Dewetting
Dewetting is the withdrawal of molten solder from a metal surface which was initially wetted by solder. During soldering, the complete wetting of an area can initially be observed, but after the original wetting had taken place, the solder withdraws again, leaving a thin coat of solder on the dewetted areas. Causes for this phenomenon are changes in the surface characteristics induced by the presence of the hot solder (high soldering temperatures and long soldering times). Zinc, for example, can diffuse to the surface, or a wettable layer has dissolved in the liquid solder or, in the structure of the layers of the surface to be soldered, a process of oxidization has taken place. The use of more aggressive flux or a further cleaning of the areas to be soldered may alleviate the problem.