All

Gold and copper form a alloy an the ratio can be mixed in practically any
proportion it is not intermetallic.  Your gold wedding ring is an example of
this and it is not brittle.  Gold and copper due interdiffuse and I beleive
that the copper is more  mobile and will do it faster, at any rate with time
the copper ends up on the surface   Kirkendall has some equations for this.
The difffusion also occurs between gold and nickel, but it is at as slower
rate than with the gold copper.  Diffusion also happen between copper and
nickel, also at a slow rate.

The intermetallics that we worry about usually have tin as one of the
elements: Au/Sn, Ag//Sn, Ni/Sn, Pd/Sn an etc and they occur in various atomic
combinations  which has much to do with their phsical properties.  You cannot
classify one group of intermetallics as weak and one as strong since strong
and weak may occur with the same two metals in an intermeallic dependent on
composition .  There are also some Pd/Pb that might have to be considered by
some users.

It is also necessary to consider that many of the elctroplated, electroless
and immersion deposits are not pure metals and codeposited elements or
compounds may be greater than 10 percent of the weight and a considerably
greater percent of the volume. When you form the tin plus other metal
intermetallic with the plated deposit, you form intermetallic compounds of the
tin plus other metal that have great purity and you leave behind the other
codeposited material in a weak layer.  "Brittle" intermetallics often take a
bum rap when the material left behind is really responsible.

Phil Hinton

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