Static damage was Re: OT Troubleshooting
cannings at earthlink.net
Tue Mar 15 20:33:33 CST 2005
The reliance on ESD protection diodes is way over-sold. From EDN Dec,2004;
"On a schematic, ESD-protection diodes resemble the diodes engineers
sometimes use to limit overshoot and undershoot on a high-speed bus, but
with one significant difference: the expected lifetime of the diode. An
overshoot-limiting diode is huge compared with its ESD-protection cousin,
and, because of its large size, the overshoot-limiting diode can absorb
pulse after pulse of high-powered current transients, lasting essentially
forever. ESD-protection diodes, on the other hand, are small, have limited
strength, and aim to handle only a few events in their lifetime. "
At Hughes we conducted (no pun intended) failure analysis on any electronic
parts we thought may have been compromised (not just CMOS). The parts were
de-capitated and scanned (as stated in the other thread). Equipment shot
into space cannot tolerate latent defects, as in those days we didn't have a
shuttle that could bring back the close orbit satellites. With the high
density sub-mil geometry's in IC's now ESD is a huge problem. A lot of dies
have "spark gaps" incorporated into the etch. Below is some typical and high
end readings for ESD voltages (source: Fairchild ESD Application Note:
Person on carpet: 12,000 V (highest reading 39,000 V)
Person on Vinyl : 4,000 V (highest reading 13,000 V)
Person on workbench: 500 V (highest reading 3,000 V)
16 Pin DIPs in plastic box 3,500 V (highest reading 12,000 V)
I think you get the idea. The diodes are normally tested at 400 Volts. After
multiple ESD hits the diodes fail open and go to their maker.
Best regards, Steven C.
> Jim Beacon wrote:
> > From: "Dwight K. Elvey" <dwight.elvey at amd.com>
> >>>From: "Tom Jennings" <tomj at wps.com>
> >>>CMOS isn't *THAT* sensitive; easy, benign, reasonable precautions
> >>>are more than adequate. Ben, the days of early MOS/CMOS
> >>>hypersensitivity to static are long gone. Even 1970's 4000 series
> >>>had input-protection diodes.
> >> The 4051,4052 and 4053 were very static sensitive. But it
> >>is try that most were relatively insensitive. Still, if you
> >>are hitting a part with a spark large enough to make
> >>a sound, you are most likely doing some damage, even if
> >>it doesn't show right away.
> I've changed an unholy amount of these in old analogue synthesizers.
> Ensoniq Mirages seem to be the worst for it.
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