Can brass stand-offs carry power? - Page 1
Forum section: electronics / projects

author: technix
author_rank: Super Contributor
timestamp: May 14, 2017, 04:44:42 pm
content: This question originated from my modular power supply project. Here is the design I was thinking about:The backplane carries the primary bias voltage rail (+15V,) secondary side auxiliary rail (+5V,) and control signals (RS485) through a card edge connectors.The input board contains the mains input protection and filter circuitry, active PFC (+400V output,) and bias voltage on the primary side, auxiliary rail switch-mode regulator and the main microcontroller on the secondary side.The output modules contains the tracking switch-mode regulator and output linear regulator, as well as an isolated control circuit.Now since the card edge connector cannot withstand the 400V from the PFC, I am thinking about using brass stand-offs between boards as a way of transferring 400V power from one module PCB to the next. This is much easier to implement as I just need to make a M2.5 plated hole on the board (preferrably opposite end of the board from the backplane.) Now here is the questions:Is this safe?How much current can a M2.5 brass stand-off pass?

author: retrolefty
author_rank: Super Contributor
timestamp: May 14, 2017, 04:58:23 pm
content: Well copper of the same diameter can handle like 300 amps, so I'm sure brass would do better then half of that? As far a safety goes, that's an implementation issue.

author: hlavac
author_rank: Frequent Contributor
timestamp: May 14, 2017, 05:05:02 pm
content: Resistivity of brass is about 10x higher than copper, which is still less than tin.I'd be only concerned about the surprise factor (someone not expecting to find 400V on standoffs) and about how to make a good contact (I guess tightened standoffs on big tinned PCB pads should work fine)

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 14, 2017, 05:13:09 pm
content: Obviously you need to arrange spacings on both boards to provide appropriate separation from other traces. Sleeves on the standoffs would deal with many of the safety issues. The interface between the standoff and the board will probably by fine, but it depends on what your demands on it are (resistance, operation in adverse environments. Things like a toothed washer on both sides of the board might be appropriate.The final concern is over-constraining the board mechanically. Any time you have more than three points of support you are assuring that there will be stress on the board due to tolerances on length. This isn't a stopper, but another thing to worry about in your design and fabrication phase.

author: stj
author_rank: Super Contributor
timestamp: May 14, 2017, 05:47:43 pm
content: this actually used to be pretty common in large power supply's.

author: SeanB
author_rank: Super Contributor
timestamp: May 14, 2017, 06:16:17 pm
content: You would also need to get some stacked belleville springs to compensate for the board creeping with time, making the connections go higher resistance. I would also probably derate a lot, if you are at 400v then under 5A would be fine, though I would guess what would also work would be PCB mounted 1/4in spade connectors and the male side, as this also allows for a good contact area, and will handle slight misalignment, plus it is very obviously a power connection.If you are using the belleville springs/washers you will need 2 per side, arranged cup to cup so they have the most travel, and a brass washer between the standoff and the board, and another on the top by the springs, to spread the load out to a larger board area, or a third beleville spring with cup towards the board. The bellevile springs are nice, compact and very constant in pressure, and in M3 should be reasonably heap, though a lot more than a simple punched washer.

author: abraxa
author_rank: Frequent Contributor
timestamp: May 14, 2017, 06:56:16 pm
content: I'm not a metallurgy expert, so just want to throw the thought out there so you can do proper research: screwing a brass standoff against a tin-plated PCB hole may not provide sufficient surface friction to be reliable. Adding a steel washer would increase this friction but may however introduce an element that causes corrosion, so before choosing the current-conducting components, I'd check which materials are usable without corroding each other.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: May 14, 2017, 07:14:58 pm
content: Another possible practice to avoid the problems with the connection becoming loose would be to use soldered-in collared nuts on the pcb.

author: nctnico
author_rank: Super Contributor
timestamp: May 14, 2017, 07:24:47 pm
content: Quote from: technix on May 14, 2017, 04:44:42 pmYou'd have to calculate this. Another thing to consider is metal to metal compatibility. One of my products is an approx. 11kW to 15kW battery charger which is fed by 70 to 95V at 160A. This is just impractical to handle on a PCB so I ran some bussbars across the backplane. M5 Brass standoffs carry the power into the backplane at 10 points so 16A each. I used a gold plated PCB for the backplane and nickel plated standoffs. Nickel is compatible with the gold plating and the copper bussbar.As others noted you'll need compression washers to keep it from coming loose in the long run.

author: TimFox
author_rank: Super Contributor
timestamp: May 14, 2017, 08:39:38 pm
content: It is often a safety requirement that ground connections should have a wire in parallel with a mechanical mounting, so that the component case is still grounded if, for example, the motor is removed from the chassis for service.

author: David Hess
author_rank: Super Contributor
timestamp: May 14, 2017, 10:57:40 pm
content: Brass standoffs will work however their are problems maintaining a good connection to the printed circuit board.The board tends to compress over time lowering the contact force between the brass standoff and pad so a compression washer is required.Unfortunately a lockwasher cannot be used between the brass standoff and the pad because the sharp edges will abrade the thin copper off of the printed circuit board over time.

author: Smokey
author_rank: Super Contributor
timestamp: May 14, 2017, 11:49:19 pm
content: Aluminum... you want aluminum standoffs... check the tables... Brass sucks hard for conductivity...https://www.bluesea.com/resources/108/Electrical_Conductivity_of_Materials

author: wraper
author_rank: Unknown
timestamp: May 15, 2017, 12:00:05 am
content: Quote from: Smokey on May 14, 2017, 11:49:19 pmIf you want corrosion on the contact surfaces, then sure. Not to say, pure aluminum is not suitable for those but alloys may have much worse conductivity.

author: nctnico
author_rank: Super Contributor
timestamp: May 15, 2017, 12:02:36 am
content: Quote from: Smokey on May 14, 2017, 11:49:19 pmBut Aluminum is incompatible with other metals so for this application it is a very bad choice. Also aluminium may compress over time because it is very soft.

author: Smokey
author_rank: Super Contributor
timestamp: May 15, 2017, 01:03:02 am
content: Quote from: nctnico on May 15, 2017, 12:02:36 am??Compression? Use screws with lock washers (bellville like mentioned above are great). Plus if you are worried about compressing standoffs, you better worry about having a PCB getting broken from all this crazy stress that's crushing aluminum apparently.Everything corrodes at some level. You need to be reasonable before throwing out the dissimilar metals card. Will this assembly be used at the bottom of the ocean and exposed to salt water or something? When you throw out "very bad", what reaction are you saying is going to happen exactly and over what time scale?

author: wraper
author_rank: Unknown
timestamp: May 15, 2017, 01:08:59 am
content: Quote from: Smokey on May 15, 2017, 01:03:02 amGiven relatively high humidity (which is normal thing in many locations), corrosion may appear in a few months.http://en.coppercanada.ca/videos-publications/publications/pub42/homepage.html

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 15, 2017, 01:10:48 am
content: The comments here dance around the fact that you need to do a real study of the environment your boards will live in. And some real analysis of the electrical aspects.The cross sectional area of a standoff can be selected to make conductivity a non-issue.Metal compatibility could be crucial if you will be operating near a sea front with saline humidity. And may not matter at all if you are in one of the great deserts of the world.Board compression will occur, and thermal cycling of some magnitude will occur in almost all environments, but shock and vibration may be effectively absent. If you are supporting really large currents magnetostriction may also be an issue. Abrasion will be a problem only after whatever preload exists in the joint relaxes until motion occurs. Loss of conductivity is likely the precede that problem.Only you know the conditions your supply will have to live with. You need to think it through and then come up with solutions for the problems those conditions cause. People on the forum can be helpful with suggestions for these problems if you describe them.

author: Smokey
author_rank: Super Contributor
timestamp: May 15, 2017, 04:01:04 am
content: So it's the copper-aluminum interface you are worried about.... When would the aluminum standoffs be in contact with bare copper? At a minimum the boards would be HASL and have a coat of lead/tin depending on your finish. That or you could pop for ENIG and have gold on the boards.

author: David Hess
author_rank: Super Contributor
timestamp: May 15, 2017, 03:22:37 pm
content: Quote from: Smokey on May 15, 2017, 04:01:04 amGold and aluminum?https://en.wikipedia.org/wiki/Gold-aluminium_intermetallicWhen I have seen this done with aluminum, star lockwashers were used but as I posted earlier, they tend to tear up the printed circuit board.This is all possible to do reliably if proper compression washers are included but precise assembly is required.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 15, 2017, 04:03:38 pm
content: While I don't doubt that star and other lock washers can damage a board under some circumstances I have seen numerous examples of this application with no more than cosmetic damage to the copper foil. On very rare occasions I have seen penetration of the foil into the underlying PWB. In all of these cases that I can remember continuity was retained, but there certainly is the potential of a failure. Again it is a question of what this project is aimed at. Everything does not have to be designed to military or space standards.There are always many solutions to a problem. Bellevue washers are an example. Another solution to the problem of crushing the PWB is to make the through hole in the board larger and insert a load bearing sleeve in this hole. Assembly is made easier if this sleeve is machined as part of the standoff. This makes installation torques non-critical, but transfers the problem to control of board thickness and sleeve length. Another solution that may work for some applications is regular maintenance. Checking the torque on the fasteners at a regular interval. This avoids tolerance issues (other than torque) and the cost and assembly issues of extra springs. The user has to pick the poison that is best suitable to their application.

author: nctnico
author_rank: Super Contributor
timestamp: May 15, 2017, 04:23:43 pm
content: Quote from: Smokey on May 15, 2017, 01:03:02 amWhat happened to doing your own homework? Do some research into metal to metal compatibility and you'll learn it is it far from straightforward and gold isn't that unreactive as you may assume.

author: stj
author_rank: Super Contributor
timestamp: May 15, 2017, 04:48:38 pm
content: well gold & tin dont mix,found that out long ago when mating cheap simm's with expensive sockets!

author: ajb
author_rank: Super Contributor
timestamp: May 15, 2017, 05:41:36 pm
content: You can get solder-in threaded standoffs. I believe they're usually steel (at least Keystone's are), but that would solve the mechanical connection at one end. Heck, even soldering in the male end of a regular male-female standoff would probably work pretty well--you'd probably have to place it manually, but it could be reflowed with the rest of the board as long as your reflow setup can deal with the thermal mass.On the other end, I would think that someone ought to make shouldered washers that could be soldered in and then bolted through to the standoff below, such that the bushing bears all of the compressive force instead of the PCB. Haven't seen them as a stock item, though. Possibly someone makes a thin shouldered bushing that would do, but those are often sintered bronze for bearings and you'd probably have to pay for tolerances that are totally unnecessary for this application.All that said, except for some really demanding environments, you'll probably be fine with regular old standoffs, screws, and washers.

author: stj
author_rank: Super Contributor
timestamp: May 15, 2017, 06:07:43 pm
content: Quote from: ajb on May 15, 2017, 05:41:36 pmthey do, you will find them used on single-sided power supply boards in tv's & such to connect the ground to the frame the pcb is bolted to.

author: ajb
author_rank: Super Contributor
timestamp: May 15, 2017, 06:14:22 pm
content: Quote from: stj on May 15, 2017, 06:07:43 pmI don't recall ever seen that in such a consumer item--usually just a hole in the PCB with some little solder pillows on the bottom to make contact with the chassis. Also it doesn't solve the question of where they can be bought as a stock item, unless you have a link?

author: Monkeh
author_rank: Super Contributor
timestamp: May 15, 2017, 06:17:52 pm
content: Quote from: ajb on May 15, 2017, 06:14:22 pmWhat, these?https://www.toby.co.uk/content/catalogue/series.aspx?id=6&Category=159

author: Devin Black
author_rank: Contributor
timestamp: May 15, 2017, 06:31:13 pm
content: One problem with this I can think of it that if you are selling these for mass production is that these standoff and screw terminals are usually ground, so if someone troubleshooting this board would be very confused if they found a 400V rail connected to where the ground should be.

author: ajb
author_rank: Super Contributor
timestamp: May 15, 2017, 07:02:01 pm
content: Quote from: Monkeh on May 15, 2017, 06:17:52 pmSure, seen those. But they still rely on the PCB withstanding the compression caused by the screw tension (see previous comments on (mechanical) creepage, thermal cycling). I was talking about a shouldered bushing that occupies the full depth of the hole in the PCB (plus the shoulder on one side and some extra on the other) and thus would bear all of the compression in lieu of the PCB. Again, probably overkill in most normal applications, but would avoid all of the objections raised in this thread to using a simple standoff and a couple of screws through the PCBs.

author: dmills
author_rank: Super Contributor
timestamp: May 15, 2017, 08:57:18 pm
content: PEM make PCB inserable tapped brushings, and for more serious things, both Wurth and Erni have a line called PowerElements that are typically press fit with various threads for real current, I suspect both are actually identical.http://catalog.pemnet.com/category/uts-and-spacers-standoffs-surface-mount-type-smtsohttp://www.erni.com/produkte/show/category/steckverbinder-powerelementehttp://www.erni.com/produkte/show/category/steckverbinder-stromversorgungsanschluesse

author: David Hess
author_rank: Super Contributor
timestamp: May 15, 2017, 09:18:51 pm
content: I forget who now but someone has been sending me ads for giant collet pins which are intended for exactly this application. Old Intel motherboards use them to connect the processor's point of load regulator to the motherboard so the VRM module is replaceable and upgradable.technix asked about brass standoffs though.

author: Smokey
author_rank: Super Contributor
timestamp: May 15, 2017, 11:36:41 pm
content: Quote from: David Hess on May 15, 2017, 03:22:37 pmDid you see the temperatures associated with those reaction? If this dude's power supply gets to 400C there are bigger problems. This is intended for bond wires welds, not cold mechanical pressure contact.Plus 2oz copper (this is a power board after all) is pretty thick. You have to dig a washer in pretty hard to get through a 2oz outer copper layer.

author: David Hess
author_rank: Super Contributor
timestamp: May 16, 2017, 05:48:12 pm
content: Quote from: Smokey on May 15, 2017, 11:36:41 pmI have seen it with tin-gold intermetallics over the long term at room temperatures but I do not know of any examples involving aluminum-gold. For the later I, assume people know better. When I work with electrical attachment to aluminum, I use stainless steel hardware.QuoteThe washer damage was do to long term thermal cycling which rotated the star washer forward gouging out the plated copper.

author: P90
author_rank: Frequent Contributor
timestamp: May 16, 2017, 07:25:04 pm
content: why not use male and female pcb spade connectors?

author: IconicPCB
author_rank: Super Contributor
timestamp: May 16, 2017, 07:34:38 pm
content: I have just prototyped a 4 ounce PCB with screw in brass standoffs happy to handle 200 Amps.Standoffs were soldered into the PCB.No problems with standoffs... the power transistors got a bit hot when operated without a heatsink...floated away on molten solder.

author: David Hess
author_rank: Super Contributor
timestamp: May 17, 2017, 01:11:12 am
content: Quote from: IconicPCB on May 16, 2017, 07:34:38 pmI have repaired a few like that; the solder always seems to fail at the joint which makes sense since solder is not mechanically strong and leverage may be high. TO-3 parts mounted in a similar way are also known for failing at their solder joints.Did you have to do anything special to get the solder to wet the brass? There seems to be a lot of variation even with the same type of brass.

author: IconicPCB
author_rank: Super Contributor
timestamp: May 17, 2017, 03:28:11 am
content: Nothing special in the way of surface preparation.The brass stand off was pressed into the PTH and sweated both sides.It had a 35mm^2 cable to contend with. The cable had a crimp lug and the lug was screwed onto the stand off using M6 screw.I would expect the solder joint to perform well.