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These solder joints on the connector look strange to me (tarnished purple).

In a more "advanced" state, they end up looking like this:

In the last image you can see, it even "ate" the plating on the mating surface of the connector, while two of the pins are still looking normal.

What is going on here?

Bad process? Wrong solder / mix of materials? Mishandling while in storage (the board is intended to be used in an normal everyday environment, thus I would assume, it should not happen due to normal surrounding parameters)?

Edit: the boards were bought from a company as a product, so no soldering on my side involved (so far).

Edit: new images: Would the last one looks like rust(?) while the other one looks purple-ish.

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  • \$\begingroup\$ Water damage possibly? \$\endgroup\$ – Andy aka Jun 15 '18 at 9:46
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    \$\begingroup\$ I recall reading somewhere that some lead-free solders can become brittle or have melting point lowered drastically when contaminated with lead. Can't provide any details though. \$\endgroup\$ – Maple Jun 15 '18 at 9:59
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    \$\begingroup\$ is there lacquer or something on this board? Hard to tell but looks a bit like it. Strange that the purple colour only seems to be on certain surfaces, and not so much on other components. Is there some sort of coating that has discoloured? It seems to actually be on the top surface of that connector, rather on the layer of solder that sticks out from under the edges. \$\endgroup\$ – danmcb Jun 15 '18 at 10:06
  • \$\begingroup\$ There's no coating on the board, guess you are referring to the flux residue around the pads from the solder paste? \$\endgroup\$ – imp Jun 15 '18 at 10:43
  • \$\begingroup\$ Water damage would have left more traces on other areas of the board, there's nothing suspicious. \$\endgroup\$ – imp Jun 15 '18 at 10:43
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I would place the problem here on the connector itself. Just looking at the picture and the poor quality with which the solder has failed to flow around the connector pin tails tells this story.

In the past I have had some connectors that were sourced from Asia that had horrible plating and oil and other contaminants on the pins that made them almost impossible to solder. Even with copious amounts of flux the soldering job could not be done.

I suggest that you try a different supplier and run some comparative solderability tests.

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The color is reminiscent of the scum that would form on the surface of lead-free solder when I used to work with solder pots. In that case, it was the result of oxidation of the molten solder sitting for hours on end. I would say that if this is indeed lead-free, your iron/air may be set too high, which allows more oxidation to occur when the solder is still in its liquid state. As for fixing the connector, try to wick off as much as you can and reflow with your iron/air as low as possible while still allowing good whetting (using plenty of flux of course), and then cleaning the joint with alcohol when cooled.

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The assembly house probably forgot to clean off of the flux. Some fluxes actually corrode metals. The best you could do at this point is maybe clean it with flux remover and remove any residue.

In the past I have seen this with what I think was water soluble flux.

Water-soluble fluxes have high percentage of organic activators and are generally corrosive in nature. For this reason, it's always recommended to clean these fluxes immediately. If not, there is always the risk of acidic activators etching the solder joints that could result in corrosion and electrochemical migration.

Source: http://www.circuitnet.com/experts/80074.html

The assembly house probably didn't follow the process. Ask them what they use, most people use no-clean nowadays (which has its own set of problems). It is better as a designer to be cognizant of these processes, make sure you know what kind of solder and flux your boards are assembled with and the processes.

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What is the end product and where is it being used? I have seen this type of corrosion when there is some H2S (hydrogen sulfide) gas in the atmosphere. H2S is a byproduct of hydrocarbon processing (oil and gas) or anything organic that rots or ferments such as sewage, garbage, wine, beer, kimchi etc. When H2S comes in contact with water, as in water vapor in the air, if forms Sulfuric Acid which attacks most solder and anything with silver. That's why boards are often "conformally coated" with a non-hygroscopic material to keep moisture away from the solder.

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