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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).

ocrdu
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TQQQ
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    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. – Bryan Jan 12 '23 at 00:01
  • I updated the question. I plan to make my own bars – TQQQ Jan 12 '23 at 01:26
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    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. – Neil_UK Jan 12 '23 at 06:26
  • 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. – Hearth Jan 12 '23 at 15:52
  • @Neil_UK well, both, but cost is certainly more dominant. If that would be copper, would it be easier to solder? – TQQQ Jan 12 '23 at 19:18
  • @Hearth it would be some alloy, though I didn't chose anything specific yet. After all, I am an electronics engineer, for me they are all iron ;) – TQQQ Jan 12 '23 at 19:19
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    Prompted by @Neil_UK's comment I've just looked [at a massive table online](https://ndtsupply.com/media/Conductivity_Al%20Reference%20Chart.pdf) and the variability is massive (pure Al: 61% of copper, common 6061-T6: 40%, some alloys as low as brass at around 27%) – Chris H Jan 13 '23 at 13:55

4 Answers4

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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}

Davide Andrea
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    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. – Hearth Jan 12 '23 at 15:57
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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.

Tim Williams
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    So would this all be solved by copper bars instead? – TQQQ Jan 12 '23 at 19:16
  • @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. – Mołot Jan 12 '23 at 20:58
  • 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. – Tim Williams Jan 12 '23 at 21:20
  • How would you solder such a mass of copper? – TQQQ Jan 12 '23 at 21:44
  • @TQQQ With suitable equipment of course, wave soldering for example. Reflow soldering may be feasible. In my lab, I might use a hot air machine and soldering iron or two, maybe fiberglass insulating pads on the busbar to keep it hot; a torch can even be used, with great care not to burn the PCB or rosin/flux of course. – Tim Williams Jan 12 '23 at 21:57
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    @TQQQ look at how plumbers solder copper pipes with a blowtorch! not sure if you can do that without hurting the PCB, but still – user253751 Jan 13 '23 at 12:20
  • :D absolutely!! – TQQQ Jan 13 '23 at 13:01
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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.

user4574
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    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. – Hearth Jan 12 '23 at 16:13
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    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. – user4574 Jan 12 '23 at 16:23
  • Excellent, I like it when the cheap options turn out to work. Though I suppose if it didn't, conformally coating the joint would also probably be adequate, and cheaper than going to a more specialized plating. What was the aluminum plated with, incidentally? – Hearth Jan 12 '23 at 16:27
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    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. – Tim Williams Jan 12 '23 at 16:47
  • So aluminum plated will nickel? And then press the bar against the PCB gold plated pad, tighten with several screws – TQQQ Jan 12 '23 at 19:14
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    That sounds correct. – user4574 Jan 12 '23 at 19:32
  • Tin or HASL on the PCB would also be acceptable I think, with tin or nickel plating on the busbar. – Tim Williams Jan 12 '23 at 21:25
  • @user4574 for the copper bar, did you simply press it against the PCB? Did you need to somehow polish the copper bar? – TQQQ Jan 27 '23 at 05:26
  • @TQQQ The bar is pressed against the PCB by the screws. It is plated and very flat. Its polished enough that I can clearly see my reflection in the one I just looked at. But I don't have a specification for surface roughness that I can give you. – user4574 Jan 28 '23 at 04:09
  • Have you tested these over time? – TQQQ Jan 28 '23 at 05:59
  • @user4574 i have a few more question, thank you for your patience :) How do you transfer current from other elements to the bar? Do you use multiple vias and layers, or a short strip of copper on top layer is good enough? And also, do you cut the PCB anywhere to compensate for different thermal expansion? – TQQQ Jan 28 '23 at 15:40
  • In response to "Have you tested these over time?" There have been a few thousand units produced over the last few years and I have yet to hear of any failures. We did high temperature testing. But we don't have data for what happens in 10 or 20 years or anything like that. The units were designed for use in harsh environments, but I don't have data regarding how many hours our customers used each unit. – user4574 Jan 29 '23 at 02:25
  • In response to " How do you transfer current from other elements to the bar?". In our case we brought in 500A and distributed out to multiple loads that were each in the 30A ~ 60A range. At each distribution point there were many (like 10~20) filled vias positioned under the bus bar. The current was then brought out on several internal layers of 3oz copper. Etching the pads for fine pitch parts and BGAs requires 1oz or 1/2 oz copper on the outer layers, so we had 1oz outer layers. One could use 1oz instead of 3oz, but its 3X more layers and probably more cost. We did not cut the PCB. – user4574 Jan 29 '23 at 02:33
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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)

SamGibson
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J16
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    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 :) – TQQQ Jan 12 '23 at 01:27
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    Nice. There are some adequately rated ones. Soldering a THT part like that to thick wide copper traces would require proper equipment. – Spehro Pefhany Jan 12 '23 at 02:05
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    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. – user4574 Jan 12 '23 at 19:36