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Sunken Joints
On the top side of the board sunken or depressed joints can occur for the following reasons: Hole to lead ratio where the solder literally sinks due to its own weight. Some form of contamination or obstruction does not allow the solder to rise in the hole. Poor pre heat or fluxing does not allow the solder to fully wet the plated through hole. In the example shown the insulation on the body of the part was also found on the top of the leads which made joint formation difficult to achieve.