What is indicated by a solder joint that is 'dull' in appearance?

A dull appearance in a solder joint is often indicative of poor wetting or insufficient solder. When solder effectively wets the surfaces of the materials being joined, it typically results in a shiny finish. A dull joint, on the other hand, can suggest that the solder did not flow properly or did not adequately cover the contact areas, which can compromise the mechanical and electrical integrity of the joint.

Such dullness may arise from a number of factors, such as insufficient heat during the soldering process, contamination on the surfaces being joined, or not enough solder being applied. In any case, a dull joint is a warning sign that requires further inspection, as it may not provide a reliable connection in an electronic assembly.