Observation and analysis of metal oxide reduction by formic acid for soldering

A method to observe oxide layer states with an ellipsometer in real-time and the relationship between solder wettability and remaining oxide layer thickness are reported in this paper. An experiment system is constructed with a spectrum ellipsometer and a vacuum chamber which has a port to introduce formic acid gas to observe the copper oxide layer reduction process by formic acid in real-time. The oxide layer thickness is monitored with the system, and solder wettability is evaluated by placing solder on the same copper substrate. From the experiment results, it becomes clear that the process which achieves complete oxide removal is necessary to obtain favorable solder wettability.