(Philips) 74HCT08

I came across this 74 series logic chip in a 2006 satellite receiver box I am looking at. It’s 4 AND gates – thats it, as ic’s go they don’t get any simpler.  The HCT stands for High speed, CMOS, and TTL compatible input.

I have been playing with my depot chemistry, and trying ~95% Sulphuric acid (~250°C) instead of ~70% Nitric and it is has some differences to Nitric acid.  One is that it tends to be less aggressive at etching metal out of bond pads which is good but you are often left with attached bond wires you then need to pluck or remove somehow.  In this case it turned out to be a good thing. Here is the die after the first depot in Sulphuric acid

First off this die is really tiny just 600μm x 600μm!, trying to pick it up with tweezers is …fun?!  It is so small that I cannot even tell whether it is mounted the right way up on the slide until I look under the microscope. The depot looks pretty dirty and you still have all 14 bond wires attached. The bonding technology here is quite impressive, those bond wires are ~70μm diameter at the base, and minimum separation is only 40μm. The clue to how they do this is the strip of something metallic(?) left across the middle of the die. I did not realize what it was at this point. I couldn’t physically remove the bond wires (Too small) so I tried a clean-up in hot Nitric acid (Which will more aggressively attack exposed metal and typically etches out the Aluminum under the bond wires lifting them out.)

The bond wires have come off nicely, but if you look closely you can see the bond wires do not line up with the openings in the passivation (The blue squares). To be able to get all 14 bond wires on the die they have patterned an additional metal layer on top of the passivation to re-distribute the bond pads. I would not have known this had I just done the Nitric depot as the metal re-distribution layer and bond wires would have all etched away.

I was still not happy with this image, as the die looks pretty dirty, the surface looks mottled and grainy, perhaps it was attacked during the metal re-distribution layer depostion and etch.  So I thought I would have a go a de-processing it a bit and try removing the passivation with glass etch.  I have to say I am pretty pleased with the result.

Finally a clean looking die.  The bulk of the area of the die is actually the input clamp diodes that enable the CMOS to run TTL input level.  The 4 AND gates is the circuitry in the centre of the die.

I recently came across a project to depot, image and reverse engineer the whole 54/74 series of chips being done by Rob Baruch (Why? – well why not depoting imaging and analyzing old chips is fun).  As it happens Rob has already reversed engineered a variant of the 74/08 just last month.  His was a Bipolar variant 74H08 made by Nat Semi (Week 11 2004 date code)  looking at his die you can see it is totally different to this HCT version. Not a surprise really, TTL compatible CMOS vs Bipolar, but it shows the wide range of variations this simple logic family has.

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