Reverse-engineered BCD to 7 segment decoder/driver from die photo - Page 1
Forum section: electronics / projects

author: robertbaruch
author_rank: Regular Contributor
timestamp: January 02, 2017, 08:51:19 pm
content: I spent the holidays learning about VLSI and die photo reverse engineering, so I picked a relatively simple chip, the HCF4056, and reverse engineered it fromthe die photo at Zeptobars.At theGitHub repositoryare the final schematic in PDF form, the datasheet, and the KiCad files.Enjoy!

author: robertbaruch
author_rank: Regular Contributor
timestamp: January 04, 2017, 09:39:32 pm
content: I also wrote ablog postabout my experience. There are a few open questions at the end, so if anyone has any insight I'd appreciate hearing from them!

author: timb
author_rank: Super Contributor
timestamp: January 05, 2017, 12:57:19 am
content: Wow, very cool! Learning how to interpret die photos has been something I've wanted to do for quite awhile.I understand the basics (what the metallization, silicon, polysilicon, etc. layers look like), can spot basic components (transistors, resistors, caps) plus simple circuits (FETs as resistors, current sources) but it's hard to wrap my head around the die as a whole.Do you break it up into small chunks or just start in a corner and create the schematic as you trace it out?

author: richard.cs
author_rank: Super Contributor
timestamp: January 05, 2017, 02:59:59 pm
content: The only question I can help with is the one about purple around the bond wires. I suspect it's due to the use of gold bond wires on aluminium metalisation. Seehttps://en.wikipedia.org/wiki/Gold-aluminium_intermetallic, it can apparently cause significant problems, the thing to search for is "purple plague".I'm not sure why so much is visible on the pad beside the bond wire though. The blue on VCC may (guessing now) be due to a different bond wire material or different bonding process for those wires, one shows patches of purple. Maybe they just used thicker wire for those and that happens to have slightly different alloying materials?

author: robertbaruch
author_rank: Regular Contributor
timestamp: January 05, 2017, 05:26:02 pm
content: Quote from: timb on January 05, 2017, 12:57:19 amI start at an input or output and add whatever I find to the schematic as I trace it out. Mainly drawing it on paper first, rearranging, then putting it in KiCad. Of course, this is only the first die I've done, so maybe not the best way?

author: free_electron
author_rank: Super Contributor
timestamp: January 05, 2017, 05:30:59 pm
content: - input protection structure sits under the bondpad. you cant see it unless you strip the metal.- the non metallic connections are made by highly doped silicon. this chip is a single metal mask design. they use the doped silicon as a conductor.- doing this from photo is painful. you really need to see the device under microscope as you can then see a 3d image. this makes it much easier to trace everything out.- you can actually make stacked gate designs. if you need a AND gate simply put two mosfet gates next to each other. this cuts the 'well' twice. both gates need to be active for the transistor to turn on. we essentially make transistors that have more than one gate electrode.logic gates and flipflops are NOT made the way you think... a flipflop is simply two inverters that bite each others tail. the drive strength of one of them is very weak so it is easily 'overridden' from a different transistor.( the input). need that with an enable pin ? simply put a series mosfet.i'll draw a sketch .. hang on ...keep in mind that we ( semiconductor designers) can control the 'drive strength' of a mosfet. think of it as the channel resistance. we can make 'weak' channels that can be overdriven. so a flipflop is made as in picture 4. two not gates biting each others tail. one is very weak and can be overdriven from the input. only the output is strong.

author: amyk
author_rank: Super Contributor
timestamp: January 06, 2017, 03:35:46 am
content: Quote from: richard.cs on January 05, 2017, 02:59:59 pmIf I remember correctly the colours come from diffraction effects, due to the very slightly different thicknesses of the layers and the fact that dimensions here are roughly of the order of magnitude of the wavelength of light. In some dies, like the attached image, you can see quite clearly the contrast between N, P, and poly. I believe that one shows 3 NANDs and 2 NOTs.