Cleaning vintage PCB - Damaged by isopropyl?

[xfd1001]

Hello,

I recently bought some vintage instruments that needed some cleaning.
Since I don't have expertise with cleaning vintage PCB's; I did some extensive searching on the web to make sure I don't use any cleaning substances that might damage the (bakelite?) PCB material.
Every single post that I read said it's fine to clean with isopropyl alcohol and some some people suggested using dishwasher soap.

So after I had the guarantee it was fine; I carefully started cleaning with a Q-tip & isopropyl alcohol. And within minutes the spots that got in contact with the alcohol started to look damaged. 😓

Can anyone tell me if it's actually damaged and if yes; why it got damaged? How to repair the damaged surface & how to proceed without further damage?

Attached you find a picture of the damaged surface.


Thanks

[AndyBeez]

Looks like the IPA lifted a conformal coating. A coating that may have used IPA as the thinner - some 40/50 year's ago. From the vintage, maybe a resin or a wax based coat? It's possible the coating had not keyed onto the SRBP surface. So far, just cosmetic/no harm. Try diluting your IPA 1 with 3 part water - so it's not so agressive.

[Gyro]

The board itself is of course not damaged, but whatever the coating is, it is clearly soluble in IPA.

As the traces are not tinned, it was common that some form of coating would have been applied to the surface to prevent them from corroding. This was often something as simple as a layer of flux or a an IPA soluble laquer of some type.


P.S. Do you actually need to clean the board surface?

[jpanhalt]

Shellac apparently has been used as a conformal coating -- at least early on.  It is not surprising IPA damaged it.  If you want to remove it completely, the more common solvent for shellac is absolute ethanol (not denatured "alcohol" for fuel).  Woodworking suppliers may be one source.

[xfd1001]

Thank you all for the help! I wish I had read this before damaging the coating & hope others who search on vintage isopropyl cleaning will find this thread.
I guess it would be okay to just leave it like this and just refrain from further cleaning?

@jpanhalt Aside from that it looks ugly damaged now; I guess it's better to leave the Shellac on? Or would it be better to get rid of it completely and apply a new coating?
If it doesn't matter I will leave it on. Preferably restore the coating to it's original state but I guess that will be too difficult?

@AndyBeez
Yup, a bit older actually; it's an analog multimeter from 1969 that I'm trying to repair & preserve.

[TERRA Operative]

I found the same with an old oscilloscope I refurbished a while back, the PCB looked identical in construction.
I didn't take as much of the coating off, so I just left it as-is in the end, but if you wanted to make it look real nice, I'd clean the coating off as suggested above and use a modern spray coating after.

[jpanhalt]

It depends on why you are doing the restoration. 

If appearance matters,  I would remove and recoat.  Apparently, there are still shellac based conformal coatings still available for authenticity.  Otherwise I would switch to a lacquer or another varnish based coating.  Removing it could be a tedious job or very simple.  Depending on what else is on the board, a few minutes in an ultrasonic bath with denatured ethanol or pure ethanol will work.  If there are transformers and relays, etc. on the board, I wouldn't do that.  Then it would require gentile cleaning with a brush/swab.  The nice thing about shellac is that it is simply solvent based in contrast to enamels that are reactive.  Thus, applying a good shellac solvent/thinner, letting it level, and slowly evaporate may repair the finish cosmetically.  All of that assumes it's shellac.

If appearance doesn't matter, then I would leave as it is. 

[AndyBeez]

An Avo meter? Certainly it's Shellac as anything that was quality and built to last was Shellac coated. Leave in situ and focus on the parts tarnished with time. What about the wiper contacts on the other side of the board?

The PCB is resin bonded paper, essentially the same material as stripboard/Veroboard. It has that distinct 'old school electronics' odour when warmed. I should add, avoid using any volatiles such as ethanol in ultrasonic tanks, unless you feel your life needs a lot more danger.

Good luck with your restoration.

[coppercone2]

ehh conformal coating is tough. I had it damaged with IPA, denatured alcohol and even PCB flux remover, it all causes it to wrinkle. Your best bet is to spot clean carefully and then apply new coating. Even some modern conformal coats from the last 20 years will delaminate/weak from alcohol and solvents .

I say using anything but soap in a ultrasonic is asking for trouble from plastics.

[helius]

Shellac is still very widely used in furniture restoration and sometimes used to finish new furniture. Something to keep in mind is that the shelf-life of premixed shellac is limited: after months in alcohol solution the resin starts to break down, with the result that it never really dries. The longer lasting form is dry shellac flakes, which come in various grades and types of processing. Some of them have had the wax removed and there are different levels of bleaching to get progressively lighter colors. They are dissolved in warm ethanol with or without methanol but without any water etc., and shaken/stirred for a few days to dissolve as much lac resin as possible, then filtered through a paint strainer or coffee filter to remove insoluble debris.
Shellac is also soluble in mixtures of petroleum solvents, where it is referred to as "padding lacquer".