Thermal Stability of Self-Assembled Monolayer Copper Preservatives for Fluxless Soldering

Abstract

Self-assembled monolayers ofalkanethiols have been demonstrated to preserve copper surfacesforfluxless soldering. However, the performance ofthese coatings for electronics manufacturing will be strongly influenced by their thermal stability. This study systematically investigates the effect ofstorage at a range of temperatures on the fluxless solderability ofalkanethiol coated copper. In the past, experimental evidence has suggested that the keyfactor to allow goodfluxless soldering is maintaining low levels ofcopper oxide. As a result, it is predicted that changes in the degree ofsolder wetting during storage will correlate with changes in the surface oxide level. X-ray Photoelectron Spectroscopy (XPS) is used here to investigate this relationship by providing detailed information about changes in surface chemistry. It isfound that there is a well defined relationship between loss ofsolderability and storage temperature in air. This link is most apparent when solderability is measured using the maximum solder wettingforce, but also applies to the rate ofsolder wetting. A correlation between surface oxide levels and wettingforce is also shown, however the situation here is more complicated than expected and requires further investigation.