7 Oct
2012
7 Oct
'12
12:10 a.m.
On Sun, 7 Oct 2012, Mouse wrote:
Not /just/ mostly between the pins though. A common ESD event likely to do
damage to electronic components is one where the potential of the material
/inside/ the chip itself is at a different potential to other objects in
the environment outside of the chip. It might or might not help to have
multiple pins at the same potential, and the foil may just give the
discharge a larger pathway.
The video that was linked to in the last discussion illustrates this type
of discharge event perfectly. http://www.youtube.com/watch?v=imdtXcnywb8
This is also why conductive foam isn't (or rather shouldn't be) used by
itself and the foam and parts should really still be placed inside a
static _shielding_ (metalized mylar) ESD bag.
The resistance of conductive foam (typically 500k ohm to several megohms
or more) also not only helps prevent damage from a discharge to the foam,
but also helps prevent a charge from ever building up because the
conductive properties allow a potential to /slowly/ self discharge through
the larger surface of the conductive foam. Styrofoam or the pink LDPE foam
are not conductive at all so offer no way to slowly bleed off any
potential charge. Aluminum foil on the other hand, will allow a potential
to rapidly self-discharge, aka an ESD discharge event, which is what we
want to avoid.
Those old static dissipative carbon black bags on the other hand tended to
be very conductive (as low as a few kilohms or even less) which is why the
black bags are not commonly used (or manufactured) today and have largely
been replaced in industry by metalized mylar static /shielding/ bags.
The old myth that storing components or pc boards in foil will protect
them has been disproven again and again yet it still comes up from time to
time.
Aluminum foil
is a great conductor of static electricity [...]
That's why it helps. The danger is not high voltage relative to some
arbitrary ground; the danger is high voltage between elements of the
chip, mostly meaning between pins. That means that a low-impedance path
joining all pins will reduce the risk of static damage.
Not eliminate it, both because the path is not perfect - it's got
nonzero impedance at best and thus can develop voltage between pins -
and because it's possible, especially after longer-term storage, for the
aluminum foil to develop holes large enough to break contact with the
pins. But then, those are true of conductive foam, too; indeed, because
its resistance is much higher, it is less protective against a static
jolt delivered directly to a chip pin. (What it _is_ much more
protective against is a jolt delivered to the foam.)