Join Cu to Au/Pt...

[wiss]

For a project I will need to join small pieces (1 mm diameter, 5 mm long) of gold and platinum pins to a wire (Copper would be handy).

What method would be best?

Any kind of soft solder (Sn-free)?

Welding?

Conductive glue?

[helius]

search for "gold embrittlement", it's a definite problem when soldering to gold parts. Most device leads that use gold only have a very thin layer, a few microns at most, and less for "flash" gold which is the thinnest visible layer. Normally there is not enough gold in this layer to cause severe embrittlement. But when more gold is present then soldered joints will be very brittle. I don't know about platinum but expect it acts similarly to gold.
I do not have direct experience in this area, but it seems like the preferred solders for use with gold are indium-lead alloys such as In70Pb30.

[wiss]

The pins are not coated but pure solid Au or Pt, any Sn-solder is out of the picture.

[Stray Electron]

  Tektronix used to recommend silver loaded solder to avoid embrittlement when soldering to gold. In fact, back in the 1960s they used to give a small amount of silver loaded solder away with their scopes so that user's WOULD use it. IIRC it contained about 2 or 4% silver and IIRC the remainder was 67/33 lead-tin alloy.

   You should go look on Kester's (or other solder manufacturer's) website and see what they recommend. If you can't find it (which would surprise me!) then contact them and ask.

     FWIW I bought up a bunch of Kester 2% silver solder years ago and I use it on everything. I LOVE that stuff!

[LaserSteve]

Talk to Indium Corp, they are more likely to have something then Kester.

   In 60/40 Tin/Lead  solder, fine gold fires dissolve amazingly fast, faster then you can see without a high speed camera.  Twenty years ago,  I had 40 modules with broken bond wires to repair, I ended up with ~25 working.  The customer was thrilled even with that 50% rate of recovery, because this is a difficult task without the exact correct alloy.

 I ended up with a  home made tin/silver alloy that was less then optimal and turned grey when it cooled.

If I had to do it again, I'd certainly  buy the correct materials.

1 mm wire has better far  stability then the bonding wire, but I would still find the correct alloy.   

Steve
 

[wiss]

So silver saturates tin? That was useful information! 

Sn95.5/Ag3.8/Cu0.7 is available from my regular supplier, this would do?

[robrenz]

Radio Shack sold 62/36/2  silver bearing rosin core solder.

Edit: I guess that wont do you any good in Sweden 

[tautech]

RS has plenty of solders with various silver contents:
http://nz.rs-online.com/web/c/tools/soldering-desoldering-tools/solders/?searchTerm=solder

[wiss]

Radio Shack sold 62/36/2  silver bearing rosin core solder.

Edit: I guess that wont do you any good in Sweden 

Under state purchase requirements as well in this case

Absence of lead would be a plus, and since our contracted supplier carry the Sn/Ag/Cu-solder I will try that!

[LukeW]

This sounds like a job for the Big Wikipedia Table of Exotic Solders.

https://en.wikipedia.org/wiki/Solder

Au80/Sn20 is recommended for gold, as is In100 (pure indium).
They're also the only alloys of choice if you must have Pb-free.

In70/Pb30 and In75/Pb25 sound OK for gold.

In80/Pb15/Ag5 is also gold-compatible.

[LaserSteve]

Under state purchase requirements as well in this case

If it makes you feel better, so am I... Dreaded rules that industry no longer follows..

Steve

[Alex Eisenhut]

  Tektronix used to recommend silver loaded solder to avoid embrittlement when soldering to gold. In fact, back in the 1960s they used to give a small amount of silver loaded solder away with their scopes so that user's WOULD use it. IIRC it contained about 2 or 4% silver and IIRC the remainder was 67/33 lead-tin alloy.

No, Tektronix specified it to solder to the silver-plated ceramic strips they used back then. If you used too much regular solder, the strip would lose its plating and you'd have to replace it.

http://www.vintagetek.org/ceramic-strip-soldering-techniques-1964/

http://www.reprise.com/ash/clients2/parts_shop/ceramic_strips.html

The only gold back then was on the leads of the tunnel diodes, and on the early PCBs. SnPb was good enough for that.

[wiss]

Found this:
http://buy.solder.com/Indium-Wire-99-99-Indium-3-ft/P159_61/.

The sizes:
"2.815 kgs/Anode" @ 95$, is this a block of 2815 grams?
"3 ft spool" @ 95$, 3 feet of 0.03" wire for the same cost as 2815 grams? What are those units???

[T3sl4co1l]

http://buy.solder.com/99-99-Indium-Anode/P98_57/ this sounds more like it...

No idea what they mean by that.  If it's $95 worth of anode, it ain't much, that's for sure.

Tim

[wiss]

Yea, that prise makes more sense!