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Mounting Multiple PCBs in Rough Environment

Discussion in 'PCB Layout, Design and Manufacture' started by CalgaryPT, Aug 1, 2019.

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  1. CalgaryPT

    CalgaryPT

    22
    4
    May 7, 2017
    Hi everyone.

    I have a PCB mounting question. I am building a RC lawn mower. I have all the electronics working and am doing the metal work for the frame, drivetrain, etc. There are a lot of electronics: RC receiver, one LAC board for an actuator, two RC relay boards, one Arduino board, one Sabertooth 60W Motor Controller board, and a few Grove / breakout boards to control some failsafes. In addition there are lots of screw terminal strips, fuse block, etc.

    My question is how do I mount all of these different PCB boards inside a case so they standoff the case? There will be a lot of vibration for sure, and I think the RC receiver needs to be outside the case as I suspect putting it inside a metal case will affect the signal.

    But all these little PCBs need to be screwed down to something so I can affix it to the protective case. My electronics knowledge started in the 1970s and I was thinking about those old perf boards (non-solderable)—but I don't even know if these are available anymore.

    Maybe there is something more modern I don't know about? Any suggestions?
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,893
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    Nov 17, 2011
    You can use standoffs (Google) to mount PCBs in a stacked manner into a case.
    Then mount the case to the frame of the mower preferably using an elastic and shock damping construction. Have a look at an RC model shop, such constructions are regularly used to mount the sensitive receiver electronics to a model.
    Your construction will be exposed to harsh environmental conditions (cold/heat, damp/dry). Your case should be sealed against direct intake of fluids (mainly water), but you'll need to have avent to allow humidity to be removed from the case. Otherwise you will experience condensation which can create short circuits on your pcbs and in the long term corrosion. One way is to use GoreTex vents or similar material. Simply havin holes in the case id also an option if you can ensure that these holes will not allow water to enter the case (e.g. by adding external shielding).
    Another option is to cover the pcbs in conformal coating (a few layers of laquer may suffice for hobby applications) or to encase the whole construction in cast resin (epoxy). Both methods make later repair or changes to the circuits difficult, however.
     
  3. dave9

    dave9

    882
    217
    Mar 5, 2017
    Perfboard is a nightmare for vibration resistance. If you go that route, pot the entire circuit (potting compound is like an epoxy filler to great a big impervious brick). Really, any route you go, you will have a more rugged system with it potted, but I would use only solid capacitors, no electrolytics (replace any present), but of course potted, it is then non-repairable, but will also be immune to moisture, chemicals, etc. Heh, I should've read Harald's reply as he already mentioned this!

    You could experiment with attaching an external antenna lead to the receiver that is routed outside the epoxy or metal case.

    Yes perfboard still exists and isn't hard to find, but is typically still a cellulose construction that doesn't hold up well to moisture. I would sooner get a weatherproof piece of *material* (anything heat resistant so soldering doesn't melt or damage it) and drill holes where you need them, unless you pot the whole thing then moisture can't get to it.

    If you were using regular PCBs instead of perfboard, and cementing down anything tall or heavy like capacitors, there are chassis mounts out there which have vibration dampening built into them. How effective they would be on a lawnmower, I don't know, but you could use them to mount an electronics box then the PCBs inside could be further cushioned by high density foam and connected together with very high strand count (highly flexible) wire, and the same (typically) silicone insulated wire will also be more durable to use for connections external to the box.

    Here are a few random examples on Amazon, the longer you search the more you will find, maybe look next on electronics supplier sites like digikey.ca:

    https://www.amazon.ca/s?k=vibration+damping+mounts&ref=nb_sb_noss
     
  4. CalgaryPT

    CalgaryPT

    22
    4
    May 7, 2017
    Good advice from both of you. I was looking at traditional standoffs but wondering if there was something new and improved that replaced them. I have used the plastic self adhesive ones as well as screw based ones, but a few of the boards are so small, it's hard to find plastic self adhesive ones. I'm sure I will though.

    I am newly familiar with conformal coating and will definitely use this. In addition I think either hot glue or blue threadlock on the standoff screws is in order. Swapping out electrolyics isn't a great option as these are on pre-made boards I don't want to mess with. Commercial robot mowers do have lots of these on their motherboards, so I'm guessing it isn't the biggest of my challenges.

    I hadn't considered moisture resistance as a big issue considering when lawns are mowed; however storage in off-season (I'm in Calgary) will be an issue. I never used perf board in an outdoor application so never considered it as a material. But yes, cellulose would be a problem...so thanks for that. I had considered venting the case—one of my boards (Sabertooth 2x60A Motor Controller) already has a onboard cooling fan on the heatsink, so I will need some vents.

    I'm liking the vibration damping option and will do some research into this together with the high density foam idea—which is kinda where my head was at originally as this seems pretty cheap and straight forward. Also considering a steel mounting plate (maybe 22G) with standoffs. I do have a metal punch so making a custom plate might be best.

    All great ideas. Thanks for your suggestions.
     
  5. dave9

    dave9

    882
    217
    Mar 5, 2017
    I would not use adhesive standoffs nor hot glue, would passively cool the box and it would be warm to hot inside. Moisture is always a concern, why grass sticks to decks and decks rust out. Sometimes you may (I do in spring) even have to mow when the grass is still wet because it's the best gap between storms and it needs mowed.

    The standoffs themselves are necessarily tiny due to the area available on the boards to use them, so your dampening will probably need to be between the standoff and the enclosure, not the standoff itself, or a rigid enclosure mount and just dampen the enclosure.

    A fan is a big problem on a mower. I can't tell you how many times I've had grass flying around (it's a mower, lol) and dust, and leaf shreds, etc.

    I'd want to convert the heatsink to a plate that is 'sunk to a metal casing, not have it sucking air into the box. If it's not possible with that controller I would use a different controller. Since you have metalwork tools, use plate aluminum, maybe 4mm thick, bend it in a brake to a U-shape, one end on the controller where the heatsink was, the other end bolted to a metal chassis with heatsink grease on both ends. If the dimensions work out you might be able to buy U-channel thick aluminum instead of making it yourself, and lap it to a very flat finish on the mating areas.

    As far as using boards with big electrolytic caps sticking up, I would at least cement them to the board (hardware store construction adhesive would work and is cheap, or use sensor safe (non-acidic) silicone caulking) and make sure your mower blade is perfectly balanced to minimize the magnitude of vibrations, but you will still have an extreme amount.
     
    Last edited: Aug 3, 2019
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    9,893
    2,096
    Nov 17, 2011
    For vibration damping you may also want to look into vibration dampening harddisc mounts. Using a few sturdy rubber bands to hold your case may be all you need.
     
  7. CalgaryPT

    CalgaryPT

    22
    4
    May 7, 2017

    Thanks. I think I have enough suggestions now to solve the issue. A last trick I taught myself years ago was in vibration-prone environments to glue a rare earth magnet between the mounting plate and the project case. The idea here is to catch any nuts or bolts that do shake loose, thus preventing them from causing either mechanical problems or short circuits on PCBs / across traces.
     
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