Decapping and Chip-Documentation - Howto

[iMo]

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Without further experiments I can't say with certainty what really helped, but it appears that the key to success is high temperature and reflux.
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The colophony contains acids. Under ambient temp the acids are not active. With higher temp they activate.
Therefore colophony has been used as the soldering flux since ever (there are many varieties of the colophony too)..

[magic]

But you can't go too hot either, or you end up with carbonized mess like on my first attempt 4 years ago:


Further experiments will be needed to find the best temperature. Here Sacodepatatas says that it should only take a few minutes, so maybe I still didn't heat it enough. Experiments will also be needed to see if this works on anything other than AliExpress junk, because the chip I decapped was a Chinese MC34063.

It's not clear what the exact mechanism is. Maybe there is a reaction with resin acids and temperature speeds it up, maybe at 380°C epoxy pyrolyzes by itself and colophony only acts as a solvent for the resulting products. Maybe the resin acids turn into something else, more aggressive towards epoxy.

One thing I can say is that boiling for many minutes at 340°C does nothing, but at 380°C or so something starts to happen, the solution turns brown-green and loses transparency, and the chip disappears.

[RoGeorge]

But you can't go too hot either, or you end up with carbonized mess

Maybe use a cheap soldering iron, or an already corroded tip (cardridge-type, the ones that have the thermocouple inside the tip, like T12 for example), and put it inside the vial, so to boil the colophony rosin at a constant temperature fixed from the soldering station.

There are also ready made thermostated pots/tin bays, for tinning wires.

[iMo]

I put a chip (LT1021) into the flame of an ordinary gas oven in my kitchen till the epoxy became fragile (was almost red), the chip simply dropped off the crust after I pressed it with pliers a bit..

[magic]

I decapped another one, a Fairchild TL431, so this can work on brand name chips as well. Took half an hour, probably because TO92 is thicker than SO8.

Temperature is stabilized by the boiling liquid, which absorbs excess heat and vaporizes. You just need not to go too crazy with heat input so that the vapor has a chance to recondense before escaping into the air. Watch the test tube and make sure that there isn't too much foam and that the white mist above the liquid isn't rising too high. Pretty simple. Below you can see what it looks like - the green stuff is the overheated colophony (little of it left at this point), yellow is foam, white is the condensing mist which is supposed to be falling down back into the liquid.

I put a chip (LT1021) into the flame of an ordinary gas oven in my kitchen till the epoxy became fragile (was almost red), the chip simply dropped off the crust after I pressed it with pliers a bit..

You can, but anything that produces cleaner, more repeatable results with less risk of overheating the die or losing it inside a chunk of epoxy falling on the floor is welcome. Unfortunately, the chips aren't coming entirely clean form this and the colophony gunk isn't the easiest thing in the world to remove and I ended up scratching this die a little. Acids are cleaner, particularly HNO₃.

I wanted to say that one could use colophony to decap chips with the package leadframe and bondwires intact, but the damn die fell off during the boil. Well, at least I have a picture of the package

[T3sl4co1l]

Imagine future paleontologists' surprise when they find a glass vessel, containing solid amber, with a "dead bug" trapped within.

Tim