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Solder Balling
Care should be taken with some solder balls, this example is shown on a track and cannot be just knocked off. It is caused by squeeze out of the tin/lead from under the solder mask. or just simple adhesion. As the tin/lead becomes liquid during reflow or wave soldering the tin/lead expands. The solder ball can form on a track. If the solder resist is thin, solder can wet during wave contact and leave a ball.