Stacked board standoffs as power/ground paths - stupid idea or not? - Page 1
Forum section: electronics / beginners

author: HwAoRrDk
author_rank: Super Contributor
timestamp: February 01, 2016, 06:07:55 am
content: I had what is probably a stupid idea.I'm not realistically considering doing this, but my enquiring mind wants to know if it's not as dumb as it first sounds.Say I have two stacked boards that need to share a common power and ground circuit. Assume that metal standoffs will be used to assemble the boards into the stack, with screws to fasten. If I were to have unmasked annular contact areas around two of the mounting holes, and one was routed to ground and the other to power, would that be workable?There's a saying: "if it's stupid but it works, it's not stupid". Just wanted to find out if my idea fits the proverb.

author: rs20
author_rank: Super Contributor
timestamp: February 01, 2016, 06:14:59 am
content: Well, it'd probably work in practice (at least for a one-off prototype), but I suspect it's one of those bad ideas. You never know if you replace the standoffs and they're aluminium with extra thick anodisation or something -- assuming that mechanical components will be conductive is poor practice if the datasheet doesn't promise conductivity.There's also the question of whether the connection between some tarnishing copper and some oxidised aluminium will be able to reliably carry current over the years.

author: DivineChaos
author_rank: Regular Contributor
timestamp: February 01, 2016, 06:15:57 am
content: As long as you aren't working with super high voltages and amperages, and as long as they are far enough apart from each other and from the case that they may be in, I'd say go for it.Just be sure that if those standoffs were to be loose or during assembly/disassembly, they aren't live.Another thing to look out for is those standoffs becoming loose over time, creating an open in your circuit that may be hard to track down.

author: Ice-Tea
author_rank: Super Contributor
timestamp: February 01, 2016, 06:20:18 am
content: You wouldn't be the first to do itAnd I've seen standoffs specifically for this purpose.One problem I can think of is that the return path would be quite a bit removed from the "signal" (be it a power rail or not) which screams EMC issues...

author: Ian.M
author_rank: Super Contributor
timestamp: February 01, 2016, 06:24:51 am
content: Common PCB materials tend to cold-flow under prolonged pressure, so for reliable contact you'd need Belville spring washers to keep the connection tight. Otherwise, brass or plated brass will be fine, steel is a lot dodgier, except for nuts and fixing screws that aren't in the direct current path and aluminium should be avoided at all costs.

author: Ice-Tea
author_rank: Super Contributor
timestamp: February 01, 2016, 06:29:19 am
content: Ah, here it is...http://katalog.we-online.de/en/em/search/power_elements

author: amyk
author_rank: Super Contributor
timestamp: February 01, 2016, 09:12:42 am
content: Apple did it... I don't know if that counts as "stupid" or not.https://www.ifixit.com/Teardown/Apple+TV+4th+Generation+Teardown/49046

author: HwAoRrDk
author_rank: Super Contributor
timestamp: February 01, 2016, 12:44:34 pm
content: Thanks all.So, basically, it would work, but not a good solution from a long-term reliability and robustness point of view!

author: Simon
author_rank: Unknown
timestamp: February 01, 2016, 12:59:23 pm
content: Quote from: Ice-Tea on February 01, 2016, 06:29:19 amThose are not stand off's, but PCB mount studs for cables. They are particularly suited for applications where a copper layout has bee routed out of a sheet that may or may no be backed by FR4 to make a high power PCB.I'd only do it as a ground, not power and in addition a ground conductor in whatever connectoryou have between the two boards.

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: February 02, 2016, 12:04:09 am
content: Another recent project on the forums using that idea, and with a fair bit of current to boot:thermal-considerations-in-led-lamp-design

author: Ice-Tea
author_rank: Super Contributor
timestamp: February 02, 2016, 07:13:41 am
content: Quote from: Simon on February 01, 2016, 12:59:23 pmNot dure if I agree? The smd studs are used as spacers. Just like, per example, mac8 smd spacers...

author: HAL-42b
author_rank: Frequent Contributor
timestamp: February 02, 2016, 07:30:24 am
content: SMD elements carrying mechanical loads are very likely to peel off. Grrr Hisss...

author: Simon
author_rank: Unknown
timestamp: February 02, 2016, 08:02:02 am
content: Quote from: Ice-Tea on February 02, 2016, 07:13:41 amWell I'm not sure if they are the type of element the OP was planning to use. Yes you could use them for that purpose but you will still have the problems others have highlighted on one side. Their original purpose was for high power, like automotive stuff, the pins on them are square and you can push them into round holes of just the right size, if you are making a power distribution bus bar you can punch them in and not even solder them, I actually spoke to someone in their technical when my boss thought we could use them for something.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 02, 2016, 08:50:47 am
content: Yes. Use washers, especially springy (wave, cone / Belleville) types.Star washers are handy too, but don't use them if they have to be disassembled frequently -- obviously they chew up the PCB's thin copper surface a lot.Use brass (usually nickel plated) standoffs; avoid aluminum (oxidizes, soft) and steel (structurally fine, but won't handle as much current -- though still quite a lot, so don't worry too much). And plastic, obviously...Tim