Thursday, April 15, 2010

IC decapping round 4: burnt perfection

As you can hopefully see from some of the previous posts, people employ many techniques to remove the irritating resin casing. I've seen suggestions of simply burning away the casing, which I was somewhat skeptical of, thinking this would annihilate the chip. However, I am open to ideas, and thought I'd give it a spin.
First off, burning is probably a bad word. The image I had in my head of how this would work was the physically heat the chip until the resin was utterly destroyed, like burning away a piece of wood. It turns out its not hard to make the resin brittle through heating. Or in other words, all one has to do is apply a short burst of intense heat and you can crumble the casing away with minimal chip damage. A picture is worth a thousand words, lets see an example. This was the result of the first chip I tried:
It came out amazingly clean like this in under a minute. Microscope inspection seems to indicate the die is healthy. More on this later. Starting from the beginning, here is a virgin chip:
A CP82C59A interrupt controller. All torch images are of one of these, but not necesarily the exact same unit. A tube of them was being kicked around in the RPI Electronics Club junk drawer. Torch meat now:
You shouldn't heat the chip too strongly. If you get this, you've gone too far:
I actually would have never guessed you could get a red hot IC package. In any case, as will be seen in the video, the most important thing is even heating. I'm not sure what the shock temperature is, but there was a certain cutoff line where the chip was extremely brittle vs very hard. In the video I don't heat the lower half of the chip good enough and it only breaks moderately well.
After removing the torch, it will burn like a well down marshmallow for a few seconds:
It should go out by itself fairly quick, but I was waving or blowing it out. Result:
Breaking now:
This final image is the die from the video. The one from the sequence above is the one on top shown here (contrasted with the very first one I flame decapped):
The heat managed to separate the die and the carrier! So that's what happens when you apply too much heat. I wasn't paying attention at the time and I'm not sure what happened to the die. I figured it had been melted in some weird way where as it probably fell to the floor when I cracked the casing open. The first one I tried (bottom die above) couldn't have gone any better. The case split perfectly and no resin was left covering the die.
Seeing how well the die withstood the heat, I wondered how far can we go? So I torched a die red hot. Although it was not as clear under the microscope as before, it still was of decent quality. I'll try to add a pic in a bit comparing an area of the first die extracted with the one that I tortured.
Here is a short video showing an heat based extraction:

In this test setup, that blue thing is a filter system to help reduce fumes. I also have a gas mask handy for exactly this sort of work. It really does help a lot. The resin was really brittle and very little force was required to break it. However, I didn't heat it enough towards the bottom, so one side of the die was still relatively well set into the case. This can be easily fixed though and is partly due to me trying to get this on film and not doing for an ideal setup.
For some future work, I may have gotten away with this due to the relatively large traces the 2 um test die I used had. Thinner traces may be much more sensitive. Still, the results are much more promising than I was expecting. As I move down the semiconductor technology roadmap, I'll revisit this and see how well it does against finer masks.
Thanks to Will Carder for lending me the torch!

3 comments:

  1. Fuming nitric acid would probably be a better option than fire.
    Use a fumehood. Good luck.

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  2. was an early test more out of curiosity than practicality. RFNA is expensive and difficult for me to obtain. I've had some luck with making it myself, but I generally find its not worth the effort. I now use heated 70% nitric to do the bulk work. Sometimes I use a heat gun to chip away the bulk epoxy first if I don't care about damage potential. Lifting the pins provide a convenient way to rip up the top portion of the package/carrier. The bottom portion of the package/carrier usually will just be held on with some glue to the die and comes off. Obviously this won't work for BGA or such, but those are usually small enough that the nitric will quickly eat through a thin epoxy layer.
    I don't have a fume hood, but I now do this in a fairly well ventilated semi outside area. I've also found using a watch glass helps to reflux which increases chemical lifetime and reduces fumes.

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