Aluminum Faraday Enclosure Surface Treatment
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  1. #1
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    Default Aluminum Faraday Enclosure Surface Treatment

    I've designed an aluminum enclosure as well as some custom backshells for a computer that, after EMI testing, has passed DO-160 radiated and a conducted emissions requirements (category H for all who care). Now I'm onto making a few more and am juggling surface treatment options. My client would prefer an anodized exterior (as would I) and I'd like to preserve a conductive surface on the interior as well as some grounding locations/main mounting pads. Been in some talks with companies around me (Denver) and no one seems psyched about the masking required for a chromate/ano mix. High end/lowish volume product so not extremely worried about cost but time is in low supply.

    Looking for any tips/horror stories/anything you have regarding the subject. Thanks in advance!
    Last edited by BugRobotics; 01-13-2020 at 10:51 PM.

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    There are specific requirements for anti-corrosion coating the metallic aluminum surfaces that will accept EMI gaskets and/or fingerstock. Anodizing (a very good insulator) will defeat EMI protection/sealing, and cause a test fail.

    DO-160, Environmental Conditions and Test Procedures for Airborne Equipment is a standard for the environmental testing of avionics hardware. What is the actual application?

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    Quote Originally Posted by Joe Gwinn View Post
    There are specific requirements for anti-corrosion coating the metallic aluminum surfaces that will accept EMI gaskets and/or fingerstock. Anodizing (a very good insulator) will defeat EMI protection/sealing, and cause a test fail.

    DO-160, Environmental Conditions and Test Procedures for Airborne Equipment is a standard for the environmental testing of avionics hardware. What is the actual application?
    Hi Joe, thanks for the reply. This product will be installed in aircraft.

    Looking for any info regarding successful/unsuccessful attempts at (1) combining chromate and ano (and the specific masking techniques that have proven successful) and/or (2) other combinations that have proven successful over the long term (10+ years). The entire purpose of the enclosure was designed around EMI suppression to meet the stringent DO-160 requirements.

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    Don't mask /anodize them- as you said above, the labor to mask them can be a deal breaker. Look into just having them alodyned- it will preventcorrosion but at the same time the surface is still conductive. If you care about RoHS compliance, most gold alodyne is not compliant due to the chromate, but clear usually is compliant.

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    Quote Originally Posted by Dan from Oakland View Post
    Don't mask /anodize them- as you said above, the labor to mask them can be a deal breaker. Look into just having them alodyned- it will preventcorrosion but at the same time the surface is still conductive. If you care about RoHS compliance, most gold alodyne is not compliant due to the chromate, but clear usually is compliant.
    Thanks Dan. Yea, alodyne/chromate finish is what I'm after for all the internal surfaces and mounting locations and may be what I have to do across the whole box. Client was hoping for a black anodized exterior but it just may not be in the cards.

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    Bug, perhaps there's a contact adhesive sheet that will be able to cover areas that need to maintain conduction, while being chemically safe and effective in the baths? I'm thinking vinyl with silicone adhesive (something meant to be removed), or other plastic if vinyl's no good.

    So sheet over the outside, alodyne, remove sheet and place over alodyned surface and anodize. This is obviously easiest if dealing with flat planes, not sure what your enclosure has for protruding features. It also might work better going the other way, anodize first then alodyne.

    Alternatively, perhaps there's a metal foil product that can be adhered to the inside of the enclosure once a full anodize is done, with contact points to maintain panel-to-panel conduction. Not sure this could meet your requirements though...

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    Quote Originally Posted by Milland View Post
    Bug, perhaps there's a contact adhesive sheet that will be able to cover areas that need to maintain conduction, while being chemically safe and effective in the baths? I'm thinking vinyl with silicone adhesive (something meant to be removed), or other plastic if vinyl's no good.

    So sheet over the outside, alodyne, remove sheet and place over alodyned surface and anodize. This is obviously easiest if dealing with flat planes, not sure what your enclosure has for protruding features. It also might work better going the other way, anodize first then alodyne.

    Alternatively, perhaps there's a metal foil product that can be adhered to the inside of the enclosure once a full anodize is done, with contact points to maintain panel-to-panel conduction. Not sure this could meet your requirements though...
    Thanks Milland. I'm sure there is a great process out there, just can't get any suppliers around me to want to commit to any of them. When the parts are more than 50% masking they have an issue. Not sure why but it seems they may have an issue with the quality of mask (or the people applying the mask). The best option given to me thus far has been to machine the enclosure, leaving extra stock on all the alodined surfaces. The part would be anodized and then I would finish machine the areas designated for alodine and then send the parts back to get finished with the alodine. I don't have time and have already machined the parts so that won't work. The box has some heat sink features but other than it's a just a fancy box with some internal PCB mounting locations/heat sink pads/strain relief features.

    I'd be down to go somewhere else outside of my local area if anyone has any recommendations as long as the lead time could be short. I'm on a tight schedule getting this approved through the FAA so I'm looking for a sure bet.

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    Alodyne, spot mask, paint.

    That's how we used to do military test equipment. The masking is required for all exterior connectors that require a good bond.

    The stuff the auto body guys use for masking paint-on-paint graphics is called frisket film. It is a self adhesive vinyl with a lower strength adhesive. It can be cut with an Xacto knife, tracing over a paper pattern and then peeling loose from the backing sheet. Both are translucent enough to see black lines on white paper.

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    Quote Originally Posted by Scottl View Post
    Alodyne, spot mask, paint.

    That's how we used to do military test equipment. The masking is required for all exterior connectors that require a good bond.
    Thanks Scott. We are looking into paint options at the moment. Any recommendations? Currently looking at powder coat but would prefer a possible thinner coating.

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    Quote Originally Posted by BugRobotics View Post
    Thanks Scott. We are looking into paint options at the moment. Any recommendations? Currently looking at powder coat but would prefer a possible thinner coating.
    If you go with powder coat and use a film similar to what Scott mentioned, be sure the adhesive doesn't cure at the baking temp. Having the glue go from removable to rock is not fun.

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    There are craft cutters that cut films for stickers and posters.

    If a self stick film is available then the mask areas can be cut out and applied.

    Sent from my SAMSUNG-SM-G930A using Tapatalk

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    Quote Originally Posted by Milland View Post
    If you go with powder coat and use a film similar to what Scott mentioned, be sure the adhesive doesn't cure at the baking temp. Having the glue go from removable to rock is not fun.
    Thanks, yea that would suck. I use tape specifically for powder coating when doing any masking. McMaster PN 7630A23. Works awesome. Also use it around the shop as it's adhesive is strong and it's pretty resilient.

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    I work in the same field and in the past what I have seen for this process is as follows: Machine the part to size leaving approx .010" where you want the part treated with chromate conversion. Anodize the entire part. Machine .010 (or enough to clean) where you want the chromate conversion and then send it back out to be dipped for the chromate conversion. It ends up being a few more steps than what you likely planned for, but the conversion coat is where you want it and you don't have to deal with the back and forth of masking approvals.

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    Quote Originally Posted by pb1 View Post
    I work in the same field and in the past what I have seen for this process is as follows: Machine the part to size leaving approx .010" where you want the part treated with chromate conversion. Anodize the entire part. Machine .010 (or enough to clean) where you want the chromate conversion and then send it back out to be dipped for the chromate conversion. It ends up being a few more steps than what you likely planned for, but the conversion coat is where you want it and you don't have to deal with the back and forth of masking approvals.
    Thanks pb1, this is exactly what I'm being told by numerous shops. Looks like we will have to make a change to the machining process to accommodate these steps if black ano is what is needed in the end. I think for the moment we will clear chromate the entire box and then modify our coating process at a later date.

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    The thing to watch for is whether you're using sheet or plate to make the parts. If they're thick plate to start with, you can likely cut .010" off one side without issue. But if you're dealing with sheet of 1/4" or thinner you stand a good chance of the part warping after taking a cut on one side.

    Edit: BR knows his stuff, this is for less experienced folks who might read this thread.

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    Quote Originally Posted by BugRobotics View Post
    This product will be installed in aircraft.
    Yes, but what is the specific kind of application. It matters - some things are more critical, or requires special attention.

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    Quote Originally Posted by Joe Gwinn View Post
    Yes, but what is the specific kind of application. It matters - some things are more critical, or requires special attention.
    This is a computer that runs a heads up display.

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    Quote Originally Posted by BugRobotics View Post
    This is a computer that runs a heads up display.
    Depending on what is being displayed, that can be safety-critical for sure. Or merely mission-critical.

    This enclosure will get the full treatment and review.

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    Quote Originally Posted by Joe Gwinn View Post
    This enclosure will get the full treatment and review.
    It is indeed!
    Last edited by BugRobotics; 01-15-2020 at 01:38 PM.


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