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topic 7453

AuSn Eutecticum


I trying to find the minimum of a layer combination to get a AuSn (80/20) eutecticum.

Is it possible with 80nm Au and 20 nm Sn to reach a eutecticum? Or where is a physical end?

Martin Priester
- Stuttgart, Germany


Hello Martin,

I don't really understand the question. I'm familiar with eutectic and eutectoid, eutectium is similar? Are you going to deposit separate layers of Au and Sn and then heat them, to cool gradually forming laminated phases within Au/Sn crystals?

If you simply want a gold/tin alloy deposit there are electrolytes for this, and I think I have one somewhere.

Ian Brooke
university - Glasgow, Scotland


Hello Ian,

I want to deposit Au and Sn by electroplating of Ni as start layer. Actually I want a layer flash gold on Ni then Au/Sn and on the top a thin layer of Au again by electroplating. Together a thickness of 3 µm or less. My question is how thin can I do this to get a eutectoid composition?


Martin Priester
- Stuttgart


An alloy of 80:20 is by weight, 80% Au and 20% Sn. Next, use Avogadro's law that states 1 cu. cm of material contains 1 gm molecular weight of that material. Using this basic formula, extrapolate this to the thickness and figure out what relative thicknesses you need for each metal. This is your home-work for tonight.

Mandar Sunthankar
- Fort Collins, Colorado


You can deposit separate layers that will be equivalent to the 80-20 eutectic composition for gold and tin, but I doubt that you will be able to manipulate the layers to get a eutectic structure. The eutectic structure, by definition, forms from liquid. My guess is that if you wait long enough or apply heat to get diffusion, you will end up with a complex structure of intermetallic compounds.

Look at the phase diagram at to see the complexity of the allowing for these two elements. Note that the 80-20 ratio is in weight percent, so you will have to correct for density to determine the relative thickness of the layers to get the ratio that you seek.

Good luck. I hope this helps.

larry hanke
Larry Hanke
Minneapolis, Minnesota


Martin Priester, my apology for misleading you. That "Avogadro" formula is incorrect. The mistake was pointed out by Tom Pullizzi. Using just the density - volume relationship, you may find the relative thickness required for each metal. Beyond that, I do not know if it works but you may have to try it.

Mandar Sunthankar
- Fort Collins, Colorado


Hello Martin

I don't think we're being very helpful.

The information I think you are after is how thick are the phase layers of the Au-Sn alloy. I do not know but at a guess 3 micro millimeters is enough. Mircostructures can be manipulated by heat/cool conditions

However that's not likely to be the structure you will get from an electro deposit. In my experience the thermal equilibrium phases do often show up in electrodeposits, however so do other phases.

If you are very careful it may be possible to electrodeposit and then heat and cool the alloy to form the phases but then I'm sure your flash base layer of Au will diffuse into the rest of the alloy. Maybe you could live with that.

I get the distinct feeling you have so far not heard anything you did not already know.


Ian Brooke
university - Glasgow, Scotland

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