High‐Temperature Micropillar Compression Creep Testing of Constituent Phases in Lead‐Free Solder

Abstract

As the size of solder interconnects used in electronics packaging decreases, methods for characterizing the creep behavior of solder at ever smaller length scales need to be developed. Long duration micropillar compression experiments at constant loads and temperatures ranging from 60 to 160°C were used to characterize the creep behavior in pure Sn dendrites and the Sn-Ag 3 Sn eutectic constituent in a leadfree Sn-3.5Ag solder alloy. The stress exponent for the Sn dendtrites as well as the eutectic at low stresses was shown to be 1, suggesting a diffusion-based mechanism, while at high stresses the eutectic stress exponent was 4, suggesting a dislocation-based mechanism. Low activation energies indicative of a high diffusivity path (around 30 kJ mol À1 ) were seen in the dendrites at all temperatures and stresses as well as in the eutectic at low stresses. In the high stress regime of the eutectic, the activation energy transitions from 36 to 97 kJ mol À1 above 120°C, indicating a transition to a lattice diffusionbased mechanism. 1168 wileyonlinelibrary.com

Evaluating Creep Deformation in Controlled Microstructures of Sn-3Ag-0.5Cu Solder

“…In the low temperature regime (À 25-75°C), dislocations act as obstacles to reduce diffusion path, which is perhaps the result of a recovery or recrystallization assisted diffusion mechanism, while the high temperature regime (75-160°C) predominantly occurred by lattice or bulk diffusion mechanisms. This change in mechanism is supported by Lee et al 22 using Cu/Sn/Cu single-lap specimens, Mayer et al 31 who tested both pure Sn and Sn-Ag 3 Sn eutectic with micropillar compression, and Mathew et al 11 showed good agreement between solder-bump array samples under shear loading and bulk samples through tensile testing at room temperature.…”

“…Prior research 11,16,22,31 has suggested a change in mechanism at higher temperature (T ‡ 0.7 T M ) to diffusion creep, i.e., a decrease in stress exponent (n). For solder composition Sn-3.9Ag-0.6Cu, Vianco et al 16,17 indicated a change in creep mechanism for bulk solder at high and low temperatures.…”