What is the characteristic appearance of a "cold" solder joint?

A "cold" solder joint appears grainy or dull, indicating poor electrical connection due to inadequate heating during the soldering process. When solder is applied correctly, usually, it creates a smooth, shiny joint. In contrast, cold solder joints typically form because the solder did not reach the proper temperature to flow and bond effectively to the components or circuit board, leading to a weak connection that can cause signal issues in electronic circuits.

The grainy appearance suggests that the solder did not fully melt and combine with the metal surfaces being joined, resulting in low conductivity and possibly intermittent connections. Identifying cold solder joints is crucial in troubleshooting electronic equipment, as they can lead to malfunctioning devices or unreliable performance.