We are having problems with spotting of our tin plate. The parts causing the problem are made from two separate copper components that are silver soldered into a single assembly. The silver soldered areas tend to develop black spots after tin plating. We do not see this problem on all of our parts, but it happens often enough to play havoc with our production schedule. Do you have any suggestions for eliminating or minimizing this problem? K.S.
The most likely cause of the spotting you are observing is a porous tin layer on the copper. In your case the problem is only in the areas that are silver soldered. This is the "smoking gun" so to speak. Since this is the only area that is having spotting problems, you must look at what in the soldered areas is causing the problem. Most likely this spotting is caused by our old friend, "poor cleaning."
Soldered areas usually contain oxides and other materials that may not be easily removed in your cleaning process. Soldered areas also tend to be somewhat porous compared to the non-soldered areas of the part. So the place to start is with your cleaning process. You do not mention the cleaning process used, but a good place to start is to discuss the issue with your cleaning chemicals vendor.
A second area that you do not address is a barrier layer between the base copper and tin layer. To get the best adhesion and best results, a barrier layer should be used. Nickel is a good candidate for a barrier layer. A barrier layer of 9-12 microns should give good results.