In-Situ Observation of Liquid Solder Alloys and Solid Substrate Reactions Using High-Voltage Transmission Electron Microscopy

The complex reaction between liquid solder alloys and solid substrates has been studied ex-situ in a few studies, utilising creative setups to “freeze” the reactions at different stages during the reflow soldering process. However, full understanding of the dynamics of the process is difficult due to the lack of direct observation with micro- and nano-level resolution. In this study, high voltage transmission electron (HV-TEM) is employed to observe the morphological changes that occur in Cu6Sn5 between a Sn-3.0wt%Ag-0.5wt%Cu (SAC305) solder alloy and a Cu substrate in-situ at temperatures above the solidus of the alloy. This enables the continuous surveillance of the amoeba-like rapid grain boundary movements of Cu 6 Sn 5 during soldering and increases the fundamental understanding of reaction mechanisms in solder solid/liquid interfaces.