Re: component over-stress

Arthur Collard (artc@lobsang.sps.mot.com)
Wed, 13 Mar 96 10:45:01 CST

On Wed Mar 13 09:53:16 1996 George Stevens wrote:

[cut]

> I believe the comments pertaining to V at input > Vcc is a likely cause. We
> will next focus on these types of analysis during system
> power up/down and system "hot insertion" modes. Also, it sounds from the
> discussions that bus contention would result in a
> different failure mode seeing as the Failure Analysis identified a toasted
> input pin as well as it being a single driver load..
>
> Questions:
> -does latchup condition normally result in a "toasted" Vcc and Gnd structures
> on the same IC?

>From my experience latchup can be triggered at one of several entry
points (pins) but usually it is sustained until failure at a different
but common PWR/GND location on the same part. The overstress entry
point is one of the emitters of the SCR's PNP/NPN which allow the base
currents to flow. Once the currents flow the PWR and GND emitter
connections become the prefered path of current flow.

Can this effect
> be cumulative?

Possibly if a silicon junction is overstressed repeatedly. Alloying of
contacts is common under overstress conditions and especially for advanced
technologies. Usually the silicon below the contact is stressed first before
full alloying occurs SO THE ANSWER IS YES.

The failure mode in the field reports the boards
> malfunctioning and requiring manual reset
> (I don't believe they were powered down), which worked for several
> iterations, then the discovery of the toasted
> Srams.

You may want to repeat the failure by monitor the currents into Vdd and
the input pin while the input pin is stressed. You should be able to verify
latchup on a good SRAM part to see it's characteristic.

Also you may want to do an ESD test with the SRAM powered. This may help
uncover any ESD induced latchup problem. Usuall ESD is tested by applying
an ESD to one pin with another (usually a PWR or GND) pin grounded.

>
> -Are you people interested in hearing more of this saga as it unfolds??? or is
> this just nuisance?? I find your
> discussions extremely helpful........ Thanks again,
>
>

I wouldn't mind hearing about a major milestone achievment unless you have
questions and no where to turn.

Regards

AC

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