Plastic chips come in a variety of epoxies. Some sample chips:
Chip 1 was in acid for a short amount of time so its not a completely fair visual, but it looked about the same before going in. Anyway, In order of increasing glass content, these chips are 2, 3, 1. 3 is a PDIP-40 Microchip PIC, 2 is a TI chip, and I forget what 1 was although I'm tempted to say it was made by ST. Chip 1 decapsulates very qiuckly, 2 is what I'd call normal or a little better, and 2 is difficult. You can see what 1 looks like being in acid for a few minutes, here's what 2 looks like after sitting for a while (with 1):
It forms a hard sediment at the bottom of the beaker instead of really breaking up. There's so much epoxy that acid soaking in actually become noticeable instead of the chip breaking up like usually happens.
1's surface looks like this under a microscope (maybe 10X):
Notice all the junk in there. It makes dissolving it much easier since not only do we not have to dissolve that stuff, but it falls out, pitting the surface and making it corrode faster.
Here's the side of 2:
Interesting that the top and bottom look a little different, I'm not sure if its related to it being hard to dissolve as I haven't got a chance to compare it to similar chips.
And here's 3 (maybe 10X):
You can see some particulate in there (maybe not as apparent in this lower res picture?) and its lighter in color than the 2.
To finish off a few things not related to epoxies but came up during this. This thin package had the die close enough to the surface that it actually managed to poke through long before the package was dissolved:
Also when you do a large lot you get a pile of that junk leftover:
I use to use HNO3 to get rid of it, but it tends to dillute quickly and dilute HNO3 attacks aluminium. Fortunately its unlikely that you'll need to decap any valuable chip in volume so you can safely just pick these out with tweezers for most chips. I'm afraid of the chips scratching but the passivation is pretty strong.
Until next time.