What is the characteristic appearance of a cold tin-lead solder joint?

A 'cold' solder joint occurs when the solder does not melt entirely, the metals being joined were not heated sufficiently, or the components shifted while the solder was cooling. This prevents a proper metallurgical bond and results in a weak, unreliable electrical connection. When using traditional tin-lead solder, a properly heated and cooled joint will appear bright, shiny, and smooth, with a concave 'volcano' shape hugging the wire. In contrast, a cold tin-lead solder joint will have a dull, grainy, rough, or lumpy appearance. It is worth noting that modern lead-free solders can sometimes naturally cure to a somewhat dull finish even when perfectly bonded, but a 'rough or lumpy' texture is universally a sign of a bad joint in any solder type.