Sn- and Cu-oxide reduction by formic acid and its application to power module soldering

In this paper, surface analysis of a copper substrate and solder foil, and real time measurement of oxide film thickness during reduction by formic acid with an ellipsometer are described. From the measurement results, the native oxide films of the copper substrate and the solder foil are presumed to be Cu
₂
O and SnO, respectively, and their thicknesses are confirmed as 4 nm and 5.3 nm, respectively. It is also found that SnO has a higher reduction rate by formic acid than Cu
₂
O. Furthermore, the contact angle of the melted solder ball becomes smaller as the copper oxide film becomes thinner, but complete oxide removal is necessary to obtain favorable solder wettability. On the basis of these formic acid reduction data, a void rate of 1% or less is achieved in a soldered sample processed with a formic acid reduction reflow machine.