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Potting material nightmare

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deathhare

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I have a circuit board I trying to work on.
The chip has apparently failed in some way and so I need to get at the chip and replace it.
Ive heard some good tips on this site about removing the softer silicone-ish potting and I want to try that.
The one idea being using silicone sealant remover to get it off.

From all Ive read this may be the best way to go. Im only afraid that it will also remove any ID on the chip and so then I wont be able to get the right replacement chip. ;)

Anyone have any idea about that?

Thanks for your help.
 
Ive removed a lot of the stuff with a hot blade but at this point hope to be more surgical in some way.
Luckily the back side is clean.
Top pic shows where Im at with it.

Thanks for your help.

**broken link removed**

Back side..

**broken link removed**
 
There are some good silicone caulk removers on the market that are based on using long-chain sulfonic acids to break the bond to a substrate. The hydrophobic portion (i.e. long aliphatic chain) facilitates penetration of the agent through the caulk. They do not dissolve silicone caulk. However, your description of using a heat knife is not consistent with most silicones.

I suspect the potting material is not a silicone caulk. It may possibly be an RTV rubber. If so, I would suggest using mechanical means to get as much as possible removed. Then, depending on your access to relatively non-reactive solvents, use a solvent for final clean up. You might soak it in a denaturing solvent, like methylene chloride-methanol-acetone, which may make the rubbery compound easier to remove. That solvent mixture is common in solvent-based paint removers in the USA.

John
 
You might soak it in a denaturing solvent, like methylene chloride-methanol-acetone, which may make the rubbery compound easier to remove.

John

Thanks.
Would that essentially be acetone?
I tried a piece in acetone yesterday and it did nothing to the material.
 
Of the three ingredients mentioned, the most important one is methylene chloride. Chloroform might be a good substitute. Methylene chloride boils at 39.6°C and is flammable. See:https://en.wikipedia.org/wiki/Methylene_chloride

Can you get solvent-based paint strippers where you are? If the label says methylene chloride (or chloroform) try it.

John
 
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Of the three ingredients mentioned, the most important one is methylene chloride. Chloroform might be a good substitute. Methylene chloride boils at 39.6°C and is flammable. See:https://en.wikipedia.org/wiki/Methylene_chloride

Can you get solvent-based paint strippers where you are? If the label says methylene chloride (or chloroform) try it.

John

Thanks. I'll have a look at Home Depot and see what they have.
Would these strippers also damage the other components or the board at all?

edit: Yes, the printed numbers on the chip are pretty important. So maybe thats not the best way. :(
 
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Well, the way I look at it is that you can't read the numbers now either, and it appeared you had pretty much given up on just trying to scrape the potting material out.

In my experience, a lot of chips have etched numbers that are not removed except by grinding. Printed numbers may or may not be damaged or removed, depending what was used to print them. Often, when a painted stencil is removed, there is a ghost image left that can still be read.

However, I think you misunderstand the whole process. Those solvents I mentioned will probably not dissolve the potting material. They may cause it to swell and become friable so that it can be more easily removed. It is a soak, scrape, soak, scrape type of process.

I picked those solvents because I thought you might have access to them. You might also consider soaking in one of the alkyl sulfonic acid based caulk removers. I recently used McKanica with good results on silicone caulk. McKanica is carried by Ace and is quite expensive for what it is (https://en.wikipedia.org/wiki/Alkyl_sulfonic_acid). If you have access to sulfur trioxide, you could make your own by bubbling it through mineral spirits. In fact one of the other brands seems to be made that way. I can't remember that brand right now, except it was no cheaper. The Lowes stuff does not work. My first choice, though, would be to try the solvents/paint remover first.

As a last alternative, you might take it to the airport and see if TSA will help you see through the potting material. Be sure to have your attorney's cell phone number handy. ;)

John
 
Don Lancaster wrote several columns where he talked about this subject.

He mentioned also many times the help of a dentist for that. Certainly Xrays.

Look in his site.
 
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