Dissolving potting epoxy from Hybrid IC

Virtually impossible.

Even if you got the epoxy off the 'printed' resistors could easilly be damaged and wouldn't be repairable.

I'd suggest building up a suitable replacement on a small board and wiring it in.

I agree. If he know the function of the IC then he can get it made again from another design engineer. Repairing of Hybrid Circuit is rarely done unless it is very special design.

Take an X-ray and pin point a problem and then use abrasive removal technique with close control of depth of removal. Such instruments should be available commercially. These were used for reverse engineering long ago. I think they used acid etching machines with force jet of acid.

Definitely possible but I do not know about chemicals. I got few such Hybrid circuits opened up for repair. I am not able to help you, as I do not the whereabouts of that chemical expert. His name Mr. Pramod Tiwari M.Sc Ph.D, he last worked for RS Components India, my ex-college at Usha Rectifiers. Alternatively you can break small pieces using cutters and other similar tools.

I once used Glacial acetic acid to remove epoxy from a potted circuit. Very, very nasty stuff, slow going and it doesn't just eat the epoxy. Backwards engineer the function and build another one. That's what you'll have to do anyway since as others have indicated repairing the device is virtually impossible and would probably be more work.

many manufacturers have products lsuch as a concret cleaner..use it brfore constructing on the floor

maybe it's useful

I have often succeeded in de-potting such assemblies, simply by soaking them in tetrahydrofuran (a.k.a. THF) for several days. Don't agitate the clear glass jar in which you immerse the entire assembly in THF, leave it quite still. The epoxy will form a sludge at the bottom and you will have placed the assembly on a little spacer to keep it up the top. You will be able to read resistor paint-stripe color codes, nearly everything! Remove the skeleton of the amplifier from the clear THF liquid and let it air dry outdoors.

Regrettably, THF is sometimes chosen as a precursor in illegal drug manufacture, so you will need ID when purchasing it. And it's carcinogenic. And it's highly flammable. So please take care not to burn, die or get arrested. But when you're finished, filter the sludge out and the THF can be used time and again for exposing the electronic mysteries that the potter thought he had hidden forever.

Mark Bingham
http://www.fourth-axis.com

Dear Mark

Perhaps you have valuable experience and it is worth putting it here. Good. I appreciate it.

Are you owner of that Fourthaxis company?

You can get in touch with me at sst (at) sensorstechnology (dot) com

In the olden days when epoxy encapsulation was quite new, circa 1953, we used to boil the assembly in triclorethelene but this was mainly to find out what had gone wrong during manufacture and not to repair it. I am no chemist so best take advice before trying it.

Methylene chloride will dissolve the epoxy but might very well attack other components. Worth a try if the THK doesn't work.

Back in the bad old days I had fair success heating the units until the epoxy would soften enough to remove it.

Save yourself a lot of heartache, buy a new machine or modernise your process.....

If you do not have exact infos about this hybrid, I give a 1 to 1000 chance of getting it to work again....sorry.

THF is not a good choice here.

You can used NMP (M-pyrol) slight heating will soften and allow you to pick off the material.

For the experienced use fumming nitric acid (Red and strong). But it will damage everything.

The board may not be coated with an epoxied, urethanes were the material of choice for board coatings.

When I need to dissolve freshly polymerized epoxy,I use a mixture of water and aceton,with about 60% aceton.The part must be immersed into the solution,in a closed vessel,and kept for one week,or even more.It's better to apply heat(30 °C to 60°C).Caution because aceton vapors are highly flammable and unpleasant to smell.
However,25 years ago,there was on the market a FAR BETTER SOLVENT:

STIRONOL PLASTO'SOLVE

probably of swedish manufacture.This was extremly efficient on potted hybrid modules and highly cured high voltage TV transformers.
You should look after the composition of this solvent,if application of warm water + aceton during one week(in full immersion)is not enough

Thanks everybody, Acetone did the trick, 3 days at room temperature was enough, there was a large resistor on the cct which had signs of fracture, luckily this went to 2 external pins so was easy to parallel a new resistor, value found by measuring the two halves of the cracked hybrid resistor, all is working now.

Cheers...

Surface coating done of Hybrid Circuits are usually ceramic sealants of high abrasion strength, high insulation, chemically inert or non-reacting type, also seal the moisture of the atmosphere and are acid resist. Some Hybrid circuits may use transparent coating similar to Epoxies used in LEDs. Ceramics are difficult to remove even by mechanical surface scratching and may generate enough heat to kill the good properties of the working components.

Manufacturers obviously know the chemical properties and often do purchase solvent of the Epoxies if used for potting as surface varnish. One manufacturer asked me to return the damaged part such that he could analyze the defect after removing the Epoxy he used for potting.

Hybrid circuits are as good as ICs and are often use and through type. However if there is some problem in manufacturing and part repair is essential then it is done before coating is applied or potting is done.