Can brass stand-offs carry power?

[technix]

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?

[retrolefty]

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.

[hlavac]

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)

[CatalinaWOW]

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.

[stj]

this actually used to be pretty common in large power supply's.

[SeanB]

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.

[abraxa]

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.

[Neomys Sapiens]

Another possible practice to avoid the problems with the connection becoming loose would be to use soldered-in collared nuts on the pcb.

[nctnico]

This question originated from my modular power supply project. Here is the design I was thinking about:

How much current can a M2.5 brass stand-off pass?

You'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.

[TimFox]

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.

[David Hess]

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.

[Smokey]

Aluminum... you want aluminum standoffs... check the  tables... Brass sucks hard for conductivity...   

https://www.bluesea.com/resources/108/Electrical_Conductivity_of_Materials

[wraper]

Aluminum... you want aluminum standoffs... check the  tables... Brass sucks hard for conductivity...   

https://www.bluesea.com/resources/108/Electrical_Conductivity_of_Materials

If 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.

[nctnico]

Aluminum... you want aluminum standoffs... check the  tables... Brass sucks hard for conductivity...   

https://www.bluesea.com/resources/108/Electrical_Conductivity_of_Materials

But 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.

[Smokey]

Aluminum... you want aluminum standoffs... check the  tables... Brass sucks hard for conductivity...   

https://www.bluesea.com/resources/108/Electrical_Conductivity_of_Materials

But Aluminum is incompatible with other metals so for this application it is a very bad choice. Also aluminum may compress over time because it is very soft.

??
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? 

[wraper]

When you throw out "very bad", what reaction are you saying is going to happen exactly and over what time scale?

Given 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

[CatalinaWOW]

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.

[Smokey]

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.

[David Hess]

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.

Gold and aluminum?   https://en.wikipedia.org/wiki/Gold-aluminium_intermetallic

When 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.

[CatalinaWOW]

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.

[nctnico]

Aluminum... you want aluminum standoffs... check the  tables... Brass sucks hard for conductivity...   

https://www.bluesea.com/resources/108/Electrical_Conductivity_of_Materials

But Aluminum is incompatible with other metals so for this application it is a very bad choice. Also aluminum may compress over time because it is very soft.

??
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?

What 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.

[stj]

well gold & tin dont mix,
found that out long ago when mating cheap simm's with expensive sockets!
 

[ajb]

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.

[stj]

I would think that someone ought to make shouldered washers that could be soldered in and then bolted through to the standoff below

they 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.

[ajb]

I would think that someone ought to make shouldered washers that could be soldered in and then bolted through to the standoff below

they 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.

I 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?