Author Topic: Can brass stand-offs carry power?  (Read 11313 times)

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Offline Monkeh

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Re: Can brass stand-offs carry power?
« Reply #25 on: May 15, 2017, 06:17:52 pm »
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?

What, these? https://www.toby.co.uk/content/catalogue/series.aspx?id=6&Category=159
 

Offline Devin Black

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Re: Can brass stand-offs carry power?
« Reply #26 on: May 15, 2017, 06:31:13 pm »
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.
 

Offline ajb

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Re: Can brass stand-offs carry power?
« Reply #27 on: May 15, 2017, 07:02:01 pm »
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?

What, these? https://www.toby.co.uk/content/catalogue/series.aspx?id=6&Category=159

Sure, 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.
 

Offline dmills

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Re: Can brass stand-offs carry power?
« Reply #28 on: May 15, 2017, 08:57:18 pm »
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-smtso
http://www.erni.com/produkte/show/category/steckverbinder-powerelemente
http://www.erni.com/produkte/show/category/steckverbinder-stromversorgungsanschluesse

 

Online David Hess

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Re: Can brass stand-offs carry power?
« Reply #29 on: May 15, 2017, 09:18:51 pm »
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.
 

Online Smokey

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Re: Can brass stand-offs carry power?
« Reply #30 on: May 15, 2017, 11:36:41 pm »
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.

Did 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.
 

Online David Hess

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Re: Can brass stand-offs carry power?
« Reply #31 on: May 16, 2017, 05:48:12 pm »
Did 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.

I 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.

Quote
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.

The washer damage was do to long term thermal cycling which rotated the star washer forward gouging out the plated copper.
 

Offline P90

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Re: Can brass stand-offs carry power?
« Reply #32 on: May 16, 2017, 07:25:04 pm »
why not use male and female pcb spade connectors?
 

Offline IconicPCB

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Re: Can brass stand-offs carry power?
« Reply #33 on: May 16, 2017, 07:34:38 pm »
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.
 

Online David Hess

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Re: Can brass stand-offs carry power?
« Reply #34 on: May 17, 2017, 01:11:12 am »
I have just prototyped a 4 ounce PCB with screw in brass standoffs happy to handle 200 Amps.

Standoffs were soldered into the PCB.

I 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.
 

Offline IconicPCB

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Re: Can brass stand-offs carry power?
« Reply #35 on: May 17, 2017, 03:28:11 am »
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.
 


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