Electronics > Manufacturing & Assembly

Bolt-on copper on standard PCB

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ArdWar:
I'm wondering with this kind of bolt-on copper design. Can I just simply open up soldermask on a standard PCB construction and literally bolt on milled copper busbar on top of ENIG? Are there any important considerations that I should be aware of?
Do I need plating for the busbar? I'd guess there would be potentially corrosion/oxidation concern especially with the PCB-busbar interface not completely airtight.
I'm not looking for volume production here, so design for manufacturability is less of a priority. Beyond not wanting unnecessary bother for myself that is.

I was looking at embedded cooper and/or extra heavy plating before, but the setup cost alone is not exactly worth paying for couple of fast changing one off prototype.

T3sl4co1l:
Yes, but make sure the bolted connections are adequate.  It may be worthwhile for example to use solder or press-in terminals, and bolt to that.

PCB itself isn't great, because it cold-flows under pressure, and fasteners can come loose.  Hardware to maintain tension is suggested -- NOT lockwashers*, much more force and compliance range is required -- conical (Belleville) washers are the best choice.

*Split washers in particular, have very little spring force (it's a single turn of a coil spring, like who cares), and once collapsed, provide no ratcheting action either.  In fact they're basically as good as flat washers -- which is to say, more susceptible to unthreading under cyclic shear than without washers at all; washers tend to lubricate a joint, making it easier to come undone.  Star washers help, but not as much as you'd think.  There are special ratcheting washers (NORD-LOCK, etc.) that do actually improve cyclic shear resistance.  Otherwise, a plain dry bolted joint with tight fitting parts and no washers tends to be the best route.

Now, both shear and creep should be pretty small here, in a prototype situation, but keep in mind the effect of vibration and creep when moving the design into production.

Might also be able to order (or hand form) pieces with thru pins, for example waterjet a forked end then CNC bender or pliers or clamp and hammer the ends over so they can fit into holes or slots, then solder in.

Soldering at all, of course, is its own adventure... preheating to nearly the melting point is likely required, a chunky solder iron, and localized hot air might not hurt too.  Lap joints are also possible, just keep in mind the thickness of metal and its effect on the board: FR-4 and Cu have different shrink rate so it'll warp a bit on cooling.  Be sure you have enough clamping force to flatten it for thermal interface purposes, and avoid leadless components that might be overstressed or solder improperly as a result.  Or perhaps, consider a design with all the power stuff on one board, and riser/stacker boards for drive and control.

Tim

ajb:

--- Quote from: T3sl4co1l on June 12, 2024, 01:28:43 pm ---It may be worthwhile for example to use solder or press-in terminals, and bolt to that.
--- End quote ---

I would second this suggestion, since it keeps the mass of the bus bar out of the soldering process and sidesteps concerns with material creep, and the mating holes can be designed to allow for arbitrary assembly tolerances quite easily.  There are threaded SMT terminals ready for PNP assembly (example), which makes assembly easy and can keep the bottom of the PCB flat (for example if you wanted to mount the board to a heatsink like the photo in the OP).  You also get the option to space the bus bars up off of the PCB, and use the space underneath for components -- or get really crazy, and have two (or more!) layers of bus bars that can cross over/under each other. 


--- Quote from: ArdWar on June 12, 2024, 08:34:27 am ---Do I need plating for the busbar? I'd guess there would be potentially corrosion/oxidation concern especially with the PCB-busbar interface not completely airtight.
--- End quote ---

You can can make any connection gastight if you tighten it hard enough :-/O 

If you were to directly bolt the bus bar to the PCB, you'd probably benefit from controlling where the bus bar contacts the PCB, so that it's limited to the area directly clamped by the fastener.  You could do this with a washer between the PCB and the bus bar or with a boss machined into the bus bar.  Either way, that will keep the contact pressure between the copper and the PCB higher, although it does mean you have to worry about the PCB creeping again. 

I don't think I've really seen useful numbers for how much a PCB will actually creep under a bolted connection, though.  It's something that often comes up in these discussions as a concern with directly-bolted connections, but without some idea of the magnitude of the problem it's hard to know how difficult it might be to engineer around. 

T3sl4co1l:
Oh, I forgot to go back and complete my thought. Edited. :D

Not actually sure how much creepage FR-4 exhibits; it doesn't seem to be a normal documented parameter, and there's oodles of academic papers on this-and-that customized application for composites, but, I just want to know what bounds to design to, okay?..!

This,
https://journals.sagepub.com/doi/10.1177/0021998319845045
seems to suggest it's fractional percent, which feels about right.  Granted, I have no idea what resin they used, in relation to, whether it's representative of what PCBs use.  The metal-laminate structure is probably not a big factor in that, but of course it will be stiffer and stronger than nearly-metal-free PCB material.

This might also be of interest,
https://nvlpubs.nist.gov/nistpubs/jres/045/2/v45.n02.a06.pdf
you could presumably load up the board with a lot of metal -- use 4+ layers, at least heavyish copper (>2oz), stitching vias in the screw-mount pad, etc. -- and gain some stiffness that way, but keep in mind copper itself is far from the strongest metal out there.  Now, it's not dead-soft in PCB form, the purity is good but does contain some oxygen I think??, and there is some strain imposed from the plating process.  So it will be a bit stronger than annealed Cu, but under fastener loading and given the small fraction of metal in the clamped joint, probably the copper will creep as well, would be my guess.

Put another way: you can give it a better chance by maximizing metal in the joint, but it probably still creeps.

Solder creeps some as well, not to forget.  Lead-free alloys do creep less; the downside is, that strain can be transformed into whiskering, depending on alloy, treatment and time.

Tim

David Hess:

--- Quote from: T3sl4co1l on June 12, 2024, 01:28:43 pm ---Soldering at all, of course, is its own adventure... preheating to nearly the melting point is likely required, a chunky solder iron, and localized hot air might not hurt too.
--- End quote ---

I have done things like that with this soldering iron, which is still temperature controlled:

https://www.amazon.com/Weller-W100PG-Farenheit-Soldering-Degree/dp/B002I7X7ZS/

Drill small holes along the length of the copper to provide access to add solder.  Maybe put a machine screw through every other hole to hold the copper flat?

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