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Oxidized solder and IC pins - PCB rot

Crawford

Experienced Member
Joined
Oct 21, 2008
Messages
275
Location
Maryland
Folks,

I have a board I’m restoring, but this is a more general topic, so starting a new thread.

The board in question is. Vector Graphic disk controller board, and the condition is somewhat of a mystery. The rest of the system shows normal aging, and is in good condition. This board is the exception to the rest of the system, but has always been part of the system and stored with the rest of it.

The board shows heavy oxidation on the soldered joints, and the IC pins are also oxidized. Overall the board looks ‘dirty’ on the electrical parts. The tops of the IC’s and caps, resistors look okay. At first, I thought the board was filthy with dust, so I scrubbed it gently with isopropyl alcohol and dried it. It looked exaxtly the same after this washing.

The oxidation on the solder joints is both electrically and heat resistant. To get a good electrical contact, I need to scrape the pins or joints with an exacto blade. If I want to reflow the solder joints, I have to use much higher temps on the solder station, and even if I get the joints back to normal shiny-ness with good 60/40 solder, there is a nasty residue, that needs to be scraped before it will come off.

I have already replaced a couple of IC’s on this board that were faulty. These were 74ls221’s that one half of the IC failed, but the other half functioned. Don’t know if this is related to the overall condition, but the oxidation makes troubleshooting harders and IC replacement a pain.

So, Ive seen bad boards before from wet environments, or from rodents nesting in cases, but this is a mystery.

First question: is there anything that might dissolve the oxidation without eating up the solder joints or causing further damage?

Second question: has anyone seen similar damage?

Third question: Could the IC damage be related?

Fourth question: what might have caused this, in a system that shows no other damage like this?

I will try to post some pictures to illustrate what this looks like.
 
Pictures of board.

Closeup of IC pins:

0C8192BF-F12C-41AD-8CC2-196A88528D20.jpeg

Back of board:

AB4F0E74-F8F5-4957-AF87-06A2F939256F.jpeg

Comparison, good board in same system:

6D20889B-DB21-4B50-9920-7A39756F7CFA.jpeg

Comparison, good board back:

2878D0FC-F22A-480B-A672-2CD5E8A38EE3.jpeg

Hopefully it’s visible that the damaged board is oxidized, but can’t depict how deep the oxidation goes.
 
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On occasion, I've resorted to a "no clean" flux pen rather than adding solder to an already soldered joint. Seems to work without the necessity of cleaning off burned flux. Don't know if this will work for you, however.
 
I have a theory on this:

What you are seeing is Stannic (Tin) oxide which is often a light grey color, it is a thermal and electrical insulator with a very very high melting point, and has to be scraped off before you can thermally couple to the solder underneath to melt it.

Presumably the solder they used had a very high Tin content, but there was just enough Lead in it to prevent Tin whisker formation.

You can also see the same effect with good quality 60/40 solder, over a shorter time frame, where it has been in contact with leaked electrolyte from capacitors, which simply accelerates the same basic process. But in your case it represents a reaction with atmospheric Oxygen and relative humidity over very many years.
 
I've used a small drop of vinegar with a cotton swab to take care of corrosion. Make sure you clean off any vinegar residue very well afterwards.

....Hmm I don't know about that.

Stannic oxide, which I think it is, would only be soluble in strong acids, like 50% H2S04 , some suggest Hydrochloric or Hyrobromic acid. And one remark I read is that it is difficult to dissolve even with strong acids. All of these things would be highly destructive to a pcb. I cannot imagine the weak acetic acid in Vinegar getting very far (but might be worth a punt, but I think this robust oxide would laugh at it) and its probably better not to put any polar / ionic compounds (except de-ionized water perhaps) on the pcb.

The only way I see to safely deal with it, is to manually scrape the oxide off enough to be able to add fresh solder and clean it up that way.
 
Folks,

Hugo’s input seems the most logical to me. I have been scraping the joints both to re-solder as well as test connectivity.

I did find a partial solution, if I’m removing an IC. I have removed several, because they malfunctioned, or the pins were so corroded I didn’t trust they would last long.

My method is to use a Hakko desoldering gun to suck out as much of the old solder as possible. Then I use solder wick wetted with past flux to ‘hot scrub’ the top and bottom of the IC pads. The wick soaks up the rest of the bad solder and the residue. Then an alcohol wash and scraping with an Xacto blade. Finally, re-tinning with good quality 60/40. This leaves the pads and exposed traces bright and shiny. I have also learnt to buzz out all of connection before I install a replacement IC. There have been some PCB breaks under the chips.

The PCB is actually a pretty high quality. It appears that the low quality solder is the main issue here. If I had done as much scrubbing and scraping on a lower quality board, inwould have burnt off pads and scraped off or lifted traces.

I guess I’m fortunate that this is the only board of my Vector Graphic system with these symtoms. Bad batch of solder?

-Crawford
 
....Hmm I don't know about that.

Stannic oxide, which I think it is, would only be soluble in strong acids, like 50% H2S04 , some suggest Hydrochloric or Hyrobromic acid. And one remark I read is that it is difficult to dissolve even with strong acids. All of these things would be highly destructive to a pcb. I cannot imagine the weak acetic acid in Vinegar getting very far (but might be worth a punt, but I think this robust oxide would laugh at it) and its probably better not to put any polar / ionic compounds (except de-ionized water perhaps) on the pcb.

The only way I see to safely deal with it, is to manually scrape the oxide off enough to be able to add fresh solder and clean it up that way.

Completely agree... it was an alternative idea - regardless, sorry you're having to deal with your oxide challenge ;)
 
It's all in the flux.

SnO₂ reacts readily with most halogenic acids; e.g. HCl. Which is why, when I get a stubborn case of solder not sticking with rosin flux, I turn to my bottle of Harris Sta-Clean acid flux. A tiny drop is all it takes--I moisten the end of a toothpick with it and touch the joint, then solder normally. (ZnCl, HCl, NH₄Cl and alcohol, mostly). For soldering stainless steel, I use phosphoric acid. There are even fluxes (fluoride-based) for soldering aluminum, which, if you think about it, is pretty neat.

Clearly, with acid flux, you have to be careful and thoroughly clean your work. A rinse with a weakly basic solution probably wouldn't hurt.
 
It's all in the flux.

SnO₂ reacts readily with most halogenic acids; e.g. HCl. Which is why, when I get a stubborn case of solder not sticking with rosin flux, I turn to my bottle of Harris Sta-Clean acid flux. A tiny drop is all it takes--I moisten the end of a toothpick with it and touch the joint, then solder normally. (ZnCl, HCl, NH₄Cl and alcohol, mostly). For soldering stainless steel, I use phosphoric acid. There are even fluxes (fluoride-based) for soldering aluminum, which, if you think about it, is pretty neat.

Clearly, with acid flux, you have to be careful and thoroughly clean your work. A rinse with a weakly basic solution probably wouldn't hurt.

I think the best solder in the world was invented in England (I think) and it is called Ersin Multicore. It has very low chemical reactivity in the remaining flux (if you leave it) but the 5 core flux is extremely effective. It is sold in the USA under that name or Loctite brand I have noticed, so I think the flux formula was licensed to them.
 
My father bought me a 'job lot' of Ersin Multicore solder (or similar - the labels have become detached from the reels) when I was in my late teens early twenties back in the late 1970's / early 1980's (if I remember correctly). I am still using it up today would you believe!

Dave
 
It's all in the flux.

SnO₂ reacts readily with most halogenic acids; e.g. HCl. Which is why, when I get a stubborn case of solder not sticking with rosin flux, I turn to my bottle of Harris Sta-Clean acid flux. A tiny drop is all it takes--I moisten the end of a toothpick with it and touch the joint, then solder normally. (ZnCl, HCl, NH₄Cl and alcohol, mostly). For soldering stainless steel, I use phosphoric acid. There are even fluxes (fluoride-based) for soldering aluminum, which, if you think about it, is pretty neat.

Clearly, with acid flux, you have to be careful and thoroughly clean your work. A rinse with a weakly basic solution probably wouldn't hurt.

Smart, I like this approach
 
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