The spec smacks of contact requirements (5 µm Sn/1.3 µm hard Au), not 
for soldering. I'm not convinced that gold-over-tin is a good idea, as 
diffusion will cause intermetallics to form which will cause the gold to 
flake off under mechanical stress. Furthermore, 5 µm tin will not even 
act as a barrier layer to the underlying copper. I believe this is a 
formula for catastrophe in the long term. Like the other person, I'm 
convinced that this is an error, a 5 µm barrier layer of nickel under 
the gold making a LOT more sense.

Brian

Franklin Asbell wrote:
> Received an unusual request for quote with a fab note callout for the
> following.
> 
> 
> 
> 1.      Plate 200/50 microinches SnAu (very hard)
> 
> 
> 
> 
> 
> Now I read that as 200 microinches of tin plating, followed by 50
> microinches of Gold and thought to myself..huhhhh!!!
> 
> 
> 
> I contacted the buyer and he agreed it was quite unusual but they indeed
> need gold over tin. The buyer could not explain why, so I thought I would
> see if others have performed this process, and why such uncommonly mixed
> coatings???
> 
> 
> 
> Franklin
> 
> 
> 
> 
> 
> 
> 
> 
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