Chris,
A lot depends on the type of stainless steel used as the filler. Long
strand stainless filler tends to have less impact on the Izod impact
spec (gotta stop with those puns!) than short fiber stainless. But you
are correct that although any reinforcing filler in any plastic will
improve stiffness, it will also affect its impact resistance. The 35% PC
I have samples of is not all that bad, impact wise. I guess a lot
depends on what your impact requirements are.
Regarding Selective Electroless Plating. The whole point of SELECTIVE
plating is that you plate what you want, don't plate what you do not
want. The process works this way: Using normal paint masking techniques
(such as electroformed masks), a catalytic primer is applied to the
plastic in the areas where you want the plating. Then a layer of
electroless copper is deposited (it deposits only where the catalytic
primer was applied). Finally the copper is topped off with a thin, dense
barrier layer of electroless nickel to act as corrosion protection for
the thicker copper layer. Thus, can achieve the single point shielding
bond contact you need. This process is done by a quite a number of
American suppliers.
Conductive paints (such as silver or non-corroding copper) can be
selectively applied directly using electroformed masks as above. Nickel
and graphite based paints can also be used, but these are pretty low
performance (but may work for you if all you want is a bit of ESD
shielding).
You are right that using zinc plated screws in this situation would be a
definite no-no. It would form a nice corrosion couple with the stainless
fibers.
Using ultrasonic inserts is the standard way to insert threaded inserts.
As you mention, this process naturally relieves stress in the plastic
since the inserts are basically melted in place. This is why everyone
uses the process for "normal" plastic inserts. But it is NOT a good
choice for making electrical contact with your stainless fibers because
under local melting (caused by the ultrasonic insertion machine), the
fibers will simply "mush" out of the way of the insert, and there will
be no high pressure gas-tight contact between fibers and insert.
Brass is certainly not particularly compatible with stainless
(galvanically) as one can see from the electromotive series. The result
will be a questionable long-term contact. But I see no reason why
stainless inserts should be extraordinarily more expensive. After all
the actual cost of the metal in an insert is 2/10ths of nothing, and
stainless is not all that much more expensive than brass anyway. The
only real reason for increased cost is that stainless is a little harder
to machine, but once a screw machine is set up to crank these things
out, this doesn't count for too much. But I think that in any event, the
use of ultrasonic inserts is not going to be as reliable as a thread
cutting screw that will make a pressure contact to the stainless fibers.
Note that this applies to thread cutting screws, NOT thread forming
ones. Thread cutters do not significantly stress the plastic; thread
formers do.
One of the reasons why you are having fibers in your fingers, is that it
seems you are using the more common short stainless fiber filler. Long
fiber stainless filler is not only better as a shield, it avoids the
shedding of fibers almost completely.
Bob Wilson
TIR Systems Ltd.
Vancouver.
-----Original Message-----
From: Chris Wells [mailto:[email protected]]
Sent: June 3, 2002 4:20 PM
To: Robert Wilson; EMC-PSTC Discussion Group
Subject: Re: Bonding to conductive plastic - best method??
Robert - thanks for the response
The 6% SS is for ESD control not for clasic shielding.
But even with this low amount I have common mode capacitive coupling
problems between it and my PCBs.
If I bond the PCBs to the housing the problem is significantly reduced.
And so my quest into bonding the PCB to the plastic.
Even with this low amount it has made the PC plastic brittle.
I would think that the mechanical properties of 35% would be nasty.
What is the base plastic and are they SS fibers?
As to plating there are two platings here -
1) I think you are talking about plating the plastic - yeah that sounds
like a mess on many levels but I was thinking about the plating touching
the PCB mounts and not the screw to plated plastic barrier.
2) My main question was the plating of the screw - We presently use
clear zinc plated steel screws and I suspect that that is not good. I
am looking at stainless steel screws.
Inserts
I have heard that ultrasonicly welding or heat pressing will give a good
electrical connection and releave the mechanical stress relative to a
press in.
Others have said that the brass and the SS fibers are not compatible and
SS inserts expensive.
The housing is a faceplate for a membrane keypad that is panel mounted.
Only the edge frame is exposed to the user.
We have a rough surface on it and that controls the looks pretty well -
so no paint.
One down side of the bare plastic is that the fibers come off in your
skin if you rub it sort of like fiberglass.
----- Original Message -----
From: Robert Wilson <mailto:[email protected]>
To: Chris Wells <mailto:[email protected]> ; EMC-PSTC
Discussion Group <mailto:[email protected]>
Sent: Monday, June 03, 2002 12:32 PM
Subject: RE: Bonding to conductive plastic - best method??
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:[email protected]]
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:[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?
1. High profile, thin thread, plastic screw like I
described?
2. Self tapping type screw?
What should the platting be?
1. Clear Zinc
2. Zinc Chromate?
3. Nickle on brass?
4. 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]
