[time-nuts] A look inside the DS3231

[Pete Stephenson]

Hi all,

A few days ago I reported the results from letting a DS3231 RTC run for
a year, and how the chip kept time well within the published specs.

Since I had acquired several DS3231s from dubious sources (Asian vendors
on a major auction site) as part of an RTC module that fits on the
Raspberry Pi's header pins, I was doubtful of the authenticity of the
chips. I decided to sacrifice one in the name of science and decapped it
at home using alternating heat (a lighter) and cold (a glass of cold
water) to embrittle the epoxy casing, then sanded down the back of the
chip on fine-grain sandpaper to expose what I hoped was the back of the
internals (so as not to damage the die itself).

Other than inadvertently sanding through half of the crystal's housing,
thus breaking one of the forks of the crystal, this was a success. (I
was prepared to decap one in acid had my attempt at physically removing
the epoxy package failed.) I slightly scratched the die itself while
separating it from the epoxy, but the die itself is clearly visible.
Based on a sample size of one and the markings on the die itself, it
appears the chip is authentic. The markings on the outside of the epoxy
package look a bit dubious and not like typical Maxim laser-markings, so
it's possible the chip was re-labeled at some point. I'll contact Maxim
to see if they can look up the lot information.

I used my 2 megapixel USB microscope to take some images throughout the
process that you might find interesting. The microscope has limited
resolution, particularly at high magnification, so some of the photos
may not be perfectly clear. I have access to a Zeiss petrographic
microscope at my work and will see if I can get some better images
tomorrow. I'll try to get high-quality images of the whole chip and
stitch them together into a larger composite.

Anyway, the photos are available at http://imgur.com/a/0zudj -- I will
add more photos from the petrographic microscope tomorrow. I focused
mainly on the markings on the die that indicated it was, in fact, a
Maxim chip but if there's any other region of the chip that you'd like
images of, please let me know and I'd be happy to take some more
pictures.

I hope you find this as interesting as I did.

Cheers!
-Pete

-- 
Pete Stephenson