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Keywords:ESD, static, Mac II


This may give you sweaty palms the next time you open your Mac II case.
One of our local Apple dealers says that, at the factory service training
sessions, he was told that the Mac II motherboard was *extremely*
sensitive to damage from electrostatic discharge (ESD).  Now many of us
already know that CMOS IC's can be damaged by ESD, and take routine
precautions when handling them.  The service training instructors
apparently are stressing this in a big way: they claim that ESD control is
especially important around the Mac II because it contains VLSI chips with
unprecedentedly high gate densities (for a PC).  To underline this
statement, they told the trainees that damage could be caused by waving a
charged piece of material (eg. plastic film) over the board, or (if you
wear the proper shirt material) by raising your arm.

Apparently, some ESD-induced damage is not immediately evident.  One
common damage mode involves an increase in gate leakage current - the
chips function properly until the leakage eventually deteriorates into a
catastrophic failure.  The service instructors said that in in a Mac II,
such damage is undetectable at first, but may show up four months later in
erratic program behavior and data loss, possibly culminating in machine
crashes and a bill for a new motherboard.  I had heard about such
incipient ESD damage, but never took it very seriously until now.

What to make of this?  Well, now that we finally have open Macs there are
going to be a lot of people out there gleefully popping in boards and hard
disks, and most of these folks are not well trained in ESD precautions.
If the boards are as hyper-fragile as the above story suggests, then
there are going to be a lot of IIs coming down with degenerative chip
disease, and a lot of unhappy owners.  Apple must know that, so perhaps
the service instructors were being a bit over-dramatic.   

But it makes sense to play it safe.  Most readers of this group are likely
to be familiar with ESD precautions, but here are some tips for those few
who may not be.  With the cover in place and the machine properly
grounded, it is reasonably well protected.  You might consider a grounding
floor mat, etc. if there are a lot of static snaps from office carpeting
and chairs.  If and when you go inside the case, it is wise to have the
machine grounded (plugged into a grounded outlet), to put it on a
conducting grounded surface, and to keep a grounding strap on your body (I
run a wire with alligator clips from my metal wristwatch band to the
chassis).  Move around as little as possible while you are installing
boards, disk drives or accessories and handle them only as much as needed.
Finally, I seem to remember that polyester clothing is more ESD prone than
cotton.
 


-- 
Julian Vrieslander    (607) 255-3594
Neurobiology & Behavior, W250 Mudd Hall, Cornell University, Ithaca NY 14853    
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