If parts are turning up where they shouldn’t be, check the feeder and paste.
Chip components sometimes appear on the surface of a board completely out of position. On some occasions they may be under other components or, as in FIGURE 1, even shorting under parts. If it’s a random chip component, then there is little chance of finding the cause. If all the parts are the same resistor or capacitor, it may be a feeder problem where the parts are being dropped after pickup, or are jumping out of the tape due to static, which can occur on small parts (below 0201).
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.