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Topic: Immersion Tin Circuit Board Oddness (Read 2921 times) previous topic - next topic

ron_sutherland

Apr 28, 2015, 11:00 pm Last Edit: May 23, 2015, 09:18 pm by ron_sutherland
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.

https://youtu.be/Hb0VoQ-xQhU
my projects: https://github.com/epccs

Coding Badly


Were the boards in a high humidity environment?


ron_sutherland

#2
Apr 30, 2015, 10:43 pm Last Edit: Apr 30, 2015, 11:27 pm by ron_sutherland
No, it was in Arizona, not outside but not always room temperature either. Dry most of the year but we do have a season with some rain which it went through (the due point gets above 50°F for 8 to 10 weeks), but that was some time ago. The circuit failed on Wednesday and got replaced. But I have seen some old Immersion tin plated circuit boards look like that in the past, and never could figure out what it was. If it was biologic it would probably not like the Tin pads. Attached an image of a new board with bright immersion tin pads.

On that "FailedBoard2..." image attached in post #1 is what might also be creep corrosion. This* article is saying that is from humidity and a galvanic reaction between silver and copper to make the "chia pet" around the via. But now I am confused rather this is Immersion silver or tin, maybe the gray is just tarnish.

* http://pcb.iconnect007.media/index.php/article/60339/what-is-your-pcb-iq-pcb-final-finishes/60342/?skin=pcb
my projects: https://github.com/epccs

Coding Badly


Everything I can find on the internet indicates the enemy of tin pads is a sulfur-rich environment.  Maybe the manufacturer is inadvertently leaving sulfur compounds on the boards.

Where are you getting the boards?



ron_sutherland

It was in a plastic case with what looks like rubber shock absorbers attached.  I am thinking both may be the source of the sulfur. The board was from a USB WD hard drive, and looking over the the other boards I had attached to the same system (they look to have an OSP finish) I see no tarnish/gray-tin or chia pets (dust does not count). Looking around on the internet I see lots of references to plastic outgassing sulfur compounds, but rubber is an even a bigger problem. 
   
I have images of the new board, so in a few months I will try to remember to open it and get another set of images, to see if the problem duplicates.
my projects: https://github.com/epccs

PaulS

Quote
the due point gets above 50°F for 8 to 10 weeks
What is the dew point while that is happening?
The art of getting good answers lies in asking good questions.

ron_sutherland

#6
May 02, 2015, 02:47 am Last Edit: May 11, 2015, 08:54 pm by ron_sutherland
For the last three months we have been dry (due point less than 35°F) except a few storms that moved through. So I don't think it is taking much moisture to grow the "chia pet", but perhaps the dry slows it down.

edit: reported RH (20%) as due point, it is now fixed.
my projects: https://github.com/epccs

LMI1

I have had problems with soldered PC boards too. After some months everything still looks good but there is no electric contact. The wire or whatever just lifts off and leaves a dull grey pad.


ron_sutherland

#8
May 13, 2015, 08:26 pm Last Edit: May 13, 2015, 08:55 pm by ron_sutherland
@LMI, that prompted me to do some more searching and Intermetallics* started to show up in the results. It is another possibility for the gray pads on my boards, but may fit what you have seen even better. Intermetallics does not seem to be driven by moisture so sounds like a good clue.

* http://www.ami.ac.uk/courses/topics/0156_intm/

Actually if I read this correctly a lead-tin solder may stop continued growth of the Tin intermetallic. It says, "Tin is depleted by the formation of intermetallics, so in lead-tin solders there will be a resultant lead-rich region" which sounds like the lead rich layer acts like a barrier that limits intermetallic growth. It also talks about ENIG boards nickel forming a barrier to intermetallic growth.  And from what I see Electroless nickel is not common with Immersion Tin/Silver.
my projects: https://github.com/epccs

LMI1

#9
May 13, 2015, 10:01 pm Last Edit: May 13, 2015, 10:09 pm by LMI
That Intermetallic link is interesting. There are some interesting topics one directory up
http://www.ami.ac.uk/courses/topics/
I hope they are useful.

I am a bit worried about this, low reliability is not good at all. Leaded and unleaded tin and mixing them does not make things easier. Those problems I described were in leadless PW boards and made with professional equipments. I dare not think what happens when if I use leaded solder with some leadless new boards and components.

Isaac96

Actually, what is happening is the tin is changing its crystal structure. Keeping it in a cool place causes this.
Now you have "gray tin"!
Do not PM me for help. I will delete immediately.
CONNECT THE GROUNDS!

After Tuesday, even the calendar goes W T F

LMI1

#11
May 22, 2015, 05:22 pm Last Edit: May 22, 2015, 05:23 pm by LMI
I see, but at least my boards were kept indoors, probably over 20 degrees centigrade. I thought that the tin pest begins below 16 degrees centigrade or so.

The solder these days is leadless, that is tin, but isn't there still some other subtances in the solder tin which prevent tin pest.

Edit: Tin rot=>tin pest
http://en.wikipedia.org/wiki/Tin_pest

ron_sutherland

#12
May 22, 2015, 08:53 pm Last Edit: May 23, 2015, 08:23 am by ron_sutherland
Also, my board was above 20 degrees C, so the gray pads are sounding less like tin_pest in my case.

I think that both tin_pest and intermetallic formation result in a change in the crystal structure. In the case of intermetallic formation, copper is entering the matrix but since it does not fit perfectly it causes things to reorganize. From reading various articles, I gather that a tin alloy does not suffer from tin_pest, but it is not yet clear to me what ratios are needed. From the reading it also looks like intermetallic growth is required for the solder to wet, but then needs to be limited by a nickel layer or a lead rich layer. 

One thing I have concluded is that if I want to solder a copper wire (which does not have a nickel layer) I will need to use a massive glob of lead free solder so that the wire is fully surrounded, or use lead solder.

So this is making me wonder if lead is really a problem in electronics, I fully understand why to remove it from petroleum gasoline, and plumbing (although this may explain a hot water line that pulled apart, which was also very odd). 
my projects: https://github.com/epccs

Isaac96

Wow, this gets stranger and stranger! If you want to solder a copper wire, just tin it first.
Do not PM me for help. I will delete immediately.
CONNECT THE GROUNDS!

After Tuesday, even the calendar goes W T F

ron_sutherland

#14
May 23, 2015, 07:46 pm Last Edit: May 23, 2015, 09:22 pm by ron_sutherland
@Isaac96, haha, yes it seems to be getting stranger to me also, so why would tin help since that is what makes the Intermetallic in the first place... see link in post #8
my projects: https://github.com/epccs

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