IC decapping, round 3: chemical etch

In my last post, I discussed some of my early experience with sanding. After sanding, I tried some simple chemical etching. Unfortunately, my memory card died and I lost some cool stuff such as a microscope etch video, but I will hopefully be able to get some new, better images and videos later.
In any case, there were two areas to try: etching the case and etching the die. Etching the resin case is usually done with nitric acid. Travis Goodspeed has a good blog post about doing this at room temp here. I was able to get 70% nitric considerably easier than RFNA, so I decided I'd try that first. If you use non fuming nitric acid (but still required to be high conc I think), you must strongly heat the mixture. But, I did try a few other things first to see if there were alternatives. The first thing I tried was just to let it set overnight. This ate away the metal leads pretty quickly, but not the resin. The next thing I tried was to adding HCl to form aqua regia, hoping such a strong mixture would eat it away. Alas, this did not help either. Out of simple ideas, I then moved on to heat it to 80C using a hotplate. Unfortunatly, I didn't have a fume hood availible (someone is workong on getting me access to one), so I could only do it for a short time before having to cut if off due to fumes. I did not see signigant chip etching on the time I was working.
However, since I had other ways to get to the die, I figured I'd still try chemically etching the die. Using one of my cermic packaged dies, I was able to easily place HF into the die cavity so I could watch it etch under the miscrope. Defintly want to get another video of this. I was able to get a clean etch of what seems to be the top layer off and reveal the intermediate connection layer with vias. If I let it sit for longer, I imagine I could easily eat layer by layer. The top layer was taken off quite quickly even at the low concentration of acid.
I later tried to see if HCl would etch the chip. I saw initial high activity followed by not little activity. I then tried to put fresh HF on the chip and saw no reaction. I'm wondering if a protective SiCl4 or w/e coating formed on the surface, making the chip highly reistant to further chemical attack by a dilute acid. If the concentration was stronger it may have been able to dissolve this coating if thats what had happened. I ran out of decapped chips and couldn't easily decap another one at the time, so I'll go back to heat gunning some caps off. I have a stash of centrifuging tubes that I've realized work well for clean storage of dies.
I need to work on getting better storage for my HF acid. I've ordered a set of plastic test tubes with rubber stoppers for stage. Ideally I wanted screw cap, but couldn't find them at a quick look and stopopers is probably just as good. I had two different types of glass test tubes I stored it in intermitantly and one was corroded by it and the other wasn't. I'm not sure if this is simply because it had been used up by the time it got to the second test tube. In any case, it couldn't etch the IC after this, so I assume it was just that it got rapidly used up.