Void formation on PCB surface finish during reflow soldering

Printed circuit board (PCB) surface finish and its reactive wetting process were identified as a main cause of voiding. During reflow soldering liquid flux residues adhered to the interface PCB / liquid solder alloy form voids while the solder alloy is in a liquid state. In contrast to that no voids are found when no reactive wetting occurred. Furthermore, we observed that the number of detected voids after the reflow process strongly depends on the tin layer thickness: Many voids are formed in the case of a thin tin layer, no voids are found in the case of a very thick tin layer. A model is established to explain this phenomenon.