Re: [SI-LIST] : How board over-shoot under-shoot influence a

Chuck Hill (chuckh@altaeng.com)
Sun, 21 Mar 1999 23:36:33 -0700

Yehuda,

There are six distinct effects I can think of:
1. Excessive can cause latchup in CMOS (although not likely since latchup
is usually controlled).
2. When there is no clamp diode to the supply rail, high E fields can
create "hot carriers" in MOS devices which slowly degrade the device by
permanently shifting the thresholds.
3. When there is a clamp diode to the substrate, if enough current is
injected, the device becomes non-functional in the region around the
substrate junction nearest the pad since it is forward biased. This occurs
for a period of time, and then the device regains functionality.
4. High E fields on MOS gates can cause migration of impurities in poorly
manufactured devices which leads to permanent threshold changes (shouldn't
happen in today's devices).
5. High ringing means lots of reflections that persist over time. These
reflections can affect subsequent switching and the exact timing of the edges.
6. Of course if the MOS gate, or PN junction is avalanched, then
destruction can occur from excessive local heating.

The high voltage of the spike occurs at very low current since the inputs
are mostly capacitive.

Charles Hill, consultant
Alta Engineering
chuckh@altaeng.com

At 12:00 PM 3/21/99 +0200, Yehuda D. Yizraeli wrote:
>Hello SI experts,
>
>
> Its good design practice to have a non reflective board design, however i
>would like to understand the nature of the failure caused by the reflections.
>
> One failure mechanism is double triggering, but assuming the reflections
acuse
>an overvoltage (over the power supply value, or under ground value) BUT
not ringing through
>thetriggering point, how can a chip be affected by this ringing....? Does
it make the input
>buffer levels lower (hey hey hey, its spikes, short term influence,
doesn't it???).
>
>
>
> Any help, article and/or any pointer is appreciated.!!
>
>
>
>
>
>
> regards, yehuda
>
>
>
>
>
>
>--
>-------------------------------------------------------------------
> Yehuda D. Yizraeli
>
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