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I am going to use aluminum bus bars in a high-current PCB. What is the best technique to establish contact between the PCB and the bars?

I plan to make my own bars. The current is generally going to be around 100-150 A per single connection (a bar will be connected in several places).

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    \$\begingroup\$ Are these off the shelf bus bars or are you making them? Solderability is generally poor but varies by alloy. This is an interesting question. \$\endgroup\$
    – Bryan
    Jan 12, 2023 at 0:01
  • \$\begingroup\$ I updated the question. I plan to make my own bars \$\endgroup\$
    – TQQQ
    Jan 12, 2023 at 1:26
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    \$\begingroup\$ Are you planning on Al for weight or cost? If weight, go ahead. If cost, then reconsider. The ability to solder copper in a standard process will improve performance, and could well reduce the total system cost notwithstanding the higher cost of material. \$\endgroup\$
    – Neil_UK
    Jan 12, 2023 at 6:26
  • \$\begingroup\$ Are you using an aluminum alloy, or pure aluminum? Pure aluminum has great electrical conductivity, but alloys designed for structural use (most aluminum on the market) can be significantly worse. Not unusably bad (this is aluminum, not steel), but maybe not as good as you're expecting. \$\endgroup\$
    – Hearth
    Jan 12, 2023 at 15:52
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    \$\begingroup\$ Prompted by @Neil_UK's comment I've just looked at a massive table online and the variability is massive (pure Al: 61% of copper, common 6061-T6: 40%, some alloys as low as brass at around 27%) \$\endgroup\$
    – Chris H
    Jan 13, 2023 at 13:55

4 Answers 4

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If you want the bar to be flush against the PCB, consider broaching PCB PEM nuts.

Broaching PEM nut

If you want to raise the bus bar, consider pressfit threaded terminals.

https://www.digikey.com/short/2hm8p02f

pressfit threaded terminal

If you want the bus bar to slip onto the PCB, consider busbar clips.

Molex PowerPlane

{Molex}

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    \$\begingroup\$ Just for the sake of completeness, I'd add that it is possible to solder aluminum for the ultimate in corrosion-resistant contact, too, but it requires using uncommon (read: expensive) zinc-based solders, extremely aggressive (read: must be thoroughly cleaned off the PCB lest it destroy the board) flux, and quite possibly an inert nitrogen atmosphere (read: expensive soldering equipment). I would recommend doing just about anything else before trying to solder aluminum to a PCB, I just think it's worth noting that it is technically possible. \$\endgroup\$
    – Hearth
    Jan 12, 2023 at 15:57
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    \$\begingroup\$ @Hearth Just plating the bar first makes soldering it a lot easier. \$\endgroup\$
    – user4574
    Dec 15, 2023 at 4:28
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Beware of creep and oxidation: high conductivity aluminum alloys are soft and may creep under clamping force. Regardless of alloy, beware of differential thermal expansion, which will cause the mating surfaces to shift as they heat up and cool down, causing wear and oxidation.

Once the aluminum surface oxidizes, that's it, you have an insulator in the affected spot. Over time, connection resistance increases, until eventually it overheats, and damages surrounding circuitry, or melts, or starts a fire.

Joints must be clamped tightly, gas tight or greased (with a suitable anti-ox formulation), stuff like that.

Don't take my word for it, or anyone else's un-cited words; I strongly recommend looking up proper industry-recommended practices, or code standards, for aluminum wiring. Afraid I don't have experience in this field [power transmission and use of aluminum conductors] so I don't have any to recommend (anyone chime in in comments?), but this may also vary by region (permitted equipment wiring practices) so you are best looking up regulations in the place(s) being marketed to / installed in.

Regarding creep, consider: Copper wires resist mechanical creeping - Leonardo Energy They give several academic references that may be of interest.

Also keep in mind, PCB material is subject to creep, or cold-flow. Preferably, use soldered or press-fit terminal blocks; if clamping board material directly, use a bolting solution with lots of compliance, such as using Belleville washers.

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    \$\begingroup\$ So would this all be solved by copper bars instead? \$\endgroup\$
    – TQQQ
    Jan 12, 2023 at 19:16
  • \$\begingroup\$ @TQQQ if you can solder them then yes. If you can't solder them, then at least you would experience less of a problem with copper to copper connection. Copper in the bars and on the PCB will have almost the same thermal expansion, for example (affected slightly by PCB substrate and bonding to it, and yet so slightly by different level of impurities). Copper is also regarded as much less prone to creeping. Thus, risk of oxidation would be much lower. And it oxidizes slower. Of course, precautions are always a prudent idea. \$\endgroup\$
    – Mołot
    Jan 12, 2023 at 20:58
  • \$\begingroup\$ Yes, I would prefer copper myself, preferably tin plated. Or brass or bronze I suppose, given enough cross section of course. Then bolted or soldered connections are fine, with no particular precautions. \$\endgroup\$ Jan 12, 2023 at 21:20
  • \$\begingroup\$ How would you solder such a mass of copper? \$\endgroup\$
    – TQQQ
    Jan 12, 2023 at 21:44
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    \$\begingroup\$ @TQQQ look at how plumbers solder copper pipes with a blowtorch! not sure if you can do that without hurting the PCB, but still \$\endgroup\$
    – user253751
    Jan 13, 2023 at 12:20
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I have done this to mount a 500A copper bus bar to a PCB, but the techniques should also be valid for aluminum.

  • First of all, the bus bar should be plated. Aluminum especially, is going to develop an insulating film if you don't plate it.

  • The pad on the PCB side should also have a plating that will not corrode if left exposed for extended periods of time (so not bare copper).

  • To make electrical contact with the PCB, we included a large, exposed, plated metal area under the entire bus bar (with a bit of clearance on either side to account for tolerances).

  • We used several screw holes in the bus bar that would clamp the bus bar to the PCB. In our particular case, it was four holes down the center of the bar, and the PCB used PEM nuts to receive the screws on the board side.

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    \$\begingroup\$ Would you say that standard ENIG is adequate for plating the PCB? Or would you go with something more mechanically robust? I'm assuming tin or tin-lead HASL is inadequate as it'll still oxidize over long periods. \$\endgroup\$
    – Hearth
    Jan 12, 2023 at 16:13
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    \$\begingroup\$ We used standard ENIG on our boards. We produced thousands of units and have not had any problems reported over the years they have been in service. \$\endgroup\$
    – user4574
    Jan 12, 2023 at 16:23
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    \$\begingroup\$ Gold plating in contact with bare aluminum is a bad idea: "purple gold" intermetallic will form. Tin or nickel plated aluminum should be fine. Note that PCB material cold-flows over time, so do something to maintain clamping tension -- Belleville washers are a good idea. \$\endgroup\$ Jan 12, 2023 at 16:47
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    \$\begingroup\$ So aluminum plated will nickel? And then press the bar against the PCB gold plated pad, tighten with several screws \$\endgroup\$
    – TQQQ
    Jan 12, 2023 at 19:14
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    \$\begingroup\$ That sounds correct. \$\endgroup\$
    – user4574
    Jan 12, 2023 at 19:32
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Could you use something similar to the below picture. Providing that the screw Terminal is within the current limits & the PCB is designed correctly.

I haven't personally used them in my designs, but have seen them in a number of high current products.

The pictured screw terminal is manufactured by Wurth Elektronik, but multiple manufacturers have similar products & the likes of Mouser/Digikey stock them.

PCB THT Screw Terminal

(Image source: RS Components)

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    \$\begingroup\$ Right. I tried using these once, then i discovered that not every press can fit them in. My manufacturer in China just shrugged on me :) \$\endgroup\$
    – TQQQ
    Jan 12, 2023 at 1:27
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    \$\begingroup\$ Nice. There are some adequately rated ones. Soldering a THT part like that to thick wide copper traces would require proper equipment. \$\endgroup\$ Jan 12, 2023 at 2:05
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    \$\begingroup\$ I have also used some of these Wurth red-cube type press fit terminals. They work well and have good current ratings for their size. In our case we needed some custom ones (identical to existing, but longer), and they were happy to make them for not too much extra cost. \$\endgroup\$
    – user4574
    Jan 12, 2023 at 19:36

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