Effects of microstructure on creep deformation of Sn-3.5Ag alloys

Creep data of Pb-free solder alloys vary widely even for a given composition. Main culprits are differences in the solder microstructure. The role of β-Sn granular and particle size is particularly interesting as it is determined by the solidification rate of the solder from the melt. In the present work, creep tests were conducted under uniaxial tension by using the Sn-3.5Ag alloys with varying microstructures. The minimum strain rate (ε̇) showed d^1.8~2.5 dependence where d is the granular size of the primary β-Sn phase. This is pretty much opposite to the grain size effect in creep where ε̇ decreases inversely (D^-2 and D^-3 respectively) with the grain size (D). The inverse relationship originates from the diffusive matter transport through the bulk or along the grain boundary, but the granular effect observed in the eutectic solders has no resemblance to the former. Here, primary β-Sn granules are surrounded by walls of creep resistant eutectic regions which are mixtures of the β-Sn phases and Ag₃Sn precipitates, and the stress which drives the creep deformation of the eutectic phase increases with the granule size in a manner similar to that of the Hall-Petch relation.