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

Xin F Tan,Hiroshi Maeno,Kazuhiro Nogita,Stuart D Mcdonald,Michael J Bermingham,Flora Somidin,Syo Matsumura

doi:10.3390/ma15020510

Abstract

The complex reaction between liquid solder alloys and solid substrates has been studied ex-situ in a few studies, utilizing 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 at micro- and nano-meter resolutions. In this study, high voltage transmission electron microscopy (HV-TEM) is employed to observe the morphological changes that occur in Cu6Sn5 between a Sn-3.0 wt%Ag-0.5 wt%Cu (SAC305) solder alloy and a Cu substrate in situ at temperatures above the solidus of the alloy. This enables the continuous surveillance of rapid grain boundary movements of Cu6Sn5 during soldering and increases the fundamental understanding of reaction mechanisms in solder solid/liquid interfaces.

Highlights

In compliance with the Restriction of Use of Certain Hazardous Substances (RoHS) Directive of the European Community, Pb-free solders have been developed
Using in situ heating/cooling high voltage transmission electron microscopy (HV-TEM) techniques, the stabilities and phase transformation kinetics of Cu6Sn5 in solder joints have been studied by Somidin et al [4,5,6]
The in situ observation in this study shows the Cu6Sn5 grain does not have a scallop morphology during dissolution, and the scallop morphology is formed 7wohfe1n0 Cu is deposited onto the Cu6Sn5 grains during cooling

Summary

Introduction

In compliance with the Restriction of Use of Certain Hazardous Substances (RoHS) Directive of the European Community, Pb-free solders have been developed. The soldering process involves heating the solder alloy above its melting temperature to react with the Cu substrate to form an intimate bond. During this process, Cu6Sn5 and Cu3Sn intermetallic compounds (IMCs) form at the interface between the solder melt and the Cu substrates [2,3]. The solid-liquid interfaces of pure Al, In and Sn, along with alumina and a few binary alloys [13], Pb-Sn [14] and Pb-free solders [15] on Ni-P substrates were studied by the same group under in situ heating TEM. But for the first time, using HV-TEM, we observed the morphological changes in Cu6Sn5 that has formed through a reaction of SAC305 and a Cu substrate via in situ observations at elevated temperatures up to a peak of 218 ◦C, just above the solidus of SAC305

Materials and Methods

Conclusions