What visual indication suggests that solder has overheated?

The dull surface and grainy appearance indicate that solder has overheated. When solder is heated beyond its optimal temperature, it can lose its desirable properties, resulting in a texture that is no longer smooth and shiny. Instead, the overheating causes oxidation and the formation of dull, rough grains on the surface, which can lead to poor connections and reliability issues in the solder joint. This grainy appearance is a clear visual sign that the solder has been compromised, distinguishing it from properly melted solder, which typically maintains a smooth and shiny finish. Understanding these physical characteristics is crucial for achieving reliable solder joints in electronic components.