potting contacts - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: December 08, 2018, 06:37:23 pm
content: I am looking at a few things that do not need to be waterproof but it would be a nice option. But my design relies on boards that are plugged together. I think this is a bad idea for potting, am I right? is it the sort of thing that a soft potting will work with?

author: Gyro
author_rank: Super Contributor
timestamp: December 08, 2018, 08:54:22 pm
content: I agree, itfeelslike a bad idea. I don't know for sure though.With soft compound there might be a chance of compound working its way under the contacts if they shuffle due to vibration. With more rigid compound, it might 'set' the contact spring shapes. There's also the possibility that anything (moisture, uncured constituents?) released from the compound might contaminate the contact surfaces without anywhere to escape to.

author: coppercone2
author_rank: Super Contributor
timestamp: December 09, 2018, 06:35:05 pm
content: you could extend some wires from the modules rather then direct fit

author: Simon
author_rank: Unknown
timestamp: December 09, 2018, 06:57:18 pm
content: Quote from: Gyro on December 08, 2018, 08:54:22 pmI was thinking soft compound would allow spring contacts to keep sringy but hard could be a problem with varying temperatures.

author: Simon
author_rank: Unknown
timestamp: December 09, 2018, 07:00:33 pm
content: Quote from: coppercone2 on December 09, 2018, 06:35:05 pmI could have one board slotted and insert the other card into it and solder but with a double sided card that now puts a hole right though the board I can't run traces under the joint, I'd have to take the tracks around the side. Which is a possibility.

author: coppercone2
author_rank: Super Contributor
timestamp: December 09, 2018, 07:22:43 pm
content: you are changing the tension and probobly the oxide growth rate etc determined by the manufacturers at some tension level. and the resistance/power handling/dissipation.i think it will be more stiff since the spring has less effective room to travel/free space is now gummy.

author: Simon
author_rank: Unknown
timestamp: December 09, 2018, 07:46:37 pm
content: Yes but the spring will already be in the right place and the potting would sit behind the spring pushing the contact but yes, quite an unknown. I guess the safest method for a potted environment is to solder the boards together.

author: David Hess
author_rank: Super Contributor
timestamp: December 09, 2018, 07:58:58 pm
content: Potting is definitely a problem with mechanical connections. If there is no alternative, sometimes the connections are sealed with tape or something like RTV silicon before the potting compound is applied.

author: Simon
author_rank: Unknown
timestamp: December 09, 2018, 08:06:19 pm
content: yes that is a good idea

author: coppercone2
author_rank: Super Contributor
timestamp: December 09, 2018, 09:01:26 pm
content: basic model in my head:if the spring is tensioned before the compound is applied then it will get into the spring and contract around it and reduce the spring tension, if the compound shrinks at all. Over time it may break itself free and restore higher tension as naturally expected.If its not tensioned before it it potted and it is tensioned after potting the spring tension will be increased because the spring will want to be where it is rather where it is not. **sounds good but cold flow can occur over time and decrease the tension, I am not sure if the dynamics of this system ensure 'tracking' of the correct torque from creep/cold flow. I think it depends on the onset of deformation.i would like to see a small load cell be clamped in the assembly then potting compound applied to see what happens. i think it would be very illuminating of this tough mechanics problem.I think using a gas tight ferrule on the wire would be a good idea too if its not already some how crimped or solid. so compound can't wick in between the strands and mess with that.

author: coppercone2
author_rank: Super Contributor
timestamp: December 09, 2018, 09:23:11 pm
content: maybe kelvin wire resistance measurement to the junction then categorize itI assume though you are not using a phoenix connector that you tighten down with a screw right?I have no idea of what kind of wizardry could be used to determine the mechanical contact force there. All I can think of is making some kind of microwave test, maybe the contact pressure differences can be compared by reflection? and you can deform them to known sizes using shim stock or something to get a comparison of how much its effected. It does not sound like its worth the engineering effort to measure it . I think a ultrasonic probe would be effected to much by the potting to give a useful measurement via conductance comparison.for a microwave test I would mate it, heat it with high current then TDR it with something fast and then try to look for rectification behavior/harmonics.You might also want to look at incomplete mates (put some crap to clog the connector progressively) to get ideas of what low tension looks like.I wonder if there is a standard method for this.Another idea might just be to run it hot and overloaded in a corrosive hot high oxygen/moisture environment cycle it a bunch and look at resistance change and compare it to normal failure of the same connector. Like random duty cycle square waves with long time constants to something high power and if its a bus connector vary thermal differentials (i.e. high current all connectors, high current some connectors but not others, dance it around). You can do it with a bunch of relays and some old oscillators. Obviously don't moisture seal it, but the effect of hot corrosive gas and temperature cycling should give you an idea of how the self cleaning and contact sealing is effected.

author: Simon
author_rank: Unknown
timestamp: December 09, 2018, 09:32:00 pm
content: I was thinking a PCIe connector as they are cheap and the mating half is free. I don't think I'd pot one. I'd have to come up with something else. probably redesign the boards with smaller contacts of the same type both sides and via's that way the connection goes straight through.

author: coppercone2
author_rank: Super Contributor
timestamp: December 09, 2018, 09:32:53 pm
content: that seems to be a good connector based on how much abuse my graphics card has taken over the decade

author: Simon
author_rank: Unknown
timestamp: December 09, 2018, 09:40:19 pm
content: yea but no one pots them

author: Zero999
author_rank: Super Contributor
timestamp: December 10, 2018, 10:48:44 am
content: Quote from: Simon on December 09, 2018, 09:32:00 pmHow about cutting a slot in one board and soldering the edge of the other board to it?

author: Simon
author_rank: Unknown
timestamp: December 10, 2018, 11:21:45 am
content: Quote from: Hero999 on December 10, 2018, 10:48:44 amYes I think that that will be best for a potted version