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 To: Chris Wells ; EMC-PSTC Discussion Group 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 To: EMC-PSTC Discussion Group 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]
