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Circuit Board Mounting

DanH

Legacy Member
Mentor
A best practices question please...

I want to mount this circuit board in this box:



I can machine hard mounting points, or some other elaborate mounting solution. However, is it permissible to simply place four blobs of an "electronics safe" silicone RTV in the bottom of the box, then float the board on it?
 
What loading will the device be subject to?

My default inclination is to say yes, just use the silicone...
 
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RTV or hot melt, if it's an indoor thing. The fancy way would be to remove the existing mounting bosses, then drill and use nylon standoffs and screws. Depends on the environment in which it will live.
 
Stand off

RTV or hot melt, if it's an indoor thing. The fancy way would be to remove the existing mounting bosses, then drill and use nylon standoffs and screws. Depends on the environment in which it will live.

Agreed. Local hardware stores usually have Delrin or Nylon stand offs. Usually a star washer is installed on the top of the board before the nut.
 
Don't know about 'best practices', but how about 5min epoxy (or JB Weld) 4 studs to the box? Insures ability to service later.
 
A best practices question please...

I want to mount this circuit board in this box:



I can machine hard mounting points, or some other elaborate mounting solution. However, is it permissible to simply place four blobs of an "electronics safe" silicone RTV in the bottom of the box, then float the board on it?

Not sure on your time frame. If you draw an adapter in cad shoot me the STL file and I will print the adapter for you or if you want draw up the entire box with mount points like you want and I will print it.

No charge. I already owe you for your many contributions :D
 
Normal practice

If it was mine, I would "Dremel off" the existing molded in attach points and drill holes in the base to match the mounting holes in the circuit board.

Countersunk screws and the previously mentioned nylon spacers from the Ace Hardware aircraft aisle finish the job off.

It probably would take less time than reading VAF for the day...:)

PS, is it a flap/cowl flap position indicator?
 
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Silicone and other adhesives are used to mount and stabilize electronics all the time. Most cell phones today are stuck together with adhesives, not screwed together. One of the ways to make something as environmentally robust as possible is to just bury it in epoxy or silicone. Silicone is nice because it has a bit of give and vibration damping. The only real downsides to these options are maintenance.

Do whatever is fastest.
 
Thanks gents.

PS, is it a flap/cowl flap position indicator?

Voltage monitor, with a built in display and settable upper and lower limits in 0.1V increments. The blue relay can operate whatever, here a panel mounted LED under the #2 ignition switch. I'll set the low limit at 13V, so the LED will come on prior to start, just like an alternator light. The idea is to keep an eye on the diode-isolated #2 battery charging for the EI.

My 2010 vintage GRT Sport monitors its own power inputs, but doesn't have an available input to alarm an independent bus voltage. I suspect some EFIS units can do it. Seems like a useful feature as the world moves toward more EI and EFI usage.

Not too worried about maintenance. You're looking at a whopping $7.
 
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I would go with Gil's idea.

It looks like the mounting holes would interfere with the existing molded standoffs. Cut/grind those off with your dremel then use some 0.25" standoffs to mount the board. You'll probably have to use a nut on the top of the screw unless there is enough height in the box to use longer standoffs. If you can fit longer standoffs then I would use threaded standoffs and a screw through the back of the box and a screw through the circuit board mounting holes. Use locktite on the screws.
 
Seems like you need a gratuitous red, white, and blue wire attached to the box so, during a pre-flight with a new passenger, you can suddenly yell "Oh, no, the self destruct mechanism has activated! Quick, hand me those side-cutters... was it the red wire or the blue wire???? I've got to think quickly!!!!"
 
As it's cheap and surface mount (not easily repairable part) I would just rtv it in place. I would use five blobs one in the center of the board for board flex. Don't scrimp on the blobs. We use that method in production all the time, works.

Bob burns
Rv4 n82rb
 
It would be good practice to put some type of support under that big round thing and another under the large IC. The idea is to help prevent board flexing, which over time causes fatigue of the connections into the board.

Believe it or not, there's a whole book on the topic for engineers, "Vibration Analysis For Electronic Equipment," by Dave S. Steinberg. It's actually one of the better books about vibration analysis generally, and is oriented to the working engineer.

Dave
 
Why the box? The few "non-serviceable" circuit boards I have are mounted behind the panel, with nylon standoffs to structure, in a vertical orientation.
Knowing you from your posts, I am sure there is a good reason.
 
Why the box? The few "non-serviceable" circuit boards I have are mounted behind the panel, with nylon standoffs to structure, in a vertical orientation.
Knowing you from your posts, I am sure there is a good reason.

At two bucks for a box, it seemed like a sensible thing to do. Easy to mount a box in a convenient spot, and the board is protected.
 
At two bucks for a box, it seemed like a sensible thing to do. Easy to mount a box in a convenient spot, and the board is protected.

That's half a pint of beer!
Yes, I neglected to notice you have to get to this at least once to set it up. My boards are all dumb and tucked away only needing access if they fail.
 
'Acid free' silicone sealant/adhesive is typically marked 'sensor safe'.

Machining off the existing hard points might work fine, but typically they're there to add strength at the mount points.

If you mount to a clean surface with that Goop, or other products using similar formulations (Shoo Goo, etc) you'll almost certainly break either the board or the box before the adhesive releases. I'm aware of some fairly heavy components that have been mounted on the hot side of firewalls using similar products (no hardware) that have remained in place for decade(s).

Charlie
 
i hate to mention the favorite goop of airplane builders but proseal works great for that purpose also.

bob burns
RV-4 N82RB
 
I know it's already mounted now, but here's another thought for the next person:

Use a dremel to remove the existing standoffs, and rough up a flat space on the inside of the case in a pattern that matches the mounting holes on the PCB. Then mount 1/4" threaded standoffs to the board with matching screws that won't go all the way to the bottom of the standoff. Put epoxy on the bottom of the standoffs, and press the assembly into the case. Allow epoxy to harden.

Now you can remove the screws to remove/service/replace the board, and your standoffs will remain for the next install. Obviously, don't torque too hard on the screws when installing, even epoxy has limits.
 
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