What visual characteristic indicates a solder joint is "not wetting"?

A solder joint characterized by a dry and grainy appearance indicates that the joint is not wetting properly. Wetting refers to the ability of solder to spread across and adhere to the surfaces of the materials being joined. When a solder joint is well-wetted, the solder will flow and adhere smoothly, resulting in a robust connection.

In contrast, a dry and grainy appearance suggests that the solder did not flow properly and has not formed a good bond with the substrate. This can occur due to insufficient heat, inadequate cleaning of surfaces, or improper solder composition, which can all interfere with the solder's ability to wet the surfaces effectively.

The other visual characteristics, such as a smooth and shiny surface, bright and reflective finish, or even and consistent flow, indicate good solder wetting and a strong joint. Each of these features signifies that the solder has effectively melted and has spread uniformly across the surfaces, forming a reliable electrical connection.