…Intermetallic Growth on PCB Where the tin-lead solder adheres to the copper of the lead (from plating) and to the pad (from hot air leveling or other pcb solder-coating method… During reflow, with the addition of tin-lead solders, the intermetallic grows. With other board metal finishes, other metallics form, such as AuSn when soldering components to boards with gold-flash finish (and tin-lead solder). Such growth can be as much as several microns during reflow.