[sdiy] Storing ICs (and static sensitivity) - how much to worry?

[Scott Gravenhorst]

"Needham, Alan" <Alan.Needham at centrica.com> wrote:
>
>I have a big problem with this explanation, even after inserting the
>supplied correction.
>
>An antistatic bag may measure a few hundred kOhms from corner to corner,
>I do not believe that this is enough to slow down any charge front, and
>if it did, it would result in the very thing you are trying to avoid -
>pins at significantly different voltages.
>As I understand it, the packaging attempts to provide some form of
>Faraday cage around the device; no matter what static voltage is
>applied, all the pins need to be maintained at the same potential, even
>if that is 1000's of volts above ground. Aluminium foil would be ideal
>IF you could guarantee a connection to every pin.

If I remember my physics correctly, the charge on the inside of a hollow metal sphere
is always zero regardless of the charge on the sphere itself.  This is because
electrons all push away from each other, so all of the free electrons would move to the
surface of the sphere to be farthest away from each other.  While a conductive bag is
not a metal sphere, it is still a conductive containter.  It's charge may be
significant on the outside, but inside it should be zero.  So I think that things
inside the bag would also be at zero potential having given up any charge they may have
had to the bag which then ends up on the bags exterior surface.  The charge on the
outside of the bag is easily removed by being grounded yourself and touching the bag.


>~~~Snip~~
>The materials used in antistatic bags and foam etc are conductive but
>have a relatively low conductivity. If you touch the antistatic
>material when you are statically charged the charge spreads through
>the material slowly, thus only producing a small local differential
>between pins on ICs.
>~~Snip~~
>It is the high voltages typically involved in static discharge, combined
>with a path of HIGH conductivity, that leads to high currents between
>the IC pins and damage to the circuit inside.
>~~Snip~~
>
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-- ScottG

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-- Scott Gravenhorst
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