If the sensitive assembly is insulated from the static protective
work surface, then it's not at equal potential with the operator or
anything else that's grounded around it. This does in fact violate
the core principle of effective ESD control.
Any wooden supports that we use have metal strips or conductive
tape (like metal plater's tape) wrapped around them to ground them
to the static mat or oven rack.
A grounded operator cannot remove charge from an insulator, because
the charge stays on the surface of the insulator. If you pick up a
block
of wood with charge on it, the charge stays put.
I don't think that an ionizer is always necessary at a conformal coating
operation. I've take static field measurements, and can't find any
appreciable charge generation at our conformal coat spray, so we
don't use ionizers in our lines, or ionizing air blowers at conformal
coat.
As always, your mileage may vary.
-Joe
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of - bogert
Sent: Friday, July 20, 2007 9:51 AM
To: [log in to unmask]
Subject: [TN] ESD Question Regarding ANSI/ESD S20.20 and use of
cardboard or wood
July 20, 2007
We have an OEM that has an ESD control program in place per the subject
specification invoked via J-STD-001. However, my position is that he
does
not comply with the requirements since during his conformal coating hand
spray process, he places the PWAs ontop of cardboard then sprays them
with
coating (no ionizer machine is used in the coating area). Additionally,
sometimes ESD sensitive assemblies are placed on top of cardboard or
wooden
blocks during the assembly or inspection process. In all cases when
wood or
cardboard is used, the wood and cardboard rests ontop of a properly
grounded
ESD mat, and the operator is grounded via a properly connected wrist
strap.
I maintain that this practice violates the subject specification since
wood
and cardboard are insulating materials which are not to be used at the
workstations, and if one took a surface insulation resistance
measurement
across the wood or cardboard surface, one would obviously not meet the
value
specified for a properly grounded mat.
I would appreciate any comment on the above, and whether other folks use
wood or cardboard upon which ESD assemblies rest. How do other folks
support assemblies during conformal coating or assembly/inspection if
wood
or cardboard is not used?
If the operator is properly grounded via a wrist strap, would any static
charge that may exist on the cardboard or wood surface dissipate through
the
wrist strap without damage to the product? I suspect not otherwise
there
would be no need to require that the grounded mats be used.
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