We are plating tin using a stannous tin bath over a plated copper layer. The parts are dried in a hot air dryer. Every so often we end up with a yellowish film on the finished part. Can you help us? T.M.
Your e-mail doesn’t give me much to go on, but here are a few ideas. My first guess would be that you are not doing thorough enough rinsing prior to the drying step. The tin plating bath you are using is acidic, and the dragout from the plating bath must be thoroughly neutralized before the drying step. You should be using an alkaline rinse as well as a thorough rinse process.
What type of water are you using in your final rinse? It should be good quality. City water or recycled water may contain mineral salts that can leave a residue on the surface.
One last area concerns the contamination of the tin plating with copper from the copper plating bath. Again, good rinsing is critical to prevent dragout from the copper plating bath into the tin plating bath.