I decided to start my adventures decapping chips with a very low-impact method that's been talked about: decapping with rosin. Rosin is the solid form of resin. Rosin can be most often gathered from eBay under the name Colophony, where it goes for not very much per pound.
Next I bought this stuff:
- Steinel type 3482 temperature-controlled heat gun (eBay, USD 149 plus 5 shipping
- 10-pack borosilicate test tubes, 18mm diam x 150mm long (eBay, USD 10)
- Acetone 99.5%, 950ml (Duda Diesel, USD 8 plus 9 shipping)
- Safety wash bottle, 1000ml, Karter Scientific #217H1 (eBay, USD 12)
- Test tube clamp, 45mm, Eisco Labs #CH0688A (eBay, USD 12.50)
- Ultrasonic heater/cleaner, 2L, unbranded Chinese manufacture (eBay, USD 55 plus 5 shipping)
Also a stand which a friend welded together, that could hold the test tube clamp. And finally, I already had a Fluke 87V multimeter with type K thermocouple.
Next, I stuck the thermocouple in the test tube and aimed the heat gun at it. The temperature in the air in the test tube was always below the temperature reading on the heat gun, and the higher the temperature setting, the more the deviation.
I took a chip, a 74S10 (a triple three-input TTL NAND gate, manufactured the 14th week of 1980) that I had many of, and, with a Dremel and a cut-off wheel, slowly whisked off very thin layers of epoxy off the back of the chip until the back end of the die carrier became visible. This allowed me to cut off the non-die portions of the package.
Next, I placed three crystals of rosin in the tube, put the nozzle of the heat gun almost against it, and started it up. I put the thermocouple touching the glass, where the nozzle was aimed. The thermocouple registered 175 deg F [80 C] when the crystals started to melt -- the heat gun was set to 230 F [110 C] at this point. At 240 F [115 C] on the thermocouple (280 F [140 C] on the heat gun) the crystals were thoroughly melted into a brown-yellow viscous liquid. At higher temperatures, the rosin became thoroughly liquid.
I made a little basket out of ordinary metal window mesh. I put the chip in, and lowered it into the liquid. I also decided to put the thermocouple in as well. At a measured temperature of 373 F [190 C] (550 F [290 C] on the heat gun) a few small bubbles started to stream from the chip. I left it there for 30 minutes, then increased the temperature to measure 399 F [205 C], and left it there for another 30 minutes.
Then I removed the chip with basket, placed it in another test tube, let it cool a bit, put some acetone in to cover the chip, and placed it in the ultrasonic cleaner filled with distilled water, for five minutes.
After this, the chip was clean, but there was no evidence of the epoxy being affected. Even the lettering was still intact.
I reheated the rosin, put the chip and basket back in, and raised the temperature to measure 447 F [230 C] (600 F [315 C] on the heat gun). I saw what appeared to be silvery metallic dust circulating in the liquid. I increased the temperature to 570 F [300 C] (800 F [425 C] on the heat gun). At this point the rosin liquid was boiling, but not vigorously. I left it for 60 minutes, then removed and cleaned in acetone and ultrasonic as above.
I found that the chip had cracked along a line apparently just above the die, so I removed the epoxy half and put the chip back in the basket. I had also noticed that the thermocouple read hotter near the top of the liquid (for example, 625 F [330 C] at the top but 560 F [295 C] at the bottom of the test tube) so I affixed the thermocouple so it was measuring the top of the liquid.
This time I set the measured temperature at 650 F [345 C] (900 F [480 C] on the heat gun) and left it in for 15 minutes. At this point the color of the liquid was brown-black. I removed the chip and cleaned again.
This time, I found that I could just pick the package apart.
The die was separated, but had cracked in one corner. There was also epoxy in the middle. I believe the metalization layer seems partially gone. it looks like copper-colored metalization is covering silver-colored metalization. The die itself appears black. I've most often seen a ring of metalization around the outside of the die, but it seems that while the ring is there, the metalization is not. I fear the metalization has been partially removed.
It is possible that the multiple heat-cool cycles cracked the die. Or, it could have been due to the high temperatures (up to 650 F [345 C]).
Next, I am waiting for an AmScope ME300TZB-2L-10M metallurgical microscope which should be able to take much higher-resolution images. I'm going to try this technique again with another 74S10, except this time set the temperature right up to the final setting, and leave it there for maybe an hour or two.
I've also ordered some sulfuric acid from Duda Diesel to try the hot sulfuric acid technique. When I was walking around Harbor Freight today, I noticed they had an abrasives blast cabinet that I could probably repurpose into a fume box.
Just seal it all up, replace the gloves with acid-resistant ones, add a filtered air outlet to attach a fume fan to (which I have courtesy my laser cutter). Hopefully that should eliminate the change of getting hydrogen sulfide smell all over the place.