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

Self-assembled monolayers of alkanethiols have been demonstrated to preserve copper surfaces for fluxless soldering. However, the performance of these coatings for electronics manufacturing will be strongly influenced by their thermal stability. This study systematically investigates the effect of storage at a range of temperatures on the fluxless solderability of alkanethiol coated copper. In the past, experimental evidence has suggested that the key factor to allow good fluxless soldering is maintaining low levels of copper oxide. As a result, it is predicted that changes in the degree of solder 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 is found that there is a well defined relationship between loss of solderability and storage temperature in air. This link is most apparent when solderability is measured using the maximum solder wetting force, but also applies to the rate of solder wetting. A correlation between surface oxide levels and wetting force is also shown, however the situation here is more complicated than expected and requires further investigation