Issues with metal contacts onto PCBs - Page 1
Forum section: electronics / projects

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 15, 2014, 05:40:50 pm
content: A customer of mine has proposed a mechanical arrangement whereby 15-odd PCBs are stacked on brass rods. Picture a set of window blinds.The rods will carry power & data.The proposal is for brass collars to friction lock onto surface pads on the PCBs, the PCBs will be mounted on aluminium strips with a slightly larger outline to the PCB. there are also some ntlon bushes involved but not sure of they are under any pressure,The proposal is to tighten up the whole stack with nuts a each end.I am worried about the long-term contact integrity (brass to gold plated PCB), as I'm aware of issues like PCB cold-flowing over time, as well as possible thermal cycling causing changes in contact pressure. I've suggested they put a spring in somewhere to maintin pressureI also have concerns that unplated brass against PCB pads may have long-term relibility issues.Power will be low voltage, about 8V, feeding a max of 1A to each PCB. I'm not sure 8V would be enough to get the oxide punch-though that high current contatcts rely on.Oxide on the data line is potentially more of an issue.I don't know enough about the metallurgy to say how likely it is that it will have problems over time, but just have a nagging feeling that it may do.Any experience/suggestions? Would nickel-plating the brass help?

author: Precipice
author_rank: Frequent Contributor
timestamp: November 15, 2014, 05:44:16 pm
content: Is this the one time when star washers are actually the answer?Turn your contact areas into points, and add a bit of spring to the stack?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 15, 2014, 07:11:52 pm
content: Hmm, you're probably right to worry about brass on gold. Nickel plate will solve that (diffusion barrier), but nickel is probably worse than brass for contacts.Brass is partly passivated by its zinc content (as stainless is passivated by its chrome content), and so is nickel. But nickel's oxide layer is somewhat tough and forms rectifying contacts, so it can be undesirable. I guess the zinc oxide layer is soft enough not to matter under most other applications.I suppose sending the brass through ENIG is inconvenient or expensive...I don't like star or split washers on PCB (they chew up the plating), but I'd be happy with cone/wave washers, one per PCB hole I suppose. Tighten the whole thing down and it should sit okay.I suppose there might be some advantage to using thinner rod, so it's stretchier. The difference between clamping a whole stack versus pinching each board between a pair of nuts is, the sprung length of thread is only a board thickness in the latter case, but includes the board-to-board distance in the former case. So you have some mechanical advantage, having more length of rod versus PCB stack, even though the PCB is that much more flexible.Tim

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 15, 2014, 07:15:33 pm
content: Quote from: T3sl4co1l on November 15, 2014, 07:11:52 pmBeryllium copper wave washers perhaps?

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 15, 2014, 07:20:17 pm
content: What is the motivation to do this?This scheme has serious metalurgy and mechanical issues.Either of those by themselves would be a show-stopper IMHO.And TOGETHER, they seem like a really terrible idea.There are at least half a dozen ways of doing this properly.The proposal seems like a horrible kludge.

author: David Hess
author_rank: Super Contributor
timestamp: November 15, 2014, 07:25:43 pm
content: I agree with T3sl4co1l about the star washers; they tend to chew up the printed circuit board although I have seen them used successfully this way.I have seen brass standoffs used this way in old designs and occasionally new ones for ground and power. The thing I would worry about is the temperature coefficient of expansions resulting in compression of the printed circuit board fiberglass. The old designs had brass rivets through the printed circuit board which would protect against crushing but compression washers should work also.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 15, 2014, 07:43:36 pm
content: Quote from: Richard Crowley on November 15, 2014, 07:20:17 pmArtQuoteMechanically it's generally fine for the application, problem is it's being designed by a mechanical guy who assumes that as long as two bits of metal are sort-of touching then it will be OKI have serious reservations about the electrical implications over time, I just can't tell them for certain that it won't work reliably, or offer detailed reasons.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 15, 2014, 07:52:17 pm
content: I would at least use bell washers between the standoffs and the boards to concentrate the contact pressure. I would think that would increase the long-term reliability significantly. A coil spring wouldn't provide nearly as much pressure. Or, at very least, precision grind the ends of the spacers to make them flat and perpendicular.Can you solder the spacers to the boards after mechanical assembly?

author: AndyC_772
author_rank: Super Contributor
timestamp: November 15, 2014, 08:00:09 pm
content: How long does this product need to last? Is "long term" reliability a problem, or does it need to survive an exhibition and that's it?

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 15, 2014, 08:07:37 pm
content: Indeed, as AndyC_772 observes, what is the expected life-cycle of this item?I would also consider using some kind of spring-contact soldered to the board adjacent to the spacer to exert continuous sideways spring-pressure.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 15, 2014, 08:14:14 pm
content: Quote from: AndyC_772 on November 15, 2014, 08:00:09 pmPermanent install, hung from a ceiling so access will be a problem.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 15, 2014, 08:15:42 pm
content: Quote from: Richard Crowley on November 15, 2014, 08:07:37 pmThat's exactly the sort of thing I suggested quite early on, but mechanical people have gone off on their own & come back for the PCB design, thinking it's just a case of providing flat pads on the PCB for their fixings to land on.

author: Precipice
author_rank: Frequent Contributor
timestamp: November 15, 2014, 08:18:39 pm
content: Can you decouple the mechanicals from the electricals? Pick power off the rods by some method other than the PCB landing pads? Maybe clamps or tapped holes in the rods?Or, if you're mostly worried about the data comms, then cheat and use RF, or IR between the boards, or something?I'd not normally propose either of those as 'easier than copper', or more reliable, but...And yeah, Belville washers do sound like a better plan than star washers. Will I ever find a use for them? They must exist for some purpose.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 15, 2014, 08:39:06 pm
content: Quote from: Precipice on November 15, 2014, 08:18:39 pmbeen through most of that already - need power anyway, so little point not using hardwired data. Th problem has always been the desire to combine mechanical & electrical connection, but they want an "invisible" join. Support rods are 2mm dia brass (though this may change to copper+kevlar cable) so no scope for holes!

author: Someone
author_rank: Super Contributor
timestamp: November 15, 2014, 08:45:36 pm
content: Are we missing the obvious of actually soldering these rods onto the pads? Or is there too much thermal cycling?

author: nctnico
author_rank: Super Contributor
timestamp: November 15, 2014, 09:07:43 pm
content: Quote from: mikeselectricstuff on November 15, 2014, 05:40:50 pmI had a similar problem where I had to bolt a busbar to a backplane. I used nickel plated brass standoffs and I used another nickel- plated washer + star washer to retain pressure. The reasoning to use nickel plated brass standoffs is because the gold plating on a PCB is copper nickel gold. Large capacitors are often bolted to a PCB so cold flowing shouldn't be an issue.In your situation I'd insist on having the stack being pressed together. Maybe even use threaded pieces (round standoffs) to build up the stack.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 15, 2014, 09:08:09 pm
content: If the support rods are only 2mm OD, then they amount to just a large "component lead".Can the boards be "threaded" on to a continuous rod and soldered into place?Or drill and tap these things and solder them onto the boards.Then use a set-screw to bite into the rods (or flex cables?)

author: Dave Turner
author_rank: Frequent Contributor
timestamp: November 15, 2014, 09:32:57 pm
content: Mike - if I understood correctly you mentioned suspended and inaccessible. At what distance from an observer? Without knowing full details I'd agree with precipice and Richard C have the way to go.Are you after the electronic version of this?http://www.amusingplanet.com/2012/08/magical-floating-faucet-fountains.html

author: lovro
author_rank: Contributor
timestamp: November 15, 2014, 09:49:35 pm
content: Saw this a few days ago in Electronicahttp://www.mac8japan.com/Maybe use some of their solderable pcb threads / spacers?

author: David Hess
author_rank: Super Contributor
timestamp: November 15, 2014, 10:24:05 pm
content: Quote from: Richard Crowley on November 15, 2014, 07:52:17 pmQuote from: Someone on November 15, 2014, 08:45:36 pmI have seen this done and it did not help at all. Standard solder does not work well for mechanical joints. With time either the pads got ripped off of the board if the solder held or the solder joint cracked. Similar problems can occur with designs which use brass rivets for through holes.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 15, 2014, 11:22:49 pm
content: Quote from: Someone on November 15, 2014, 08:45:36 pmbeen there, done that. Never again ( for a few reasons)

author: sparx
author_rank: Contributor
timestamp: November 15, 2014, 11:31:34 pm
content: Could you mill a hole into the edge of the pcb, and use something like a brass pin with a hole through it perpendicular to its body, to support the pcb on the rail? Therefore stopping the issue of pcb plating on rails?My description was pretty crap:Also, conductive lube?

author: kizzap
author_rank: Unknown
timestamp: November 16, 2014, 02:09:29 am
content: Can you use something like a spring nut, where you have a nut threaded onto the brass rod that is threaded through, and said nut has a spring welded/soldered/permanently joined to it, which you can tighten down onto the PCB. If you get really keen, you can solder the spring to the PCB and having a nut behind the PCB will lock the PCB in place, while allowing it to move if it needs to.

author: free_electron
author_rank: Super Contributor
timestamp: November 16, 2014, 02:49:28 am
content: PEM nuts may be the way to go.these are 'pulled' into a plated hole and 'bite' into the plating.the nut itself is threaded. you can get these with a counter nut. KF2 or KFH serieshttp://www.pemnet.com/fastening_products/circuit_board.htmlanother solution is a soldered nut that is guaranteed to get good contact.we use PEM nuts ll the time for the gournd contact on burn in machines. these things go through thermal cycleing thousands of times without losing electrical contact.but ! you need to design the correct hole in the PCB and you need to employ thick plating ( 2 ounce copper so you have 1 ounce in the hole) you also cannot rely on the hole containing the nut to contact the inner layers. you need additional via's around the hole to make that connection ( standard construction of a screw hole with 6 via's in its annular ring )wurth has those solderable nutshttp://katalog.we-online.de/en/em/search/Stromversorgungselementeanother idea would be to solder on a round collar to the pad ( like the wurth ones ) and use a turret spring inside the bushing. the turret spring goes over the rod . hmm kinda difficult to explain. envision those turned-pin ic sockets. that insert is called a turret spring.Multicontact makes those in various shapes and sizes.http://www.multi-contact.com/products/productline/7look at that sa400. that is what i mean. they exist in different sizes. all you need is to pick one that clamps over the brass rod , and inside the bushing soldered on the board. these things are used inside connectors that are to be semi-permanently mated.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 16, 2014, 07:50:54 am
content: PEM and other brands' nuts are available as soldered (shoulder, barrel or both), blind or through thread, etc.In a recent project, we put some on a board to hold a daughterboard; seems to work, though it hasn't been through thermal yet.They're not quite as available as general hardware from McMaster is, so as you build a design, be careful what you're working yourself into, if all the parts are quite available or not.Tim

author: David Hess
author_rank: Super Contributor
timestamp: November 16, 2014, 10:46:41 am
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 16, 2014, 11:30:04 am
content: ^ Wave washers, unless you mean like the waffle style gasket stuff.Tim

author: David Hess
author_rank: Super Contributor
timestamp: November 16, 2014, 11:48:45 am
content: Quote from: T3sl4co1l on November 16, 2014, 11:30:04 amIt 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.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 16, 2014, 11:58:53 am
content: By wave washer I assumed something like this

author: David Hess
author_rank: Super Contributor
timestamp: November 16, 2014, 12:03:45 pm
content: The other ones I have seen for EMC applications look like little coiled wire donuts.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 16, 2014, 12:57:43 pm
content: Quote from: David Hess on November 16, 2014, 11:48:45 amOhhh, 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.htmTim

author: nuno
author_rank: Frequent Contributor
timestamp: November 16, 2014, 02:39:30 pm
content: For "un-screwing" washers, seems that the best is Nord Lock.

author: nctnico
author_rank: Super Contributor
timestamp: November 16, 2014, 04:51:45 pm
content: Quote from: David Hess on November 16, 2014, 11:48:45 amSearch for 'crinkle washer' and you'll find lots of them.

author: tonyarkles
author_rank: Regular Contributor
timestamp: November 16, 2014, 05:16:00 pm
content: I totally agree with your apprehensionI 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]

author: David Hess
author_rank: Super Contributor
timestamp: November 16, 2014, 06:16:14 pm
content: Quote from: T3sl4co1l on November 16, 2014, 12:57:43 pmExactly.Quote from: nctnico on November 16, 2014, 04:51:45 pmI did but nothing I saw matched.tonyarkles, I have seenoldceiling 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 16, 2014, 08:22:50 pm
content: Quote from: nctnico on November 16, 2014, 04:51:45 pmNo, 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/3505717Tim

author: SeanB
author_rank: Super Contributor
timestamp: November 16, 2014, 08:44:47 pm
content: Google serrated washer, possibly these?https://www.google.com/search?q=serrated+washer&safe=off

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 16, 2014, 08:46:38 pm
content: 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

author: SeanB
author_rank: Super Contributor
timestamp: November 16, 2014, 08:53:06 pm
content: Belville washers? You can get them in beryllium copper, but they are both expensive and have a fatigue life.

author: Precipice
author_rank: Frequent Contributor
timestamp: November 16, 2014, 09:31:35 pm
content: Quote from: mikeselectricstuff on November 15, 2014, 08:39:06 pmI 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?)

author: richard.cs
author_rank: Super Contributor
timestamp: November 16, 2014, 09:47:55 pm
content: 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.

author: Tothwolf
author_rank: Regular Contributor
timestamp: November 17, 2014, 01:12:53 pm
content: 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.

author: Tothwolf
author_rank: Regular Contributor
timestamp: November 17, 2014, 01:27:28 pm
content: Another thought, but maybeMicro Jackswould 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.

author: calzap
author_rank: Frequent Contributor
timestamp: November 18, 2014, 04:12:06 pm
content: 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

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 18, 2014, 07:01:06 pm
content: Quote from: calzap on November 18, 2014, 04:12:06 pmNot a big weight - 5 rods holding eleven 25x850mm strips of 0.8mm PCB on 3mm aluminium.

author: Dave Turner
author_rank: Frequent Contributor
timestamp: November 18, 2014, 11:44:13 pm
content: MikeHow 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.

author: langwadt
author_rank: Super Contributor
timestamp: November 19, 2014, 12:13:15 am
content: Quote from: mikeselectricstuff on November 18, 2014, 07:01:06 pmsmd wire lugs mounted like the one in the middle of this pic?

author: nctnico
author_rank: Super Contributor
timestamp: November 19, 2014, 12:40:42 am
content: Epic fail. Solder used in electronics isn't suitable for joints which have mechanical load on them.

author: pickle9000
author_rank: Super Contributor
timestamp: November 19, 2014, 02:48:23 am
content: 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.

author: calzap
author_rank: Frequent Contributor
timestamp: November 19, 2014, 03:27:10 am
content: 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

author: pickle9000
author_rank: Super Contributor
timestamp: November 19, 2014, 03:47:17 am
content: Quote from: calzap on November 19, 2014, 03:27:10 amI was thinking about the silver type.http://www.2spi.com/catalog/vac/silver-filled-grease.shtml

author: langwadt
author_rank: Super Contributor
timestamp: November 20, 2014, 08:45:22 pm
content: Quote from: nctnico on November 19, 2014, 12:40:42 ameverything smd has some mechanical load, it is just a matter of how muchAnd with spacers between the pcbs the solder wouldn't have to take any mechanical load, but the screwwould provide a more reliable electric connection that just sticking it through a plated hole and hoping forthe best

author: Rigby
author_rank: Super Contributor
timestamp: November 21, 2014, 05:01:31 am
content: Wave washers will work if you use 2oz copper on the PCBs, you use a conductive, non-evaporating grease on every washer, and you have something to take up any slack that might appear in any of the rods.Unless the boards are going to flap in the wind, they could probably go 5 years before anyone would even need to inspect those joints again. Maybe more.The issues mentioned are real (don't solder any of it, especially) but I think if some sort of method to maintain the pressure can be devised, that it will be more than reliable enough for a stationary art installation.Worse contact decisions have been made in various vehicle designs over the years.My best advice would be to do some self namedropping and contact some engineers that design connectors at Molex or something.

author: timb
author_rank: Super Contributor
timestamp: November 25, 2014, 12:08:12 pm
content: Can the rods be hollow? If so you just run wire through them.Sent from my Smartphone

author: IHITech
author_rank: Newbie
timestamp: March 14, 2015, 04:26:30 pm
content: Yes there is a lot of good comments here on the SMT / SMD issues. You are really hitting the issues.Solder joints on PCB terminals, especially surface mount ones, have to be protected from long term forces due to solder creep over time (a good collection of the science behind all this athttps://lugsdirect.com/Precautions-when-using-SMT.pdfIf these applications in this blog were UL tested and inspected (UL are THE standards people to aspire to) the inspectors would look for wire tie down in all cases so the SMT lug would not creep over time. There is even a UL standard to make that easier called UL486E. This is a derivative of the mega standard UL486A-B for almost all wire connector terminals in the USA (and similar standards under IEC for most non US countries).This "concession" standard UL486E allows much lower pull out forces for factory installed wiring that would be coming off a boxed "delicate" device that is not up to having wires anchored and yanked on it. So a wire strain relief is provided and the device is allowed to pass with low short term wire pull numbers. For SMT /SMD, close to zero pull is the right number to design the application and wire tie down around to nix the creep issue completely.Theshort termscrew tightening and loosening torque forces are not a creep issue. Only one of needing a decent soldering job.So the bigger issue by far is the creep in the PCB plastic itself since any bolted through connector concept is doomed to creep loose as the FR4 moves under the screw pressure. THT (Through Hole) is always first choice for PCB power lugs selection since you can then use multiple layers to share current and have better mechanical anchoring. If you have to use SMT / SMD then zero out the wire pull force and use heavy foils. And do not overheat the foil "glue" when soldering on. A foil blister reduces the ability of the foil to conduct its heat through the foil/plastic interface.The springs and Bellville cupped or wavy spring washers have some merit for test use, low contact force, non soldered connections but tend to be universally under engineered and fail prematurely. For a long term solution the spring has to be fairly strong, never run out of take up room as the PCB gets thinner (creep stroke) and have dielectric lubricating "grease" to 1. lubricate to eliminate fretting 2. eliminate air (oxidation, oxides) . Counter intuitively the conductive particle filled "greases" are not the answer for long term life. But that is whole new story.