Author Topic: Grounding aluminium PCB substrate  (Read 2184 times)

0 Members and 1 Guest are viewing this topic.

Offline Amper

  • Frequent Contributor
  • **
  • Posts: 266
  • Country: 00
Grounding aluminium PCB substrate
« on: January 30, 2019, 06:40:56 pm »
Hi!

Im recently doing quite a few high power circuits on aluminium backed substrate and since its mostly lower voltage stuff id like to ground the pcb or even use it as a groundplane.

Is there any industry accepted method for doing so? Id like to glue the pcb to a heatsink with thermally conductive epoxy, screws are usually difficult inn my applications.

I thought about using soft aluminium solder like its some times used on lithium pouch cell terminals. Just drilling through the dielectric in the required place like a via, put a drop of the special solder in the crater and reflow the entire board after assembly of the components.

Does anyone have experience with this kind of think?

cheers A.
 

Offline viperidae

  • Frequent Contributor
  • **
  • Posts: 306
  • Country: nz
Re: Grounding aluminium PCB substrate
« Reply #1 on: January 30, 2019, 11:41:00 pm »
To solder aluminium, don't you need to remove the oxide coating first? The one that forms in seconds when exposed to air.

Googling how to solder aluminium comes back with results that suggest it's quite difficult without specialised machines
 

Offline Amper

  • Frequent Contributor
  • **
  • Posts: 266
  • Country: 00
Re: Grounding aluminium PCB substrate
« Reply #2 on: January 30, 2019, 11:53:29 pm »
There is special solder with an unusual flux core that can break that oxide layer and solder aluminium like bare copper using a normal soldering iron.
 

Online Alex Eisenhut

  • Super Contributor
  • ***
  • Posts: 3072
  • Country: ca
  • Place text here.
Re: Grounding aluminium PCB substrate
« Reply #3 on: January 31, 2019, 12:08:15 am »
You can buy flux for lead soldering to aluminum and it works quite well, but it's hell on the iron tips.
Hoarder of 8-bit Commodore relics and 1960s Tektronix 500-series stuff.
 

Offline ConKbot

  • Super Contributor
  • ***
  • Posts: 1312
Re: Grounding aluminium PCB substrate
« Reply #4 on: January 31, 2019, 06:38:44 pm »
You want a "direct thermal" "direct copper" or "copper pedestal" MCPCB. It is used in better quality MCPCBs for LEDs. The thermal pad in the center of LED footprint is copper plated onto the core directly, rather than isolated by the dielectric layer. This will give you a nice selectable copper pad rather than having to deal with nasty corrosive fluxes for aluminum.
 

Offline Amper

  • Frequent Contributor
  • **
  • Posts: 266
  • Country: 00
Re: Grounding aluminium PCB substrate
« Reply #5 on: January 31, 2019, 09:19:36 pm »
mhhhhh but they dont come cheap. Im looking more for some kind of homebrew solution.
 

Offline Ian.M

  • Super Contributor
  • ***
  • Posts: 11532
Re: Grounding aluminium PCB substrate
« Reply #6 on: January 31, 2019, 10:11:51 pm »
Any flux that can solder aluminum without mechanical abrasion through the solder pool will destroy ceramic bodied components (Aluminum oxide is the ceramic alumina) and trace contamination with its residue will corrode copper tracks, iron and nickel component terminals etc.

OTOH, for hand assembly or rework, if you scrape the aluminium board edge while its wetted with a  few drops of a high smoke-point oil to prevent the oxide film quickly reforming, then immediately solder through the oil film with a scraping action with the bit, you can easily get the aluminum to tin with ordinary Sn60Pb40 or Sn63Pb37 RMA flux core solder.  Once one spot tins you can easily expand the tinned area sideways by keeping it fluxed with rosin, and rubbing with the bit, adding fresh solder as required, as the solder alloys with the aluminum, disrupting the aluminum oxide film at the edges of the tinned area. 

Wipe off excess solder while molten, degrease and deflux, then solder copper tape over the tinned board edge and an adjacent ground track to link them.
« Last Edit: February 01, 2019, 05:35:08 am by Ian.M »
 
The following users thanked this post: SeanB, cdev

Offline rounin

  • Regular Contributor
  • *
  • Posts: 102
  • Country: us
Re: Grounding aluminium PCB substrate
« Reply #7 on: February 01, 2019, 05:11:48 am »
I've used KappAlloy brand ZincTin solder with the Kapp Golden Flux (it is kinda nasty stuff) to solder copper/tin to aluminum. Was actually pretty easy, but it did somewhat nasty things to the iron. Use an old iron : ). I was soldering leads to giant size Al battery tabs. Didn't really have an issue with the oxide reforming, but I soldered through the wet flux.

I don't remember if it was  KappAlloy9, KappAlloy15, etc. Probably KappAlloy9 though, that seems to be the "default" one.
https://www.kappalloy.com/solder-alloy/kappaloy-solders/
 

Offline Amper

  • Frequent Contributor
  • **
  • Posts: 266
  • Country: 00
Re: Grounding aluminium PCB substrate
« Reply #8 on: February 01, 2019, 07:13:30 pm »
Mh, somehow i completely overlooked the corrosion issue. But as long as i wont solder the pars with this solder? My idea would be to first etch the pcb, then drill the "vias" to the ground plane, put a snip of solder in the holes and reflow. Then clean thoroughly and assemble the parts and bus bars and reflow again. As i understnad its only the flux and not some special alloy that will degrade the parts.

Does anyone know if the dielectric is suceptable to the flux too?
 

Offline Amper

  • Frequent Contributor
  • **
  • Posts: 266
  • Country: 00
Re: Grounding aluminium PCB substrate
« Reply #9 on: November 30, 2019, 05:19:14 pm »
Hi!

Just a short update,  i finally got around to trying this method and it worked out nicely. I just took a standard 3mm drill bit and sunk it through the ground plane into the aluminium until the cylindrical part was about to start. Then i preheated the board to roughly 150°C and tinned the dimple with the aluminium solder, then whiked it away again and filled the dimple with normal electronics solder (good ol 6040). The connections seem to be reliable for now, the future will show how they will hold up, but since no flux is left there should not be an issue. Soldering into a freshly drilled hole probably also helped.

Maybe for larger numbers working in argon atmosphere will completely remove the need for all of this, or just spot welding some sort of cup.

Anyways, grounding the substrate turned out to be absolutely necessary.
 

Offline coppercone2

  • Super Contributor
  • ***
  • Posts: 6546
  • Country: us
  • $
Re: Grounding aluminium PCB substrate
« Reply #10 on: November 30, 2019, 07:11:00 pm »
aluminum solder is shit, sand it flat, use the proper aluminum solder and ultra smelly agressive flux and scrape it with a sacrificail iron to get some kind of wetting, then sand the nasty oxide off both tinned surfaces, hold them close, reflux, remelt, should hold (poorly), getting it to 'flow' is like convincing a cat to do something (maybe dripping hydrochloric acid on it and washing would work well )

I never got it to 'look nice' its like a bad weld job where you need to sand it afterwards



I kind of wonder how a rivet would work if you put a star washer for it to compress against , something like pop rivet, crimp, star washer, aluminum

flux matters:
https://superiorflux.com/portfolio-item/tensile-strength-testing-3003-aluminum-soldered-lap-joint/

what I like doing with aluminum braze is low strenght frames for filters to put over chassis holes. you glue window screen material into an aluminum frame (you can cut the aluminum flat bar with bolt cutters, arrange it in a shape on a fire brick and braze/solder it together very quickly, since you are putting glue and filter material over it, it does not need to look nice, and materials use is minimal since you are not cutting holes in sheets etc use thicker flatbar so the edge joint has some strenght and cut it on 45 degrees, but I am setup to do this quick with a proxxon mini-saw

i put a rubber gasket under neath I cut out of things like foormat or toolbox mat liner material around the frame.

obviously not a EMI screen
« Last Edit: November 30, 2019, 07:30:30 pm by coppercone2 »
 

Offline Amper

  • Frequent Contributor
  • **
  • Posts: 266
  • Country: 00
Re: Grounding aluminium PCB substrate
« Reply #11 on: November 30, 2019, 08:48:16 pm »
Mh, i didnt have any of those problems. The solder i have is used just like normal electronics solder with flux core. It boubles sligtly more but it wets  the surface beautifully and the joint is just as good as with any lead free solder on copper.

What i do wonder though is if doing the entire thing under argon could make flux redundant entirely, in this case it would be possible to do this in the pcb manufacturing process commercially by just drilling half way and setting large bga balls down on the holes to melt later.
« Last Edit: November 30, 2019, 08:52:34 pm by Amper »
 

Offline coppercone2

  • Super Contributor
  • ***
  • Posts: 6546
  • Country: us
  • $
Re: Grounding aluminium PCB substrate
« Reply #12 on: December 01, 2019, 03:18:30 am »
can you post a picture of a beautifully wetted aluminum solder joint, it might make me reinvestigate the process, right now I am totally fed up with it
 

Offline Amper

  • Frequent Contributor
  • **
  • Posts: 266
  • Country: 00
Re: Grounding aluminium PCB substrate
« Reply #13 on: December 01, 2019, 11:33:00 am »
Here you go, some copper i found on the table soldered to a piece of .1mm aluminium drill cladding. Soldered with Rexin 240 using a normal soldering iron, didnt ruin the tip, though i clean it after soldering.

 
The following users thanked this post: SilverSolder


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf