What is a cold solder joint?

Put simply, a bad solder joint that is failing to allow the flow of electrons, this can cause an electrical circuit to fail completely or intermittently. Usually, if in doubt just go ahead and re-flow the joint to make sure it's a good connection.

Visible signs of a cold solder joint:

• For lead/tin solder, the surface of the joint should be shiny. For lead free solder, the surface will be dull.
• The surface of the joint has a crack.
• The solder doesn't go all the way through the mounting hole (for through-hole designs)

Almost all of the time this problem can be resolved by just re-flowing the solder at the joint, to do this you just need to melt the solder again with a soldering iron and then test the circuit to see if this solved the problem.