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After about 8 month since the electronic components were soldered to a PCB using Pb-free tin, a strange substance appeared in some of the contacts (see attached image)

enter image description here.

The PCB may have been exposed (OR NOT) to some humidity but not to other substances during its working time

What could such substance be? Would it cause short-circuit between the component pins?

It looks like some mold. But why there?

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  • \$\begingroup\$ Hard to say; I do know that SAC solder corrosion is galvanic in nature when exposed to high humidity and some available impurities (dissimilar metal corrosion). \$\endgroup\$
    – Peter Smith
    Commented Jun 5, 2019 at 13:40
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    \$\begingroup\$ That looks a lot like mold or silk. Evidence of insects or moisture? \$\endgroup\$
    – hekete
    Commented Jun 5, 2019 at 13:52
  • \$\begingroup\$ Any chance you could get us a closer up (or microscope) image? \$\endgroup\$
    – Jack B
    Commented Jun 5, 2019 at 14:04
  • \$\begingroup\$ @hekete yes, it looks like mold. But what caused to appear there? \$\endgroup\$
    – BamsBamx
    Commented Jun 5, 2019 at 14:29
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    \$\begingroup\$ Some kinds of solder flux can get fun chemical reactions with whatever else might be present. I had something similar myself recently, where some RoHS flux reacted with silicone and created a strange white powder all over the place. \$\endgroup\$
    – Lundin
    Commented Jun 5, 2019 at 14:31

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Clean it off with flux remover, then look at the pins and see if they are corroded (material removed or pockmarks). If it is corroded then the chip might be toast.

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