Hello kind AAC people,

I am fairly new to electronic design and repair. The story begins with a need for some quality cable and a soldering iron. From there, the plot thickens with a home-made power supply (I figured if electronics weren't meant to be for me, I might as well be dead, right? lol). I have also read the AAC text book, which is absolutely one of the best resources available on the entire internet. That's my foundation of knowledge at this time; I am comfortable around a circuit board and its components these days, but I'm still a bit green with the details.

I've now in my hands an old synth that I am trying to restore. The main board looked to be in fair shape, except maybe covered in flux or some other yellow/brown coating. I q-tipped it with 90+% rubbing alcohol and now it's shiny green. With that said, two IC chips are exhibiting some sort of corrosion or oxidation (it was there before the q-tipping), and I'm wondering how serious of an issue this is.

Can the oxidation affect the traces the legs are bound to? Is the oxidation detrimental to the chip's functionality?

From what I've read on similar threads, it seems folks have tried to salvage chips with oxidation on them, so maybe it's not a deal-breaker of an issue. I've attached a picture of one of the chips (ic22) in question. I thank you in advanced for your expert opinions on the matter and answers to my question.

 

Hello,

It depends how deep the corrosion is.
Most times the corrosion is on the surface and can be brushed off with a glass fiber pencil, like in the following link:
http://www.ebay.com/itm/Fibre-Glass...Tools-Fiber-/281178279715?hash=item4177880f23

Bertus

 

Hi and welcome to AAC!

You should remove as much of the corrosion that you can, possibly even going so far as removing it so you can get at the underside of the pins. If the corrosion encroaches into the package, the seal will be compromised and will lead to premature failure of the IC.

 

The yellow-brown coating (if uniform over both sides of the whole board) may have been a conformal coat, whose purpose is to protect the board from the effects of moisture and oxygen. It likely should not have been removed.
But the corrosion on the pins should not be a problem that will affect the operation of the board as long as the solder joints are still okay.

 

thank you everyone for your replies so far.

The yellow-brown coating (if uniform over both sides of the whole board) may have been a conformal coat, whose purpose is to protect the board from the effects of moisture and oxygen. It likely should not have been removed.

dang off to a not so good start. I take it I should be able to find some sort of product to re-apply?

 

Those chips are just tri-state TTL buffers on what looks to be some sort of bus with pull up/down resistors so it's insensitive to this level of surface discoloration. Use a clean horsehair brush to get under the leads with a IPA cleaner. The level I see here is mainly cosmetic and once cleaned it should be OK.

 

That greenish color on the leads looks like corrosion attacking base metal of the lead frame. It needs to be removed before it gets into the package. I'd use something abrasive like scotch brite pads, wire brush, or fine grit sandpaper to remove as much as you can and then apply solder to protect from further corrosion.

 

if the corosion is near an electrolytic cap, check for leakage of the cap, some of them have very corosive stuff inside. then change the cap and chip. I have even seen the goo eat through plated through holes on the board.
basicly, if the board is critical to life (you or someone important to you) replace the chip, otherwise clean it and use conformal coaing to prevent more corrosion.

 

all the caps look ok, although from what I've read, I understand caps can fail without showing it to the naked eye. Is a bad cap the same thing as a cap leaking its inner liquids (I'm presuming that's some sort of acid)?

mind you, my original hunch was that it was moisture that caused the corrosion on the chips as the metal case itself was badly rusted. not that anyone cares, but now that's looking a bit better after applying some absorbic acid and sanding it down.

 

Is a bad cap the same thing as a cap leaking its inner liquids (I'm presuming that's some sort of acid)?

Caps have multiple failure modes. The electrolyte is very corrosive. It will take wax off linoleum floors like it was acetone (ask me how I know).

mind you, my original hunch was that it was moisture that caused the corrosion on the chips as the metal case itself was badly rusted. not that anyone cares, but now that's looking a bit better after applying some absorbic acid and sanding it down.

If it was badly corroded, I'd remove the chip, sand all surfaces of the leads, and tin them with solder. Pay particular attention to the point where the leads exit the epoxy package. If moisture gets in the package, corrosion will attach the IC.

 

thank you everyone for your replies so far.



dang off to a not so good start. I take it I should be able to find some sort of product to re-apply?

If the coating came off that easy, its probably what didn't prevent that corrosion.

The clever bit is identifying whether acid or alkali is the culprit, and cleaning it with an opposite solution to neutralise it.

Originally, boards were cleaned during manufacture with solvent, but that was banned to protect the ozone layer. Now boards are cleaned with water. I'm most cases you just have to make sure all the water is out of any nooks and crannies before powering up, but you have to watch out for things like DIL switches, relays and sensors etc. Some have a plastic seal pull off tab that keeps the water out, others are simply not fitted till after washing.

Conformal coatings were discussed here a couple of months ago, so you should get some suggestions what/where to buy.

 

The tree and the woods.

Whatever affected any of the chips, has been working on the whole board as well. Better to inspect the whole thing.