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Help me get 35 Amps onto my PCB!

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frogblender:
Howdy... I need to get 35 amps onto my PCB.  Attached pic shows my first stab at it.   The other end of the busbar is attached to a another PCB in the same manner.

The busbar is huge and can easily handle 35amps.

The 7mmDia PCB pad is the part I am worried about...  It'll have local topside flood, and a bunch of vias (both in the pad, and in the topside flood (not shown) surrounding the 7mm pad.

Anyone:
a) have any better ideas?
b) knowledge on whether the 7mm pad can handle 35amps?



Wolfram:
That busbar looks awfully big for 35 A. Having rigid busbars connect to PCBs can lead to a lot of potential problems. Differential thermal expansion between the copper and the PCB can cause a lot of stress on the interconnection, which can lead to failure over time. I use flexible wires for up to around 70 A, and even going beyond this is easily done by using multiple wires in parallel. Wire with high temperature insulation can help buy some margin here, 125 or 155 degree rated XLPE is a good match for the temperature handling of normal PCB materials. Busbars are viable for higher currents, but need to be designed with flexible sections to limit strain in every plane.

The connection between the insert nut and the board itself can easily be designed to handle 35 A. Make sure the whole current path is as wide and short as possible, and increase the copper thickness in case you need more margin in your design. Skip the thermal reliefs if practical, that way your wire can help heatsink the interconnection and plane.

Edit: I just noticed that the PCB itself will be compressed by the screw and the nut. PCBs will creep at higher temperature under compression, and once you lose compression of the joint then you also lose the integrity of the electrical connection. Consider either pressfit terminals where the PCB isn't compressed by the screw, or soldered threaded terminals.

frogblender:

--- Quote from: Wolfram on October 05, 2021, 04:58:25 pm ---Edit: I just noticed that the PCB itself will be compressed by the screw and the nut. PCBs will creep at higher temperature under compression, and once you lose compression of the joint then you also lose the integrity of the electrical connection. Consider either pressfit terminals where the PCB isn't compressed by the screw, or soldered threaded terminals.

--- End quote ---
Thanks Wolfram.   I've heard that belleville washers (cup-shaped, such that as you tighten them down, the cup shape starts to flatten out, and it acts like a spring) are often used with busbars, presumably to maintain force even if things creep over time.  Ever hear of this?

m98:
There are dedicated Connectors like Molex PowerPlane or Würth PowerOne for such applications.

Wolfram:

--- Quote from: frogblender on October 05, 2021, 05:17:37 pm ---
--- Quote from: Wolfram on October 05, 2021, 04:58:25 pm ---Edit: I just noticed that the PCB itself will be compressed by the screw and the nut. PCBs will creep at higher temperature under compression, and once you lose compression of the joint then you also lose the integrity of the electrical connection. Consider either pressfit terminals where the PCB isn't compressed by the screw, or soldered threaded terminals.

--- End quote ---
Thanks Wolfram.   I've heard that belleville washers (cup-shaped, such that as you tighten them down, the cup shape starts to flatten out, and it acts like a spring) are often used with busbars, presumably to maintain force even if things creep over time.  Ever hear of this?

--- End quote ---

Of course I've heard of belleville washers. The problem is that the PCB will continue creeping as long as there is an applied force, and quantifying how much mechanical compliance you need to always maintain an acceptable lifetime, while ensuing that the pads don't disconnect from inner planes during compression, sounds a lot more difficult than using a proper interconnection in the first place. If you don't care about reliability then anything goes of course. Belleville washers are useful when making interconnections between materials that don't flow under compression, like connecting solid metal pieces together.

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