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Dewetting may occur after wetting. Metallurgical reactions precipitate a continuous change in the energetic state of wetting boundary surfaces. When molten solder cools, its wetting energy decreases while its cohesive strength increases. As a result, the molten solder pulls back and solidifies into large polyhedral beads. Small amounts of reaction products of tin and lead are left behind on the previously wetted surface, making it act as if it was tinned. However, at high magnification it is visible that the wetted surface was not flawlessly covered with solder. Dirt or debris trapped under the surface layer is a frequent cause behind dewetting.