What does shiny areas on the inside of a soldered joint indicate?

Shiny areas on the inside of a soldered joint typically indicate a proper flow and adequate bonding of the solder with the base materials. When solder is heated and flows nicely, it can create a smooth and shiny finish as it solidifies. This shiny appearance is often synonymous with a good metallurgical bond, which happens when the solder adequately fills the joint without any significant deficiencies.

If there were inadequate amounts of solder used, it may result in matte or dull areas where the solder hasn't properly filled the joint, indicating a poor connection and potential weak spots. Therefore, shiny areas generally suggest that enough solder has been used to properly form the joint, reflecting the effectiveness of the soldering process.