For a little project I need to decap a PIC16C57C but keeping it alive for analysis. I finally manage to have a clean “localized” decaping but I did many test before. If you have stuff needed to work with HNO3 it will be easier, there are many article on that point on the mega Interweb. In my case I’m more comfortable with H2SO4 mainly because it’s easier to source (drain cleaner that’s what I use) where Nitric Acid is regulated (explosiv manufacturing ?). So my today results seem promising, the decaping is really clean.
Before having this results I made many tests, bare die decaping with Sulfuric Acid is not a problem, basically you throw the chip inside an Erlenmeyer flask, you can see example here. Beware if you don’t control the temperature and do that on cooking hotplate put the acid in a “big” flask to avoid acid to flood your hotplate. My previous tests yesterday were total failures you can see on the picture that the epoxy is still here (and that after more than one hour cooking… DIE is probably dead ;)).
In that case, only the near surface of the resin is attacked by the H2SO4 and the only way to go deeper is to remove all that burned epoxy by mechanical means (knife, cutter, …) it’s slow and pretty dangerous for the DIE (thermal stress and scratch risk). It seems once the epoxy is burned with hot acid it doesn’t react well anymore I tried to fix those burned chip today and didn’t success (I’ll put them on an acid bath). My idea was to control the temperature and not letting the acid boiling with burned epoxy on it. you can see on the following picture what you see when it’s happening.
If you see that try to immediately rinse the chip with acetone (but not on the hotplate !!!). The improvement I made since previous test is to had a thermo-couple in the heatsink I use as a chip support. It doesn’t give the exact temperature but you can with some try and fail test determine the good one following what you put on your heatsink. In that case the good range is between 175° and 200° Celsius. You want to have some fumes but no bubbles !
At that time the temperature was 195° Celsius and I rinsed it just a second later, as you can see there are no big dirty bubbles and fumes are limited. Don’t forget you can move the heatsink on the hotplate to change the temperature quickly between the center and the border. At that temperature I let each drop of acid between 20seconds (for the first one) to 1 or 2 minutes on last ones.
Those two pictures show you how to NOT work 😉 You can notice the difference between the epoxy on this chip and the burned one. And finally a picture of the best chemical lab setup in the world \o/
Remember if it stinks don’t blame on the H2SO4… Take a shower !