Soldering: Copper to Aluminum

I'm attempting to build a project using the Capacitive Sensing Library. As suggested in the last paragraph of "Grounding and other known issues" section, I put a foil ground plane under the plates I'm using.

However, I get highly variable results. It was very difficult to get the copper wire to solder to the aluminum plates I am using. First, I cut the top and bottom off of an aluminum can then cut that cylinder so that I could flatten it out. I cut that into plates 3 cm by 3 cm. Then I sanded until I had a corner with no paint then tried to solder to the bare section. It took several tries, but eventually to the wire soldered to the plate.

It easily broke off. I used a 25 watt soldering iron. Would it help if I step up to my 45 watt iron? Should I cut the corners so that I could crimp before soldering? Is the plate just too big so that it dissipates the heat too quickly? I've also tried soldering to aluminum foil, with the same result. I've got some thin steel that I could try, if that would solder better. I'm seeking advice before I experiment further.

FYI: I tried the example program for this library (first paragraph), with no changes. I taped the plates down with a wire in contact with the bare corner on the plate, and used 41 MΩ resistors. The three plates that I tried were highly variable and seemed to perform better when they had better contact with the wire. That is why I need to know how to get a good solder connection.

Heat dissipation may be part of the problem, but more likely is the aluminum oxide layer, which reforms very quickly after sanding it off. One way to beat it is to electroplate the aluminum with copper, and then solder to that. Or you could use a mechanical connection with nut and bolt through the aluminum to squeeze the wire against it. Or try the steel.

Aluminium is extremely difficult to solder, due to the layer of aluminium oxide on the metal surface.

You need to use a formulation of solder especially made for aluminium.. Both the metal composition and the flux contained within are specific for the job.

I once had some limited success (back in the 1970s) brazing aluminium to aluminium, but have never even attempted since.

The topic of soldering aluminium is not even covered on the IPC website.

John Hales CIS, Certified IPC Specialist.

It took several tries, but eventually to the wire soldered to the plate.

No you didn't. What you did was to stick the wire onto the aluminum with the flux. This is why it broke off easily.

The simplest way of connecting aluminum to copper wire, is through a solder tag bolted onto the plate. Use an M3 or 4BA nut and bolt. Solder up the tag first before bolting it onto the plate otherwise the plate will suck all the heat out of your iron.

The three plates that I tried were highly variable and seemed to perform better when they had better contact with the wire. That is why I need to know how to get a good solder connection.

Even with a good contact the cap sense library method of sensing capacitance is not very stable.

The easiest way to connect to alu is using a sheet (or a box) and use a ring terminal and a sheet-metal screw (self tapping or self drilling)

Self tapping screws are useless on aluminium got from a drinks can. Way too thin.

Use copper/brass foil.

I have read about, but have not tried this. And you need a pretty good soldering iron that will maintain the temperature. Put a puddle of automobile oil in the area you want to solder. Auto oil will not burn off very quickly. Then use a sharp knife or blade of some kind to scrape the aluminum oxide off the metal under the oil pool. The oil will keep the oxygen away from the aluminum. After scraping as best as you can, begin to use the iron and lead based solder in the oil pool and heat the aluminum until you can melt the solder.

I have seen the so-called aluminum solder used to repair aluminum engine parts. This was a sales demo at an air show in Arlington Washington. I think the solder was zinc based and it did seem to work. A propane torch was used, not a soldering iron.

Paul

Aluminum is difficult because of its high conductivity for heat. It also has a lower melting point than most common metals. As soon as you get a little puddle of molten aluminum then it melts the rest of the structure very quickly.

There must be some solder formulations but they may be toxic and unavailable for the general public.

I like the idea of folding over a corner and crimping the wire.

If you really must use a thin aluminum can as a base, use knife to cut a small “V” is the aluminum, and a hole in the V big enough to feed the copper part of your wire in.

Fold the V upwards, put wire through the hole and wrap back on itself then solder away. Your mostly just making a secure physical connection but that usually works for a quick hobby connection.

There are aluminum solder kits see Amazon (Al Solder Kit) )

to use aluminum can parts I would attache a wire using a screw /star washer / nut. If you want you could cover them with bearing grease or Vaseline (Vaseline could run if hot).

Else I would look at copper or brass flashing from a store like home depot (as was suggested previously)

OR… Maybe tin cans could be used, you should be able to solder to them.

Standard solder does not work on aluminium at all. Special zinc-based solders for aluminium require far higher temperatures, its basically brazing.