When a solder joint is reheated without applying flux, the solder might get dull. Or, when I solder a joint (maybe with the temperature set higher than enough), the solder looks shiny when melted, then turns dull upon solidifying. The solder joint might still have good electrical connection, but I just wonder what makes the joint not shiny.


marked as duplicate by winny, Sparky256, PeterJ, DoxyLover, Voltage Spike Dec 27 '17 at 5:30

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    \$\begingroup\$ Is it Lead-Free solder? That will turn dull once cooled. \$\endgroup\$ – Tom Carpenter Dec 23 '17 at 11:16
  • \$\begingroup\$ I tried it with tin-lead solder, the same thing happens \$\endgroup\$ – domino Dec 23 '17 at 11:17
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    \$\begingroup\$ The dullness is surface oxidation see this question \$\endgroup\$ – RoyC Dec 23 '17 at 11:52
  • \$\begingroup\$ @RoyC - No. What the OP is talking about is the change which occurs upon transitioning from liquid to solid. \$\endgroup\$ – WhatRoughBeast Dec 23 '17 at 16:34
  • \$\begingroup\$ The joint where I buy my solder is dull because it is run by a bunch of old guys that have no sense of humor. Oh wait.. that's a different question. \$\endgroup\$ – Trevor_G Dec 23 '17 at 16:51

What you are seeing is a change in surface texture. This occurs when the solder transitions from liquid to solid. In the process, the solder forms into lots and lots of tiny grains, and the surface becomes rougher than it was as a liquid. This surface roughness reflects light in all directions, and the result is a surface which is not dark (since ambient light is also diffused) but which looks dull. These grains are not crystals as such, but rather occur due to impurities in the solder, which form dislocations which prevent the solder from forming into a single crystal in the process of solidifyng. In general, this occurs in any impure substance which is cooled below its melting point quickly. If you were to take extremely pure solder, melt it into a vacuum to prevent surface oxidation, and then cool it very slowly, you could produce a material with a very smooth surface and high reflectivity. It wouldn't be a perfect surface, since solder doesn't have a defined crystal structure, but the grain size would be much larger.

  • \$\begingroup\$ I use a weller soldering iron that is set at 700 F ( and it is locked, can't be adjusted). I think that temperature is a little too high. I noticed that when I solder a joint and pull off the iron very quickly, the joint is shiny. And if I hold the iron at the joint for little longer, the solder takes a little longer to turn solid solid and it gets dull. I think it has something to do with the temperature of the molten solder. \$\endgroup\$ – domino Dec 24 '17 at 1:06
  • \$\begingroup\$ Long heat application allows copper to alloy (dissolve) into the solder, which is not good. It embrittles the solder, and even a small motion during freezing will cause cracks. Remove the old solder and apply some new solder, to get a shiny joint. \$\endgroup\$ – Whit3rd Dec 24 '17 at 9:50

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