Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting (wettability) is determined by a force balance between adhesive and cohesive forces. Wetting is important in the bonding or adherence of two materials. So what’s the result of insufficient wetting? Here EPCB lists 3 examples.
Insufficient Wetting (Pad): Suppose a solder has wetted leads nicely, but it has not formed a good bond with the pad. This can be caused by a dirty circuit board.
Insufficient Wetting (Pin): If a solder in a joint has not wetted the pin at all and has only partially wetted the pad. In this case, heat was not applied to the pin and the solder was not given adequate time to flow.
Insufficient Wetting (Surface Mount): Assume that we have three pins of a surface mount component where the solder has not flowed onto the solder pad. We can heat the solder pad with the tip of the iron, then apply solder until it flows and melts together with the solder already on the pin.