How to tell if a CMOS chip is toast?

[Martin Czech]

Again appologies,  have to answer that too: ;->

No, in general it is very hard to find all possible damage.
For us diy enthusiasts only input leakage, excess supply current
and obvious malfunction are possible to find. Ouput
damage is hard to find, because of feedback. Of course,
a fully blown output stage will be observed...

Let me say that not only CMOS chips suffer from ESD, bipolar structures
suffer as well. Todays gate oxides are pretty much perfect, they can
stand enormous E-fields for some 100ns, which is enough to survive ESD. I
see photos of destroyed chips all day, 1 out of hundred may have a gate
oxide pin hole, and these devices are all CMOS.

This is because junction breakdown will always come earlier then gate
oxide breakdown today. I don't know if this is still true for 0.1um
or even smaller technologies, but this is not our field of interest 
here.

Some tests are very easy to perform, the DUT is measuring itself.
You need no expensive test equipment, just a few circuits with sockets.

You can check for supply current. Easy. Use a voltage follower circuit,
plus input to gnd. It is no good idea to check for supply current if
the inputs are floating!

You can check for correct behaviour with noninverting or inverting
circuits. You can check for difference gain. This is a bit more
tricky. Using a 10MHz sine wave generator you can even check bandwidth
and slew rate.

You can check for input bias currents.  Basically you use a noninvertig
or inverting integrator setup with a good styro cap. The caps have to
be discharged via push buttons and some 100 Ohms resistor, then you look
at the rate the output voltage ramps up or down.  Then you can calculate
the input bias current.

As long as the chip has standard pinout, a few cheap circuits can give
a lot of information.

The op amp cookbook by Mr. Jung shows all this.

(In your special case I'd guess that it is input leakage).

Note that modern devices are quite robust. Total failure is seldom the
case, degradation is your main trouble. A zapped input stage may show
strange offsets, and drift etc. etc. Very unpleasant, because you may find out
very late, after hours of search, especially if your are hunting some kind
of drift.

Anti ESD lab equipment is a good investment. About 75 Euro will do

Table mat, wrist strap, soldering iron with ground connection,
some conductive foam plates to put circuits on and to discharge circuits
that you're going to touch. That's all you need.

m.c.

:::Date: Fri, 5 May 2000 03:15:55 -0400 (EDT)
:::From: Paul Wilkinson <synthdiy at mail.com>
:::To: Synth synthdiy <synth-diy at node12b53.a2000.nl>
:::Subject: How to tell if a CMOS chip is toast?
:::Content-Transfer-Encoding: 7bit
:::X-Originating-IP: 63.193.120.159
:::
:::Is there an easy way to tell if a CMOS chip is toast?  Or do I have to
:::figure it out based on the chips behavior?
:::
:::- Paul
:::
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