening behavior of Ag
₃
Sn and Cu
₆
Sn
₅
precipitates in ball grid array-sized joints of Sn-3.8Ag-0.7Cu solder attached to Ni bond-pads with four different thermo-mechanical histories is reported. Because of the substantial numerical superiority of Ag
₃
Sn over Cu
₆
Sn
₅
, it was inferred that the evolution of mechanical properties during aging is controlled largely by the coarsening of Ag
₃
Sn. An effective diffusion length (
x̅
) for Ag diffusion in Sn was defined, and it is shown to adequately describe the thermo-mechanical history dependence of Ag
₃
Sn particle size. The shear creep behavior of these joints was experimentally characterized, and the entire creep data were fitted to a unified model combining exponential primary creep and power-law steady state creep. The parameter
x̅
was then incorporated into the creep equation to produce a unified creep model, which can adapt to thermo-mechanical history-dependent microstructural coarsening in the solder. Predictions using this creep law show very good agreement with experimental creep data for several different test and microstructural conditions.