Impression creep characterization of rapidly cooled Sn–3.5Ag solders

The creep behavior of solders crucially influences solder joint reliability in microelectronic packages. This, in conjunction with the ongoing industry-wide transition to lead-free solders, has resulted in significant current emphasis on developing approaches for expeditiously creep testing miniaturized samples of a wide range of solder compositions to determine their suitability for packaging applications. Here we characterize the creep response of Sn–3.5Ag solders via impression creep testing. The tested microstructures represent a rapidly cooled state, resembling the highly refined microstructures often found in tiny microelectronic solder joints. It is shown that in the as-reflowed state, the solder creeps by a viscous glide-controlled mechanism at low stresses due to the presence of a non-equilibrium amount of dissolved Ag in Sn, and a particle-limited climb mechanism at high stresses. Following slight aging, which allows the dissolved solute concentration to drop to near-equilibrium levels, the low stress mechanism changes to dislocation climb-control. Based on these observations, a unified view of creep of Sn–3.5Ag solders is presented, which is consistent with observations of a transition from low to high stress sensitivity with increasing stress, which has been reported in several studies.