Effect of Composition and Thermal–Mechanical History on the Creep Behavior of Sn–Ag–Cu Solders—Part I: Experiments

Solder joints experience cyclic shear stresses during power/rest cycles while being subjected to temperatures ranging from 0.44 to 0.8 T
_m
(where T
_m
is the melting temperature). This makes these alloys susceptible to significant microstructural changes, which, in turn, influence the creep response and thereby the thermal cycling reliability of microelectronic packages. In this paper, the correlation between the microstructural condition and the resultant creep behavior of two Sn-Ag-Cu alloys, namely, Sn-3.0Ag-0.5Cu and Sn-1.0Ag-0.5Cu, subjected to different thermomechanical cycling and isothermally aging processes, has been studied. The creep behavior of the solders was compared with the response of pure tin using the impression creep technique. The key microstructural feature that evolves during both aging and thermomechanical treatments is the Ag3Sn particle size (and, hence, the interparticle spacing) in the eutectic constituent. The creep rate was found to linearly increase with particle size for Sn-3.0Ag-0.5Cu, the microstructure of which contains a continuous matrix of eutectic surrounding proeutectic β grains. The behavior of Sn-1.0Ag-0.5Cu is more complicated since the eutectic is not continuous, and the proeutectic significantly contributes to the overall creep response. The results are utilized in the accompanying paper to develop a composite model of creep that can account for both types of microstructures.