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A good solder connection is shiny, and we're taught that dull solder indicates a cold solder joint. Why is this?

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Sometimes you can see a difference in the appearance of the metal of a bad solder joint. I think this is due to something having gone wrong as the solder was solidifying, like one part of the joint was moved with respsect to another.

It seems to me though that the majority of bad solder joints don't look different on the surface of the solder. If there is a visual clue, it's usually that the solder didn't wick into the joint properly. It will look a little beaded up instead of having formed a nice miniscus with the edges having flowed along the metal of the joint.

Another point is that good solder joints don't actually look nice and shiny. The solder will look shiny when molten, but develops a slightly dull finish upon cooling.

All in all, I think what you are referring to is more superstition than fact.

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I have seen that before, but only when solder was used on something that really needed more flux for how oxidized it was. – Kortuk♦ Jan 22 '12 at 23:29
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@Olin I agree completely. This is even more true with lead-free solder. When I'm evaluating the soldering quality I look for inconsistencies in a group of joints, not how any one specific joint looks (Of course, I mostly evaluate machine-soldered PCB's, not hand soldered). – David Kessner Jan 22 '12 at 23:41
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It's a government conspiracy, as indicated by HAM radio Technician class question T7D09: What is the characteristic appearance of a "cold" solder joint? / A grainy or dull surface. – Mark Harrison Jan 23 '12 at 10:08
A good solder joint using leaded solder does actually look shiny. – Brad Gilbert Jan 23 '12 at 19:07
Note also that while a Cold Solder Joint is almost guaranteed to be a Bad Solder Joint, there are several other types of Bad Solder Joints in addition to Cold Solder Joints. Olin describes "Good" and "Bad', but does not mention "Cold". – mickeyf Jan 23 '12 at 23:31

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