The role of lengthscale in the creep of Sn-3Ag-0.5Cu solder microstructures

TL;DR

This study investigates how microstructural lengthscale influences creep behavior in Sn-3Ag-0.5Cu solder, revealing that finer microstructures enhance creep performance and alter deformation mechanisms across temperature ranges.

Contribution

It provides new insights into the effect of microstructural lengthscale on creep mechanisms and deformation behavior in SAC305 solder.

Findings

Refined microstructure improves creep lifetime and ductility.

Creep mechanism shifts from obstacle-controlled dislocation to vacancy diffusion with temperature.

Microstructural lengthscale affects recrystallized grain size and deformation heterogeneity.

Abstract

Creep of directionally solidified Sn-3Ag-0.5Cu wt.% (SAC305) samples with near-<110> orientation along the loading direction and different microstructural lengthscale is investigated under constant load tensile testing and at a range of temperatures. The creep performance improves by refining the microstructure, i.e. the decrease in secondary dendrite arm spacing ({\lambda}2), eutectic intermetallic spacing ({\lambda}e) and intermetallic compound (IMC) size, indicating as a longer creep lifetime, lower creep strain rate, change in activation energy (Q) and increase in ductility and homogeneity in macro- and micro-structural deformation of the samples. The dominating creep mechanism is obstacle-controlled dislocation creep at room temperature and transits to lattice-associated vacancy diffusion creep at elevated temperature (T/T_M > 0.7 to 0.75). The deformation mechanisms are investigated using electron backscatter diffraction (EBSD) and strain heterogeneity is identified between b/-Sn in dendrites and b/-Sn in eutectic regions containing Ag3Sn and Cu6Sn5 particles. The size of the recrystallised grains is modulated by the dendritic and eutectic spacings, however, the recrystalised grains in the eutectic regions for coarse-scaled samples (largest {\lambda}2 and {\lambda}e) is only localised next to IMCs without growth in size.