Less-than-optimal preheat can lead to moisture-related defects.
Typically, when copper, organic solderable protectant (OSP) coated boards are left exposed after soldering, the copper oxidizes and darkens in color. It will quickly become unsolderable but does not corrode. If exposed to activated flux residues that have not deactivated after preheat and soldering, it is possible to see green verdigis on the copper surface. FIGURES 1 and 2 show a printed circuit board that has been soldered, and the exposed copper on the topside of the board around the pad shows the green deposit referred to as green verdigris.
When micro-voids appear on solder joints, it might not be the solder to blame.
Maintain the right soldering profile to prevent caps from popping.