Soldering tungsten to silver wire for electrode

[Michael Zions]

Interesting situation here. I want to use a tungsten electrode to stimulate neurons. (The electrode is actually an Austrian shirt pin from WW II, I think.) I'm uncertain of the provenance of these pins or whether they are pure tungsten or an alloy.

Anyhow, I have a silver wire which I need to solder to this tungsten, although I can fall back on copper wire. But the tungsten is just not wetting. At any temperature and with several regular electronic solders, the surface blackens immediately and the solder balls up and falls away. I've tried brushing on rosin and it hasn't helped, nor has sanding the tungsten first.

So, at this point I'm considering a. finding some jewelry solder with high silver content. I have no idea if this will help but it's an option. b. using plumbing solder with an acid core. Again, maybe this would help but I don't know. c. Third option is to chemically treat the surfaces first - but with what? An acid? A base? Some other solvent?

Has anyone here successfully soldered tungsten into a good electrical circuit?

Looking forward to suggestions and ideas.

[Robert Joseph Korn]

Can you connect it by spot welding?

Kind of like how the tungsten filament I'm a light bulb is connected

Wrap the wire around it then hit it with a pinch type spot welder

Best regards,

Bob

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Robert Joseph Korn

Director of Enterprise Operations and Infrastructure

Enterprise Systems Group

Pratt Institute

rk...@pratt.edu

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[Justin Eastman]

http://hackaday.com/2011/11/06/diy-spot-welder-can-join-anything-together-even-copper/

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Subject: Re: [NYCR:Microcontrollers] Soldering tungsten to silver wire for electrode
From: ko...@pratt.edu
Date: Wed, 26 Feb 2014 15:19:40 -0500
To: nycresistormi...@googlegroups.com

[Robert Joseph Korn]

Or maybe copper or tin plating the tip first?

A good conductive wire glue?

Suspend it between good binding posts?

Best regards,

Bob

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Robert Joseph Korn

Director of Enterprise Operations and Infrastructure

Enterprise Systems Group

Pratt Institute

rk...@pratt.edu

[Robert Joseph Korn]

Just google "mot spot welder"

Or 

microwave oven transformer spot welder

Really easy to make

I just bought one from Harborfreight since I'm lazy

They have 110 and 220 models for the same price

It can weld 1/2 steel rod so it may be a bit much for the ip's use

http://m.harborfreight.com/240-volt-spot-welder-61206.html

Best regards,

Bob

--

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Robert Joseph Korn

Director of Enterprise Operations and Infrastructure

Enterprise Systems Group

Pratt Institute

rk...@pratt.edu

[Michael Zions]

Fantastic ideas. I'm going to try spot welding if plating doesn't work first. I tried plating last night but got too thin a film to solder. I'll try again in a few minutes.

[Robert Joseph Korn]

If it's as hard as  a tig welder electrode you can grind down the ends to a point and screw on a brass or soft steel nut then solder to that.....

Best regards,

Bob

--

--

Robert Joseph Korn

Director of Enterprise Operations and Infrastructure

Enterprise Systems Group

Pratt Institute

rk...@pratt.edu

--

[raphael]

Also consider ultrasonic welding. I don't know it it works with all
metals. Also, I have no idea where to do it on a small scale.

Or a simple crimp connector?

Keep us posted on your results!

[Michael Zions]

Crimp connector is a problem because of the low voltages we use and the precision we need. Think 500 millivolts tops, usually more like 100 millivolts, along with just how very fine the two wires are. The silver wire and tungsten probe are both around 30 AWG. I'm not confident that a crimp won't raise impedance or allow corrosion inside and the resistance will drift after a while. I intend to use the electrode produced by this project for at least a year. I do use crimps on other, less mission-critical stuff.

The spot welding ideas will probably be useful and I can get away with a really small welder, again because of the tiny wires. I don't want to vaporize the silver, which has happened in the past when using a small torch.

So, an update on the copper plating: It seems to be progressing well. I have DC voltage running through a copper sulfate / water solution, with the tungsten as the anode and a bundle of copper wire as the cathode. I initially used way too high a voltage and was electrolyzing the water; I got fizzing bubbles and a rapid buildup of some dark grains on the anode. I finally found some nearly-dead AAA batteries and a battery holder and ran that for an hour; I found a faint coppery sheen on the tungsten but it still rejected solder. I've moved the voltage back up a bit and seem to have found a sweet spot. Now there is a healthy layer of copper being laid down and I will try to solder onto that after the weekend.

Thanks all, and I'll post an update on Monday.

On Wednesday, February 26, 2014 12:42:29 PM UTC-5, Michael Zions wrote:

[raphael]

Amazing!

Post some pictures! This sounds like a really good process to share.

[Robert Joseph Korn]

Beware the spot welding method on something that small. From what I understand the reason you wrap the steel wire around the tungsten is because you never really melt tungsten in a spot weld. You just melt the wire around it and encapsulate it. 

I am told I over engineer everything so If it were me I'd plate with copper then dip in tinnit pcb solution then wrap the wire around the plated tip and cover with a crimp made from a small brass tube then solder it through the open end of the tube with silver bearing solder then heat shrink or plasti-dip the joint so I don't have to see it 

That way you get good electrical and mechanical bonding and corrosion resistance

you can get tiny brass tube in model or art stores  for 30ga even a piece of copper capillary tube from an old fridge would be good to make a crimp from

Best regards,

Bob

--

--

Robert Joseph Korn

Director of Enterprise Operations and Infrastructure

Enterprise Systems Group

Pratt Institute

rk...@pratt.edu

--

[Michael Zions]

I'm going to lay in a supply of brass tubing for crimping, or maybe even some brass sheet so I could roll my own size sleeves.

I'll post pics on Monday, but the best update - ten minutes before I left, right after I packed my bag - I pulled the tungsten from the electroplating solution and found a uniform layer of shiny copper on it. I warmed up an iron - and it soldered beautifully! I now have at least one ready electrode, plus a method to make many more. I'll record specifics when I repeat this on Monday.

I am still going to make a spot welder, though.

Michael Zions

[Michael Zions]

Ok, I've delayed updating this thread because I don't know what went right. Seriously.

First, what I know - I have twice successfully electroplated what appears to be copper onto what I believe to be tungsten wire, and I know that after I plated it I was able to solder it easily. Before that it would just reject solder.

I dissolved copper sulfate in water the first time; the second time around I did the same but eventually added some dilute acid. Both times I used a couple of nearly-dead AAA batteries to provide a weak ~1 volt current between a copper wire cathode and a tungsten anode.

But here's the problem. On both occasions, I met with no success for a long while at the beginning. The tungsten was clean; on the second attempt I scrubbed it with alcohol and scraped it down as well. But I had it plugged in for hours with no visible copper deposition. Someone half-jokingly suggested that I replicate the steps which had led to my initial success; I had once tried connecting a 9-volt battery and was quickly met with a rapidly growing black deposit on the tungsten, as well as a great deal of bubbles from the electrolysis of water. With my best scientific reasoning I figured "why the hell not" and hooked up a 9-volt. Of course there were bubbles, of course the black deposits appeared. After a minute I wiped the tungsten clean and reconnected the weak AAA batteries, exactly as before.

Within seconds, I had a layer of copper.

What exactly is going on? Did I blast something clear of the metal surface? Did I inadvertently reverse some corrosion, or force copper ions into the solution? Why in the world would a brief pulse of relatively high voltage make a difference?

For what it's worth, I have pictures. The first attempt was in a glass beaker, the second in a Tupperware, but I think I will go back to glass. I'll post the pics in a separate reply to keep things neat.

[Michael Zions]