Electrically attaching to Aluminium

[Curious P]

Hi guys,

Not sure whether this belongs in elec or mats/chem so I thought I'd put it here.

Basically I am trying to extract current from a thin aluminium layer.
Due to the Al-Oxide layer, simply soldering an external lead isn't working.

I'm able to etch away (some?) of the oxide layer but even then, soldering is still very dificult/impossible. Ultrasonic soldering has had some (limited) success but the yield has been very poor (1 in 10-15 attempts actually gets a decent bond)
The solder's I've tried are the lead free variety (99.5% Sn, rest Cu) and regular 60/40.

I've read of some special Al-solders but they seem to be 60%+ Pb? (trying to stay away from it)


Another approach I've looked into is electrolysis or electroless plating. However a few of the companies and businesses I've contacted have told me they don't do it and it's very difficult.
In saying that I haven't actually tried it myself (most promosing might be electroless nickel plating?)

Any ideas out there / Any approaches I'm missing?

Thanks for reading

 

[berkeman]

The two possibilities that come to mind are welding and mechanical contact. You can weld aluminum with some special welders (TIG maybe, I'm no expert on welding by any means):

http://www.lincolnelectric.com/knowledge/articles/content/alum.asp

Is your Al layer thick enough to weld to, or is it too thin for that?

The other possibility is mechanical contact, like with spring fingers or a screw-type clamp. The contact needs to be high pressure enough to be gas tight (so Al oxide cannot form), and it is a good idea for the mechanical contact to be at multiple high-pressure points, rather than spread across an area. That's why a star washer is a better choice for an electrical contact at a bolt hole, rather than a flat washer.

Would either of those work? I may move this thread to more of a chemistry-oriented forum for better views at some point.

 

[zeitghost]

The people who make electrolytic capacitors must have this one licked...

The leadout wires are presumably pressure/temperature ?spot? welded to the aluminium foil inside the capacitor.

How much further that gets you I don't know.

I do remember brazing two bits of aluminium together, and they stayed together for 10 years before the flux ate into the joint & it fell apart.

 

[Curious P]

Appreciate the responses guys.
The layer is about 2microns thick, supported by glass.
I don't think anything that melts the material would be suitable

Mechanically compressing sounds interesting. I will look into that, but another problem is that the Al is about 1mm wide (by an arbitrary length)

If you think it belongs in a Chemistry based forum that's fine, thanks.


Thanks for the info on the Al electrolytic caps (at least someone has a use for that oxide layer!), I'll check out how they are connected there aswell.

Any more practice or suggestions are much appreciated

 

[berkeman]

I'm moving this thread from General Engineering to Materials & ChemE to see if there are other ideas for Curious P.

 

[chemisttree]

I believe you would need to galvanically coat the aluminum with zinc first, a process known as 'zincating'. The procedure would be something like 1) wash with mild alkaline cleaner (sodium bicarb with a little sodium carbonate added) 2) dip rinse quickly in water 3) followed by an acid wash (3 sec, dilute nitric) and finally 4) immersion into a stirred zinc oxide solution (30 sec to 1.5 minutes). This will leave you with a very thin zinc coating on the aluminum which can be electroless coated with gold or nickel. Zincating solutions and procedures should be commercially available in kits such as that by http://www.caswellplating.com/kits/zincate.htm"

 

[f95toli]

Appreciate the responses guys.
The layer is about 2microns thick, supported by glass.
I don't think anything that melts the material would be suitable

Sounds like your samples are very similar to the ones I am using (although my samples are of course patterned). Anyway, the best way to do this is to use photolithography to patterns some gold pads on your samples, this is the standard method of doing it and had the advantage that the contace resistance is pretty low.
How are your films deposited? Would it be possible to deposit e.g. 100-200 nm of gold on top of the Al in-situ? Maybe with a thin sticking layer of e.g. Ti. You can then etch away Au from parts of the sample and define the pads that way. Alternatively, if you have access to a deposition system with in-situ ion beam milling (to remove the oxide) you might be able to deposit some gold on existing sample. This would be the simples way of doing it and you might even be able to use a mechanical mask (i.e. no need to spin resist etc)

If you don't have access to a clean room I would also suggest some mechanical method, if you are only interested in room temperature measurments even a simple probe station with sharp probes might work.