Issues with metal contacts onto PCBs

[David Hess]

I just remembered that I have seen special hardware for this type of application.  It was like a lock-washer but thin and constructed like metal EMC gasket material out of a springy but hard copper or steel alloy.  The spring steel ones were plated with something that had a dull copper look but the others were probably beryllium copper or maybe bronze.

[T3sl4co1l]

^ Wave washers, unless you mean like the waffle style gasket stuff.

Tim

[David Hess]

^ Wave washers, unless you mean like the waffle style gasket stuff.

Tim

It was different from a wave washer and more like a really thin lock washer with the "locks" twisted at an angle so the edges would dig in above and below the washer but they were very springy and would easily twist flat under compression unlike a real lock washer.  I was not able to find an example online which does not bode well for availability but mentioned it because someone else may know where to find them.

[mikeselectricstuff]

By wave washer I assumed something like this

[David Hess]

The other ones I have seen for EMC applications look like little coiled wire donuts.

[T3sl4co1l]

^ Wave washers, unless you mean like the waffle style gasket stuff.

Tim

It was different from a wave washer and more like a really thin lock washer with the "locks" twisted at an angle so the edges would dig in above and below the washer but they were very springy and would easily twist flat under compression unlike a real lock washer.  I was not able to find an example online which does not bode well for availability but mentioned it because someone else may know where to find them.

Ohhh, like the EMI strip that's got the kinked over corners kind of springy, yeah I think I've seen those before, but damned if I can find any hits... hmm...

Did find a neat article about split washers though.  Turns out there's just as bad as I always intuitively figured: they loosen just as bad as a flat washer, and a joint clamped with no washers at all holds up better!  That they've been in use for over a century is kind of mind boggling, let alone that they're still available.
http://www.boltscience.com/pages/helicalspringwashers.htm

Tim

[nuno]

For "un-screwing" washers, seems that the best is Nord Lock.

[nctnico]

^ Wave washers, unless you mean like the waffle style gasket stuff.
Tim

It was different from a wave washer and more like a really thin lock washer with the "locks" twisted at an angle so the edges would dig in above and below the washer but they were very springy and would easily twist flat under compression unlike a real lock washer.  I was not able to find an example online which does not bode well for availability but mentioned it because someone else may know where to find them.

Search for 'crinkle washer' and you'll find lots of them.

[tonyarkles]

I totally agree with your apprehension

I recall a restaurant that I used to eat at in University and they had a large two-layer metal grid of round metal rods that all of the lighting was screwed to. One day I was staring up at them and realized that there was no power running to the lights at all and that they must be getting power from the grid itself; that's when I realized that it was a two-layer grid and... well then it all made sense. Point being, there is some precedent that this is a feasible approach.

Aesthetic concerns aside, would it be possible to decouple the physical mounting and the power flow? Use whatever mechanical means to mount the board to the rods, and then a separate wire that clamps onto the rod?

[I just remembered that I have some lights downstairs that have a similarish mounting system as what you initially described, I'll post a few pictures once I figure out how to get them off my phone and into the forum]

[David Hess]

^ Wave washers, unless you mean like the waffle style gasket stuff.

Tim

It was different from a wave washer and more like a really thin lock washer with the "locks" twisted at an angle so the edges would dig in above and below the washer but they were very springy and would easily twist flat under compression unlike a real lock washer.  I was not able to find an example online which does not bode well for availability but mentioned it because someone else may know where to find them.

Ohhh, like the EMI strip that's got the kinked over corners kind of springy, yeah I think I've seen those before, but damned if I can find any hits... hmm...

Exactly.

Search for 'crinkle washer' and you'll find lots of them.

I did but nothing I saw matched.

tonyarkles, I have seen old ceiling lamps constructed this way with live 120 VAC exposed on the power conveying structure.  I assume newer one use a step down transformer and low voltage lamps.

[T3sl4co1l]

^ Wave washers, unless you mean like the waffle style gasket stuff.
Tim

It was different from a wave washer and more like a really thin lock washer with the "locks" twisted at an angle so the edges would dig in above and below the washer but they were very springy and would easily twist flat under compression unlike a real lock washer.  I was not able to find an example online which does not bode well for availability but mentioned it because someone else may know where to find them.

Search for 'crinkle washer' and you'll find lots of them.

No, those look like twisted wave washers...

Like this stuff, but instead of on one edge of a C-shape, imagine the folded corner tabs bits arranged around a circular washer.
http://www.digikey.com/product-detail/en/0097055217/903-1231-ND/3505717

Tim

[SeanB]

Google serrated washer, possibly these?

https://www.google.com/search?q=serrated+washer&safe=off

[T3sl4co1l]

Nah, those aren't nearly flexible enough, or are just variations of traditional lock washers (a means to increase friction but not to produce spring force).

Tim

[SeanB]

Belville washers?  You can get them in beryllium copper, but they are both expensive and have a fatigue life.

[Precipice]

Support rods are 2mm dia brass (though this may change to copper+kevlar cable) so no scope for holes!

I missed this earlier...
I think you're doomed, unless you decouple mechanicals from electricals. Whatever you build will fret and fail, won't it?
How about soldering (or silver-soldering?) a beryllium-copper spring between the rod and the PCB? Build some slack, and some stable current path, into the system? It shouldn't be too ugly. Make it dismantlable by soldering the spring to the rod, and bolting the PCB side, if you need?
I just can't see a compression stack working if you're wanting to use a kevlar (or even something better? Spectra, or whatever's currently spiffiest?)

[richard.cs]

If you end up using star washers on a multilayer board make sure you don't route any tracks under the area they'll dig in to. Other than that I've successfully used star washers on pcbs,  mainly when I've wanted RF boards well-grounded through their mounting holes. They're not good if you want many mating cycles though.

[Tothwolf]

Rather than rely on brass to gold, why not use tin plated brass eyelets in the pc board instead? Keystone makes all sorts of different size eyelets so surely they'd have some just the right size for the brass rod. [Keystone also sells some rather expensive tooling for mass production with eyelets, but they also offer some inexpensive tooling for small production/prototype type use.] You could then also use an internal star lockwasher between the brass bushings and eyelets for an even better electrical contact without worrying about the washer digging into the pc board itself.

I would suggest soldering the eyelets after pressing/rolling their edges though. If you are already stuck with a gold plating on the contact areas, you'd probably want to strip it off with a quick solder/desolder before installing eyelets to avoid any chance of gold embrittlement of the solder. If you don't have to be RoHS compliant (does the brass rod they plan to use contain lead?) then you might even be able to use leaded solder for the eyelets.

[Tothwolf]

Another thought, but maybe Micro Jacks would be the solution? If you have to use 2mm brass rod, maybe something like the 1641A/1641B or 1687/1688 with the bottom ground off would work? If you could use a slightly larger brass rod, the 1640A/1640B might be even better.

[calzap]

I'm assuming the PCBs will be suspended from above because cable was mentioned as an alternative.  2 mm brass rods seem pretty thin to support 15 PCBs overhead (presumably over people).   Will it be subject wind (including ventilator drafts)?  Hope the mechanical folks have done their homework.  Wouldn't want a repeat of the Kansas City Hyatt Regency disaster (it's in Wikipedia).  There are considerable differences in tensile strength of brass formulations, but the electrical conductivity doesn't vary much.

Mike in California

[mikeselectricstuff]

I'm assuming the PCBs will be suspended from above because cable was mentioned as an alternative.  2 mm brass rods seem pretty thin to support 15 PCBs overhead (presumably over people).   Will it be subject wind (including ventilator drafts)?  Hope the mechanical folks have done their homework.  Wouldn't want a repeat of the Kansas City Hyatt Regency disaster (it's in Wikipedia).  There are considerable differences in tensile strength of brass formulations, but the electrical conductivity doesn't vary much.

Mike in California

Not a big weight - 5 rods holding eleven 25x850mm strips of 0.8mm PCB on 3mm aluminium.

[Dave Turner]

Mike

How about a plated mounting hole in the PCB, 3.6mm in diameter, with a 0.8mm concertina spring taking up the slack in horizontal from the 2mm diameter brass rod and the plated hole all within the depth of the PCB. As a spring this would be self-wiping thus maintaining good contact. You'd just need to work out the best way to prevent vertical displacement.

[langwadt]

I'm assuming the PCBs will be suspended from above because cable was mentioned as an alternative.  2 mm brass rods seem pretty thin to support 15 PCBs overhead (presumably over people).   Will it be subject wind (including ventilator drafts)?  Hope the mechanical folks have done their homework.  Wouldn't want a repeat of the Kansas City Hyatt Regency disaster (it's in Wikipedia).  There are considerable differences in tensile strength of brass formulations, but the electrical conductivity doesn't vary much.

Mike in California

Not a big weight - 5 rods holding eleven 25x850mm strips of 0.8mm PCB on 3mm aluminium.

smd wire lugs mounted like the one in the middle of this pic?

[nctnico]

Epic fail. Solder used in electronics isn't suitable for joints which have mechanical load on them.

[pickle9000]

I wonder (with the original concept) if a conductive grease would help?

I'm not really familiar with this kind of thing (clueless in reality) but this is a very interesting thread.

[calzap]

There are anti-oxidation "greases" that are designed for use when different metals are in electrical contact, usually aluminum (not aluminium, Dave!) to copper.  However, they are designed for 120 and higher VAC.  Don't know how well they would work with lower voltages or DC.

Mike in California