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Old 2005-10-18, 20:31   #1
ewmayer
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Default CPUs as Art - How to Expose the Bare Silicon?

I have a small collection of bare CPUs which I add to from time to time - I've always enjoyed looking at the bare silicon of the chip, and the colorful diffraction patterns given off by the features etched into same. Up until several years ago it was usually pretty easy to wedge the tip of a knife blade under a corner of the metal cap covering the chip and pop the cap off, but there are 2 cases that have me stumped:

1) Heavy use of solder - if the metal cover is really heavily soldered onto the "box" containing the CPU, it's impossible to wedge anything between the cap and the box. Is it safe to heat the solder to a temperature high enough to melt it (e.g. using a propane torch)? Note that I'm not at all concerned about preserving the functional properties of the CPU, just its physical appearance. (Also the fine gold-wire connections if possible, but I fear gold may have a melting point similar to that of the solder - anybody know the numbers on that?)

2) I have some FPGA chips from a company I worked for a couple years ago (now defunct) which I'd like to mount and display. The problem here is that the chips don't have any kind of removable cap protecting the silicon wafer - instead that is covered with a flat blob of some kind of black epoxy-like goop that looks like it was just smeared onto the silicon and allowed to harden. Anybody have any idea what this stuff is, and how it might be removed without damaging the silicon beneath?

Thanks,
-Ernst
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Old 2005-10-18, 21:43   #2
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Best way to separate the different materials is freezing them to a very low temperature. Usually with dryice is most sufficient and not that difficult to get by with some chemical companies in the neighbourhood. But to start try your own freezer to -18 or -20 Celcius which just could give you enough movement in the materials that they will separate.

Heating can also be, but you always get oxidation of the materials itself and of the dust in the air that will also be burned while heating the CPU in this case ;)
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Old 2005-10-18, 21:46   #3
Peter Nelson
 
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The black stuff is commonly called "potting compound" if its what I think you mean. One purpose is to prevent precisely this kind of "reverse engineering" LOL.
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Old 2005-10-18, 21:49   #4
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I'm quite sure the gold has a much highr melting point than solder. How about a soldering iron and a small vacum pump to carefully get rid of the solder?
For the second case, I don't know what kind of epoxy they used but most epoxies have a fairly low melting point (less than 100°C) so baking the chip in the oven at low temperature should get it liquid. It's a messy job to clean it though. Some solvents might work fine too. If you want I can ask some knifemakers I know how to get rid of unwanted epoxy (epoxy is largely used for knife handle assembly).
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Old 2005-10-19, 09:44   #5
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Quote:
Originally Posted by Peter Nelson
The black stuff is commonly called "potting compound" if its what I think you mean. One purpose is to prevent precisely this kind of "reverse engineering" LOL.
You should find concentrated nitric acid will remove the potting compound very nicely, though be sure to perform the reaction in a well-ventilated area and to wear protective clothing, especially eye protection and PVC gloves, or you will end up feeling rather . HNO_3 won't attack aluminium, silicon or gold but will go through copper very quickly.

HNO_3 is the standard reagent for such purposes.

Paul

(Yes, I was a chemist in a previous life. )
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Old 2005-10-19, 10:10   #6
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Ernst, you are not the only person to collect cpus (even I do) but nice list of sites here:

http://www.cpu-world.com/links/


Including chips as "art" here:

http://www.chipscapes.com/


I have some old quartz windows EPROMs which are nice to look at with a microscope, but quite regular as all the memory locations are the same logic.

Did you know there are sometimes little logos, designer initials or pictures etched as part of the die design? eg My Texas Instruments eprom has in the corner a miniture outline of the state of Texas. This kind of thing is quite common.
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Old 2005-10-19, 10:44   #7
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There are lots and lots of little drawings hidden in microchips, some quite pretty. Here's a gallery: http://micro.magnet.fsu.edu/creatures/index.html
I think there are several other such galleries.

Alex
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Old 2005-10-19, 19:48   #8
ewmayer
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Quote:
Originally Posted by Peter Nelson
The black stuff is commonly called "potting compound" if its what I think you mean. One purpose is to prevent precisely this kind of "reverse engineering" LOL.
Yep, that sounds right. I found sevevral posts on Yahoo Groups about removing the stuff - one guy had success by soaking the part in lacquer thinner for a week, another recommended gently melting and scraping it off. Before trying Paul's nitric acid recipe I think I'll give the lacquer-thinner method a go.
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