David, You mention that you have a Steel and Aluminium to content with. My opinion about mixing metal parts in an assembly (chassis) is a sure way of promoting corrosion, especially if the installation is in a humid environment. If you want the chassis to be a reliable electrostatic shield do not have panels that are made with different metal bolted to the 'frame' or chassis. Under some codes this is a 'No Go' area.
BTW, I assume that you must be working with a small box, otherwise you would need much more than a 'rubber eraser' to clean the mating surfaces. All equipment metalwork should be electrically bonded in a manner which does not rely on 'hopeful' electrical conduction through anti-corrosive treatment like anodised aluminium and paint. Careful attention to the assembly process will weed out things like ball-bearings races, nylon runners and coasters, or other insulating materials. Conduction through painted panels should not be dependent on the gripping action of star washers. The design should be such that no currents flows in any part of the metal work. The objective is to ensure that any part of the metalwork can be relied upon as an effective electrostatic screen and not the reverse, a radiator. Tim Foo |---------+-------------------------------------> | | "Gary McInturff" | | | <Gary.McInturff@worldwidep| | | ackets.com> | | | Sent by: | | | [email protected]| | | eee.org | | | | | | | | | 03/01/02 07:58 AM | | | Please respond to "Gary | | | McInturff" | | | | |---------+-------------------------------------> >--------------------------------------------------------------------------------------------------------------------------------| | | | To: "John Juhasz" <[email protected]>, "David Heald" <[email protected]>, <[email protected]>| | cc: (bcc: Wan Juang Foo/ece/staff/npnet) | | Subject: RE: Pencil erasers for pre-EMI cleaning? (cleaning mating surfaces, chassis, ) | >--------------------------------------------------------------------------------------------------------------------------------| "Gary McInturff" <[email protected]> 03/01/02 07:58 AM I agree if this was the actual EUT under test, but if it is a peripheral that gets used lots, or if its a PC chassis for example I would clean the surfaces at any point I got nervous about the performance. One of the reasons that it is difficult to find a PC that meets the emissions limits is that after shipping or opening and closing the fool things a number of times the mating surfaces get crunged up (technically speaking) and build up a certain amount of resistance. I can't afford new peripherals every try so I just get these surfaces back to "as built" condition. If you have to do this on the EUT it should only be done if you find it a problem because it caused a failure. Then as suggested many times prior you have to go back to the design guys and tell them they have to fix it before you can certify it. I haven't had had any problems with erasers and good electrical cleaners like you can find in the lab. I would stay well short of sandpaper or anything abrasive because it can go through whatever plating is being used for corrosion protection. The problem only gets worse after that. So will many cleanings with the eraser. Gary David Spencer <[email protected]> 03/01/02 08:11 AM Hi Dave, Those were the days. Computer not working, pull out all the cards and take an eraser to the gold fingers. Put it back together and Voila! everything works. In a perfect world, my opinion coincides with those previously expressed. We should be testing with pristine equipment, as a customer can expect to receive. This means items direct from the manufacturing process. Perhaps many of the esteemed members of this group are in positions that afford them these resources. My personal experience has been one of receiving the latest prototype that has been passed around through many hands and has little cosmetic resemblance to that shipping to customers. In these situations, I believe that it is of mandatory importance, if your equipment is a card slot type or has removable parts/panels, to try to return the item to as pristine a condition as you can prior to test. There is just as much probability of connection debris preventing a ground loop that would have increased emissions/susceptibility as there is for it reducing them. To the specific question of the eraser: I heard a couple of eons ago that pencil erasures leave a contaminating residue and they should not be used, especially on gold flash, leading me to seek alternatives to the Good 'ol Days mentioned above. I use contact cleaner made for noble metals on edge fingers and frequently use a 50:50 mixture of denatured alcohol and distilled water to clean gasketing materials to remove oil and dust deposited by handling. One other design type note: If you are using steel, think about zinc plating. It is well worth the money. Your finish should not be so fragile if expected to stand up to normal wear and tear. Have a Great Day! Dave "Scott Lacey" <[email protected]> 03/01/02 05:43 AM David, Forget the pencil eraser. This is the same deal as using one to clean battery contacts. Works great - for a short time. Look on the web for Caig Laboratories, Inc. they make a series of chemicals that are meant to deoxidize metal contacts. I've used their "DeoxIT D5" for years. I call it "gremlin spray" because it gets rid of pesky intermittent connections. Scott Lacey John Juhasz <[email protected]> 03/01/02 05:08 AM I'm sure there are products on the market just for this purpose. But I would be concerned about my design if pass/fail depended upon how clean the mating surfaces are. A component substitution or some other ECO to the product down the line could put the product over the limit. Refer to the thread on test margins. John Juhasz Fiber Options Bohemia, NY Cortland Richmond <[email protected]> 03/01/02 04:36 AM David Heald wrote: >> I'm preparing for an emissions test and I had started cleaning some of my chassis mating surfaces with a pen/pencil eraser then alcohol to ensure the surface to surface contact was good. << David, This is an ever present help in time of trouble. Don't do it. (grin) Cleaning mating surfaces will help pass a test. But it is a bad idea. The purpose of a test is to catch what fails. If there's a nonconductive film preventing contact -- and this is common! -- then you need to change the process that put it there. Or find another way to make contact in that location. You want your PRODUCTS to pass, not just your test samples. It is a great troubleshooting, tool, however. I favor a paper towel with alcohol; the silicates in the paper are rough enough, usually, and the alcohol will dissolve some of the grease. Cortland Stephen Phillips <[email protected]> 03/01/02 04:04 AM I'm not trying to be antagonistic, but unless you plan to perform this procedure on all production devices - neither should you do so to the test sample(s). My 2 cents worth. Regards, Stephen David Heald <[email protected]> 03/01/02 03:43 AM All, I'm preparing for an emissions test and I had started cleaning some of my chassis mating surfaces with a pen/pencil eraser then alcohol to ensure the surface to surface contact was good. A friend then told me that using an eraser would also remove the anti-corrosive coating that was on the metal (Thanks Paul!). So I would end up with a very short term benefit, then rust. What I am trying to determine is if maybe light rubbing with a pencil eraser might only remove surface contaminants and leave the metal and coatings intact. (the pencil eraser is much less abrasive than the pen side) So the real question is... Does anyone have direct good or bad experience with the aftereffects of using a pencil eraser to clean mating edges (card faceplates in a telco box for example)? I have both steel and aluminum surfaces to worry about so info for either type is welcome. (and don't worry the different metal types are not adjacent). Any feedback would be greatly appreciated as the system is really dirty right now. Thanks and Best Regards, Dave Heald ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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