Wetting of New SnAg Surface Finish

Abstract

This paper shows how a surface finish does affect wettability. Four standard lead-free alloys selected for surface finishes on PCB (LF HASL, ImSn, ENIG, and Cu) with a newly developed surface finish based on SnAg7 alloy were compared. Wetting characteristics of surface finishes were analysed using the spreading rate method and a method based on wetting reflow pattern, both in laboratory condition. Experiments were supplemented by analysis of the effect of plasma on the flux activity, and eventually on wettability. A self-developed equipment was applied to evaluate the spreading rate. The spreading rate method allows for real-time measurement of contact angle of liquid SAC 305 solder balls on various PCB surface finishes. The method based on wetting reflow pattern by SAC 305 solder paste applied on various PCB surfaces confirms these wetting characteristics. The wettability of the new and relatively cheap SnAg7 surface finish is almost comparable with surface finish based on ENIG. This new surface finish is a prospective material that offers interesting properties for application in electronics.