Newsgroups: sci.electronics
Path: utzoo!henry
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: chips into powered sockets
Message-ID: <1990Aug14.224959.562@zoo.toronto.edu>
Organization: U of Toronto Zoology
References: <23655@dartvax.Dartmouth.EDU>
Date: Tue, 14 Aug 90 22:49:59 GMT

In article <23655@dartvax.Dartmouth.EDU> charles.hitchcock@dartmouth.edu (Charlie Hitchcock) writes:
>Now that I have an application where I want to insert
>and remove a chip into/from a powered socket, I am hard-
>pressed to explain what electrical phenomena would occur
>that could do damage.

The main problem is that many ICs respond badly to seeing voltages
on signal pins that are outside the range [power-pin voltage, ground-pin
voltage], and it's very hard to guarantee that those two pins will connect
first and disconnect last.  In particular, CMOS can be triggered into
"latchup", essentially a self-destruct mode inadvertently built into
the chip :-).  A consequence of the standard CMOS processes is parasitic
bipolar devices here and there, which form [gulp] an SCR connected
between power and ground.  Normally the gate of the SCR is reverse-biased,
so to speak, and the SCR never conducts.  Hit an input pin with a voltage
outside the power-ground range, and you may trigger it... at which point
the chip challenges your power supply to a duel, and usually loses. :-)
Equipment built for hot swapping typically uses oddball connectors
to ensure that power and ground always make first and break last.

There are also potential problems with static electricity and the like.

>What do commercial chip test systems do?

They normally have enough sophistication in their socket drivers that
they can power the entire socket down for insertion and removal, I think.
-- 
It is not possible to both understand  | Henry Spencer at U of Toronto Zoology
and appreciate Intel CPUs. -D.Wolfskill|  henry@zoo.toronto.edu   utzoo!henry