The first of your suggestions will not work with the material Chris is
using. Any polymer that has conductive material mixed in, tends to form
a thin skin of pure resin at its surface. A conductive paint or epoxy
applied to the surface will not come in contact with the metal strands,
and all you will mainly get is capacitive coupling.
Further, applying a silver-loaded epoxy to the surface of most metals
(including stainless steel) will not penetrate that material's natural
oxide layer and hence (unlike what most people I've talked to seem to
think) will NOT produce a reliable electrical connection, even when
applied directly to the metal. A reliable electrical connection to any
metal requires that one must cut through the metal's oxide layer. Thus,
your second suggestion is more workable, although he is basically using
these already (the plastic screws he refers to are very similar to sheet
metal screws).
Bob Wilson
TIR Systems Ltd.
Vancouver.
-----Original Message-----
From: Jim Conrad [mailto:[email protected]]
Sent: June 2, 2002 4:34 AM
To: Chris Wells; EMC-PSTC Discussion Group
Subject: RE: Bonding to conductive plastic - best method??
Hi Chris,
I would try a Silver loaded conductive epoxy or use SS self tapping
screws.
Jim
-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Chris Wells
Sent: Friday, May 31, 2002 10:23 PM
To: EMC-PSTC Discussion Group
Subject: Bonding to conductive plastic - best method??
----- Original Message -----
From: Chris Wells <mailto:[email protected]>
To: EMC-PSTC Discussion Group <mailto:[email protected]>
Cc: [email protected]
Sent: Friday, May 31, 2002 8:07 AM
Subject: Bonding to conductive plastic - best method??
Recently we had a good string going on conductive platics.
I would appreciate some help coming up with the ideal electrical bond to
this type of plastic.
We use a 6% Stainless Steel Fiber mix in Poly carbonate and I was
looking for the best method to electrically bond to the plastic to drain
off ESD or for high frequency bonding with the metal portion of our
product. This would include Printed Circuit Board to plastic, Cable to
plastic and metal housing to plastic type connections.
For joining metal housing and PCB connections we have been using plastic
screws that mechanically are designed for plastic.
The threads are widely spaced, tall and thin - They cut into the plastic
with out breaking the mounting bosses (studs).
The engagement with the plastic (screw thread in plastic boss hole) is ~
100% leaving little or no air gap between the screw and the plastic.
First set of Question -
If using screws what would be the best type of screw to insure good
contact to the stainless steel fibers?
* High profile, thin thread, plastic screw like I described?
* Self tapping type screw?
What should the platting be?
* Clear Zinc
* Zinc Chromate?
* Nickle on brass?
* Stainless Steel?
Is there any sort of liquid/paste like adative that could be added to
the screw hole to enhance the electrical connection?
We have experimented with metal inserts (PEM nut brand for example).
What sort of bonding does one get when molded in? I suspect poor
relative to press in.
Can the inserts be plain brass or should they be plated like with
nickle?
Depth of inserts - Longer inserts should be better but I am concerned
about cracking of the boss.
Screen matterial in the screw boss.
I have seen screen matterial embedded in conductive plastic.
Perhaps it could help screws bond to the plastic as well.
Any comments? Sounds messy.
Coatings on top of conductive plastic?
If a coating were placed on the inner surface of the conductive plastic
it might make a good hybrid sollution but costly.
Are there other considerations?
If you respond today try CC to
[email protected]
So I get it at work too (the ITE guys can't handle this subscription
list at work)
This group is the best resource I have for this sort of topic - thanks
in advance!!
Chris Wells
Senior Design Engineer
Cutler-Hammer
Pittsburgh, Pa. USA
[email protected]
