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

A cold tin-lead solder joint typically has a rough or lumpy surface, which indicates that the solder has not been melted properly or that there was inadequate heating during the soldering process. When solder does not flow and bond correctly, it creates a poor connection that appears uneven and can result in electrical failure. This rough texture is a visual cue that the joint was not made properly, contrasting with a well-made solder joint, which would have a smooth, shiny surface indicating good thermal contact and proper flow of solder.

In contrast, dark black spots usually indicate oxidization or contamination, while a bright or shiny surface reflects a good solder joint that has melted and flowed properly. Excessive solder can lead to a buildup but does not specifically relate to the cold joint characteristic, as it might not necessarily imply that the joint is cold; it could merely indicate an over-application of solder.