The problem you are encountering is made somewhat more difficult by the
very low amount of metal in the polycarbonate resin. Reasonable
shielding generally requires higher than 20% metal in the mix (35% is
not uncommon).
But to answer your questions:
- The type of screw is relatively unimportant. Any screw that cuts into
the plastic matrix, and gets through the insulating resin "skin" that
forms on the surface as a very thin insulating layer, and makes a
"pressure" contact with the stainless matrix will do nicely.
- The material the screw is made from is also not particularly relevant
from an electrical contact standpoint. Anything will do as well as
anything else (at least initially). But in the long haul, a stainless
screw is preferred since there will be no galvanic mismatch to cause a
corrosion couple. Any plating is NOT a good idea since it will be cut
through when the screw is inserted, and your contacts to the stainless
fibers will be made mainly to the bared metal substrate of the screw.
- The larger the screw the better. There is very little stainless steel
in your mix, and you need to maximize tour contact area (or use more
stainless filler).
- Conductive liquids and pastes are useless in this application. They
cannot make significant contact with the metal fibers because there is a
microns-thick layer of pure polycarbonate resin on the surface of the
plastic (to say nothing about other things like mold release).
- Molded-in inserts are not reliable. You need to make a gas-tight
metal to metal connection, and for this you need mechanical pressure.
-You also must remember that the polycarbonate resin itself (like any
polymer) is NOT gas tight, and NOT hermetic. It allows gaseous diffusion
over time, and thus any internal connection between a screw and the
stainless fiber matrix, must be gas tight (i.e. a high pressure
connection) to ensure reliability. Interestingly, a significant mode of
conduction BETWEEN fibers is by electron tunneling through the ultra
thin layer of resin that separates them.
But as a general comment, I suggest you use a polycarbonate blend with a
more reasonable amount of stainless fiber. This will obviously improve
your shielding, but more importantly it will dramatically improve your
contact reliability. Better still, is to use long strand stainless as a
filler, which has far better shielding characteristics than finely
chopped stainless (if you are not already doing so).
Do you need to paint the plastic as a result of the stainless content?
Generally, the surface finish of metal fiber-loaded resin is rather
ugly, necessitating a final painting for aesthetics. If this is so in
your case, then you really ought to consider forgetting about using
stainless filler altogether and instead selectively electroless plating
the inside instead of painting the outside. This will result on FAR
better shielding, and a better looking product. Conductive paints such
as Spraylat's non-corroding copper, on the inside will also result in
one or two orders of magnitude better level of shielding than the low
level of stainless fiber content you are using.
Bob Wilson
TIR Systems Ltd.
Vancouver.
-----Original Message-----
From: Chris Wells [mailto:cdwe...@stargate.net]
Sent: May 31, 2002 7:23 PM
To: EMC-PSTC Discussion Group
Subject: Bonding to conductive plastic - best method??
----- Original Message -----
From: Chris Wells <mailto:cdwe...@adelphia.net>
To: EMC-PSTC Discussion Group
<mailto:emc-p...@majordomo.ieee.org>
Cc: christopherdwe...@eaton.com
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
christopherdwe...@eaton.com
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
christopherdwe...@eaton.com
