why are bad solder joints dim?

[m4rtin]

If I solder two wires together and during the few seconds, when the solder cools down, I move the wires, the solder joint remains dim. As I understand, dim solder joint is not as reliable and strong as a shiny one. Why is this so? Does solder crystallize in a different way when parts are moved during the solder cooling processes?

[ddavidebor]

because the big mass of the wire and the high termal conductivity of the copper "suck out" the heat from your solder joint very fast.

[mariush]

There's a chemical reaction between the metals, you're messing that up when you move the wires. If you want to reduce this problem, you can try switching to 63/37 solder, which goes transitions much faster to solid, at a fixed temperature. The 60/40 has a wider temperature range, so you have to keep the 2 parts fixed until the solder goes outside that transition temperature range and becomes solid.

There's a very good series of soldering tutorials at Pace's youtube channel, here's the first part:



The other parts are in the same channel, easy to find.

The videos are a bit dated, but the information still applies to this day and the production quality is quite good.

[vlf3]

Dry joints also depend on using tinned copper wire or components, of the same solder alloy metals, or type !... current pre-tinned wires or components are led-free, so if you wet a joint using the old 40/60 tin-led ratio resin flux, then the joint will look dull and even not flow. 

Disturbing a solder joint while it cools, will change the crystal structure of the joint particularly, if insufficient flux is present; a good solder joint is made by firstly pre-heating the clean junction area first, and then applying the solder; correct temperature is important too for good solder flow, leaving a joint with a concave shape around the component, to PCB surface. 

Personally, I still keep to 40/60 resin flux solder, due it's low temperature flow properties... since I also keep to through-hole designs, for personal use.

[ptricks]

Unfortunately you can't judge solder joints anymore by how shiny they are thanks to ROHS, some of the lead free solders are always dim no matter how good the joint .

[marshallh]

If you want nice looking leadfree joints, look into nitrogen-assisted soldering stations. The resulting joints seem to be less prone to whiskering as well.