Hello and welcome back. In the last episode we repaired a whole bunch of pretty HP boards from
an HP 98035 clock interface module. We ended up with a whole bunch of deceased vintage components.
One of them was a one megahertz quartz crystal we also had a few dead IC chips from the early
4000 series, one of the first CMOS chip series. They were both part of a simple
clock circuit that you'd think should rarely fail and since we're curious - heck it's in
the channel name - we thought we'd take a quick peek inside this failed components and see if
we can learn something from it. We'll start with our biggie one megahertz crystal. In the circuit
our failed crystal behaved in a really bizarre way, giving a chaotic output. What in the world?
In case you don't know yet, there is an excellent video about quartz crystal oscillators and how
they work link in the doobly-doo. In the same video, we also explain and show the typical
expected response of a quartz crystal on the vector network analyzer, with a resonance,
an anti-resonance, and a big switch in phase from plus 90 to -90 degrees in between.
When I measured the misbehaving one megahertz crystal on the VNA,
I still got the somewhat familiar crystal curve. But the resonance was much weaker
and the frequency was off. Man, it's a quartz, although it's completely off! It's at 1.0015
megahertz so maybe it's dying or something. So off to the torture instruments it goes.
I chose to use the mill rather than the dremel so i could do a more carefully controlled decapping.
I used a small end mill and very light passes so as not to further damage the crystal.
And success! It looks like we were able to open it up cleanly. And I think I see what the problem is:
I think it's missing a little piece. This whole piece just broke off. There's a scallop on it.
Under the microscope, it's even more striking. Here's the big scallop, and you can clearly tell,
from the mark in the wall of the case, where it hit it. But if you look carefully, there
is another smaller nick in the crystal to the right, with another telltale hit mark on the case.
And finally it dawned on me that one of the contacts had failed entirely.
The crystal is supported by tiny springs that double as contact attachment points. Here is a
close-up of the good contact attachment point. Looks like it's held in place by silver epoxy.
Zooming back out, if you look at the extreme bottom right, you can see that it's not even
attached anymore. I'm even surprised that we could see a quartz resonance still showing up on my VNA.
The signal must have coupled capacitively to the crystal. And it's quite easy now to explain why
the frequency was higher by one kilohertz: the crystal just got lighter when it lost a chunk.
It must have been subjected to quite a jolt to fail in so many ways. But not to worry says Ken
Shirriff over on patreon: it will buff right out. Ha ha. Very funny master Ken. So now it's time
to give the loving treatment to our other failed component, an early CMOS IC from the 4000 series.
A CD4069UB inverter to be exact. Conveniently it was in an equally early ceramic package. These
are very easy to open: just give a good chisel blow near the seam and the top should go flying.
You can see the chip on its small golden carrier right in the middle.
The ceramic debris were easily cleared with a bit of compressed air
Under the microscope we go. Simple, but pretty chip. There was no sign of catastrophic damage
or even corrosion. To be honest, this was a bit expected, and I venture that this IC died the
common death of early CMOS ICs, victim of ionic migration or some other kind of contamination.
By the way, in case you didn't know, your modern Intel processor
also has a known finite lifetime too, and it will eventually succumb to electro-migration,
want it or not. I looked around for a little bit. but I really couldn't see anything suspicious.
So instead, since this is a fairly simple circuit, I called in again our famed Ken Shirriff,
so he would tell us what we are seeing here. Once again this is one of the simplest ICs
consisting of six inverters. The UB designation means that it is further simplified by having no
buffer stages after the inverter gate. This is in order to minimize propagation delays,
so this is in essence the simplest CMOS gate you can get: two MOS transistors, a p and an n,
forming a complementary pair. Hence the C in CMOS. The two input gates of the transistors
are tied together so either the p is on, or the n is on, but never the two at the same time.
Which is what gives the CMOS its low power characteristics: there is no current going
through the transistors except when they are switching, since one of the two is always off.
Although there are only 12 transistors on this die, one pair per inverter, it's still pretty
hard to figure out where they are. I sent the picture to Ken and it came back colored this way.
Do you see the transistors yet? Let me help a little. Here's the p-MOS of one pair,
and here's the corresponding n-MOS. The gate is the middle green trace.
Both gates are connected together to the input pad towards the top. Similarly,
the two drains are connected together on the yellow trace to the output pad at the upper left.
The sources are connected to their respective rail voltages:
plus for the p-MOS and ground for the n-MOS. And voila, we have made our inverter circuit. And
there is even more than you can see in this die picture. Let's have master Ken explain it to us.
So it's a very simple chip. These two transistor structures are repeated six
times because it's a hex inverter. You can see that the p-MOS transistors are much bigger than
the n-MOS transistors. They're 2.5 times the size because n-MOS transistors work much better: the
electrons in the n-MOS transistors have 2.7 times the mobility of holes in the p-MOS transistors.
The plus signs on the die don't show the positive voltage: they are fiducial markers
for the alignment of the layers. If you look very closely at the markers, you can see they could
have aligned the layers a bit better. Now each one of these inputs has two protection diodes
and a resistor. The resistor is the little part where it narrows. One protection diode connects to
the high voltage and other diode connects to the substrate. This provides protection against over
voltage and also against static discharge. The excessive positive and negative voltage will flow
through the diode, and hopefully the chip won't be damaged. You can see the part number over here:
14069. RCA introduced the 4000 series of CMOS chips in 1968. For some reason, Motorola uses
the 14000 numbers for the same series of chips. There you have it, from the master himself.
Of course, this was child's play for Ken. In his blog, he routinely reverse engineers chips
with tens of thousands of transistors, such as this Intel 8086, down to the transistor level.
If you're interested in this kind of deep dives, go to his blog at the address below.
All right, I think we have learned a lot about this simple chip,
except of course what we wanted to know - as we still have no confirmation
of what it died from. Oh well, I hope we still see you in the next episode. That's all folks!