In-situ investigation of the time-temperature dependent lattice and microstructure of Sn-Bi alloys

Abstract

Alloys based on the Sn-Bi system are widely considered as the most promising candidates for low temperature solders (LTS) in the electronics industry due to their low liquidus temperature, non-toxicity and relatively low cost. However, implementation of LTS is complicated as they exhibit different characteristics from conventional Pb-free solders. While the solid solubility of alloying additions in Sn is typically <1 wt% in the current generation of Pb-free solders, the solubility of Bi in Sn ranges from less than 3 wt% at room temperature to 21 wt% at the eutectic temperature of 139 °C. In consequence, the microstructure and properties of Sn-Bi solders change significantly under varying temperature during normal service, affecting reliability. Since these changes are time and temperature dependent, interpreting results obtained using ex-situ methods requires assumptions about the stability of the alloy during transitions from the test environment to the observation environment, leading to an incomplete understanding of these changes. To address this, in-situ synchrotron powder X-ray diffraction (PXRD) and in-situ scanning electron microscopy (SEM) techniques were developed to investigate the time-temperature dependent lattice and microstructure of hypo-eutectic Sn-37wt%Bi and near-eutectic Sn-57wt%Bi solder alloys. This made possible for the first time the observation of changes during temperature fluctuations. The study revealed that lattice and microstructure are highly sensitive to factors like temperature ramp rate and isothermal holding time, not previously considered. The results will impact the design of electronic circuitry and process parameters during electronics assembly to enhance reliability and performance.