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WARNING: This is very dangerous and should not be attempted unless equipped with proper training and equipment.

Some time ago I posted about uncapping integrated circuits and mentioned that newer case materials are more resistant to being etched by sulfuric acid on a hotplate. The reason that I came to that conclusion was that I placed a recent AD711 and LM317 in a Pyrex container with sulfuric acid and heated it without any substantial results. The writing on packages became slightly worn after an hour but that was about it.

I tried to uncap a few chips this week using a dremel motor tool and a round grinding stone but was unable to get close enough to the die without scratching it. I figured that I could grind down the case above the die (while wearing a face mask) leaving only a thin layer of packaging and then use sulfuric acid to get rid of the rest.  To my surprise, the acid worked really well and destroyed the whole package leaving only the die and the metal pins. I used the same acid as before and heated it to 60-80 degrees Celsius (in a fume hood). I think that I used a slightly cooler setting last time, so either the 10-20 extra degrees did the job or removing the surface of the package made it less resistant to the acid.

My next idea is to try to expose the die without destroying the chip. From the experience of using the dremel to expose a few dies, I can tell that if done carefully and under a microscope, it is possible to see the tops of the interconnect wires before you reach the core. That is, either use the dremel very carefully to remove just enough package so the wires only begin to appear, or start with the dremel and then use a fine grit sand paper. Once this is done, the next step would be to carefully time the etch checking periodically with the hope that the package is much thinner over the die and will be dissolved faster. This way, the die will be exposed and the rest of the package will still be in adequate shape, although the pins may have to be resurfaced so they don’t become too brittle.

I am aware that there are commercial places that will take an X-ray image of the chip to get exact depths and then do a controlled etch, and given enough time, I may be able to get access to/build such a system, but the key for me is something that is quick and easy to use. The way things are going, the dremel and sulfuric acid may be a pretty good option.

 
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