What Are the Causes of Solder Dewetting?by Ed Wagner ; Updated September 28, 2017
Dewetting in a solder joint indicates a problem with the solder pad or component part. Solder will not bond with the part. The two underlying reasons for this are corrosion and contamination, with subsequent overheating causing the dewetted condition. Careful examination, board preparation and cleaning do much to eliminate this problem.
Though rare, dewetted solder joints can occur during manufacturing, but they're more likely to result during repair. A lead on a component part will be in good mechanical contact with its underlying pad, but the solder itself will not be bonded to the pad. Eventually, the connection will become intermittent. A repair technician will find that the leads simply "pop" off the board. Dewetted joints have an irregular appearance, without the uniform concave fillet of a good joint. They may appear lumpy from out-gassing.
Look over the printed circuit board carefully using magnification. Examine the entire board, not just the repair area. Watch for any conductive debris, like solder balls or clipped off leads from previous repairs. Examine the repair site for any corroded or tarnished pads or through-holes. A dewetted pad may be salvageable if it's cleaned down to bare copper.
Corrosion and Tarnish
Use a pencil eraser to clean off any tarnish or light corrosion. A tarnished copper pad has a dark brown color rather than bright copper. Solder will not adhere to tarnish. Once it's clean, apply flux and a blob of solder. Remove the excess with solder wick. If any part of the pad did not accept the solder, carefully clean it once more with the eraser. Look at it under magnification for conformal coating or some solder resist coating. They look like clear plastic and can be removed easily with a hobby knife.
Handle boards by their edges, keeping fingers well away from repair areas. Circuit boards are readily contaminated in handling. One common source is right at your finger tips -- your finger tips. Skin has oil and salt on its surface; by touching a clean copper pad, you transfer those chemicals onto the board. Oil will interfere with the formation of a bond, while salt makes copper tarnish rapidly.
Remove salts and oils from a circuit board with cotton swabs and alcohol. Traces of conformal coating can be removed with acetone, but exercise care as acetone will dissolve some plastics. Solder resist, which is normally used to cover runs, will occasionally be found atop a pad. It can be scraped away with a hobby knife.