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Solder Skip
Unsoldered surface mount joints are referred to as solder skips where the termination does not have any solder. It is caused by incorrect chip wave height or gassing of the flux on the underside of the board. As in this case the resist thickness can also cause the problem or make it worse. Solder resist or mask should be level with the pad surface or below it for the ideal assembly conditions. If the mask is thick it creates a cavity around the pad where flux vapours are trapped forming a bubble. The solder cannot displace the vapour easily to form a joint.