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- If the intermetallic is too thin, the joint will be mechanically weak.
- If the intermetallic is too thick, the joint will become brittle.
Wetting
Over time, the intermetallic will passively become thicker on its own. This can contribute to the deterioration of old electronics.
Wetting
In soldering, wetting Wetting is a the ability property of the liquid solder to flow over and cover the desired surfaces of the components, wires and PCB pads. The degree of wetting (wettability) is determined by a force balance between adhesive and cohesive forces. Good wetting is achieved when the solder is heated to the correct temperature, there is sufficient active flux and the surfaces to be soldered are not contamined with dirt, fat or oxides. Active flux will to a large degree remove impurities, but it doesn't have infinite capacity to clean.
Flux
Flux is a chemical cleaning agent, flowing agent, and/or purifying agent (Wikipedia article). One of the roles of flux is soldering is to prevent and remove copper oxides on components and PCBs. Tin based solder attaches very well to copper, but poorly to the various oxides of copper, which form quickly at soldering temperatures. By preventing the formation of metal oxides, flux enables the solder to adhere to the clean metal surface, rather than forming beads, as it would on an oxidized surface. There are various types of flux:
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