How to Check the Solder Joints on Your Electronics Project
After you've completed a solder joint, you should inspect it to make sure the joint is good. Look at it under a magnifying glass, and gently wiggle the component to see if the joint is stable. A good solder joint should be shiny and fill but not overflow the pad.
Nearly all bad solder joints are caused by one of three things: not allowing the wire and pad to heat sufficiently, applying too much solder, or melting the solder with the soldering iron instead of with the wire lead. Here are some indications of a bad solder joint:
The pad and lead aren't completely covered with solder, enabling you to see through one side of the hole through which the lead passes. Either you didn't apply quite enough solder, or the pad wasn't quite hot enough to accept the solder.
The lead is loose in the hole or the solder isn't firmly attached to the pad. One possible reason for this is that you moved the lead before the solder had completely cooled.
The solder isn't shiny. Shiny solder indicates solder that heated, flowed, and then cooled properly. If the solder gets just barely hot enough to melt, then flows over a wire or pad that isn’t heated sufficiently, it will be dull when it cools. (Unfortunately, the new lead-free solder almost always cools dull, so it looks like a bad solder joint even when the joint is good!)
Solder overflows the pad and touches an adjacent pad. This can happen if you apply too much solder. It can also happen if the pad didn't get hot enough to accept the solder, which can cause the solder to flow off the pad and onto an adjacent pad. If solder spills over from one pad to an adjacent pad, your circuit may not work right.