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Gold dripping away...


Externet

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In 2007, the electronic industry transitioned from ~60/40 lead/tin to unleaded ~95/5 tin/silver/copper solder.

Its different behavior posed some manufacturing headaches to get used to it, the soldering iron tips grew more deposits due to temperature increase from 190C to 230C and difficult to keep a tinned-'wetting'

 

Decided to experiment, and had a soldering iron tip made of 24K gold.

It was shiny and clean at the operating temperature, did not tarnish. But it dissolved and dripped together with the molten tin instantly.

 

If the gold melting point is 1065C, how can that happen at 230C ? The puddle for shure contained the gold within the tin, liquid until solidified.

Can someone explain what reaction goes on ? Is there an industrial gold processing that makes advantage use of this low temperature melting? How difficult posterior separation from tin be ? Does it decant/sink while in the molten alloy ?

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The gold dissolved. (You already gave the answer yourself).

 

It is similar to sugar dissolving into your tea: sugar has a (theoretica) melting point far above the boiling point of water. Nonetheless, it will dissolve even at room temperature. The temperature at which a component can dissolve has nothing to do with the melting point.

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Sodium chloride melts about 800C, but you don't get surprised when it dissolves in water at 20C.

Why are you surprised that gold dissolves in molten tin?

I don't know of any use for that mixture, but mercury is used as a solvent for gold in some processes for extracting it from the ore.

The mercury is subsequently boiled off, creating an environmental screw-up.

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Gold is known for that behaviour, silver as well. Tin wets it very easily because gold doesn't form an oxide layer, but thereafter, bye-bye gold.

 

You need a different metal that forms no oxide layer but resists corrosion. One choice is palladium, used on the caps of microwaves components for being nonmagnetic. Nickel is an other, much cheaper.

 

In case nickel isn't expensive enough for you, you may try cobalt, molybdenum and few more.

 

Beware nickel layers can be electrolytic or electroless, and in one case, it contains phosphorus up to 13% - then I doubt tin wets it easily.

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