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Topic: Immersion Tin Circuit Board Oddness (Read 618 times) previous topic - next topic


May 23, 2015, 09:00 pm Last Edit: May 29, 2015, 09:29 pm by ron_sutherland
I opened the replacement WD chassis again today, and the board inside has tarnished (see images). The frame is metal this time (not plastic), and the tarnish is mostly in one area, but I don't see a chia pet (yet).


Do not PM me for help.Only links to topics.

After Tuesday, even the calendar goes W T F


Is this some kind of material problem in the solder tin. I thought that the problems in lead free solder were solved.

I think that it is not very probable that something in an organic compound like plastic would change the crystal structure of tin.


I read the plating page. I wonder. They talk about self life. My boards were soldered as new (we were in a hurry).


May 29, 2015, 10:14 pm Last Edit: May 29, 2015, 10:16 pm by ron_sutherland Reason: make into links
The original search was for why the circuit board pads had turned dark. I had turned up tin-pest, however, LMI pointed out gray pads after things lifting off which pointed me to intermetallics (link in post #8) as another possibility for the gray pads. Intermetallics form as copper get into the tin. Both intermetallics and tin-pest are crystal changes in the structure of tin (phase).
I am not ruling out tin-pest, or an intermetallic, but at this time think tarnish from sulfur seems more likely, and it happened much quicker than I had expected on the new boards. The chia pet reference is from the link in post #2. It refers to creep corrosion and involves sulfur and silver (as tarnish) that cause a galvanic reaction with copper. Sulfur is used for vulcanization* of polymers, so is found in plastics and rubber, both of which outgas** with heat and age. Creep corrosion can be clearly seen in the image of my OP, but I had no clue what it was at the time.


As far as lead-free solder being fixed, I am convinced that is wishful thinking. Even if the board pads are ENIG which can stop continued intermetallic formation, is a tin-rich solder really immune to tin pest. Here is some info on how to test for tin-pest in tin-rich soler*. Also do the parts soldered on the board limit intermetallic formation.

Another problem that keeps showing up when I search is Tin Whiskers*, which again is related to tin-rich surfaces. Some are saying these are extrusions caused by tin-pest.




As I said we were in a hurry and so the boards were not washed properly. Later we found some residue between the component legs and so on on.

I made a Google search for creep corrosion. Some results seem to fit in our case.


A little over year old circuit board looks very odd. It had Immersion Tin (edit: probably Silver)  pads, now the pads won't take solder and look like a fungus has got all over them. All I can find that describes this is "Tin Pest". Anyone else ever see this sort of thing with immersion tin.

Some of our customers require silver plating on their boards. The board house vacuum seals packages of boards in plastic, along with a desiccant package. Even so, some boards have tarnish on the silver in some places. We have used liquid "Tarnex" solution to clear the tarnish and then the boards solder as new. The package really should have nitrogen to replace all the air, but doesn't seem to be done.


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