Gold is very solderable, and just as important, very coplanar.  It is 
increasingly becoming a popular choice for SMD fine pitch board 
assemblies.  With gold, there is always a concern for embrittlement.  
However, if the thickness remains thin, this is not a great concern.

When using electroless or electrolytic nickel, you have a reasonable 
thickness for a barrier plate over the copper.  Immersion gold will 
only leave a thickness on top of the nickel of 2 to 8 micro inches.  
This is ideal for SMD attachment.  When using electrolytic gold, the 
thickness is greater and much more difficult to keep on the very thin 
side.  Therefore, if your vendor uses electrolytic gold, you must 
have a note that controls the maximum thickness to 15 micro inches.  
Over this thickness, you must be concerned about embrittlement.  With 
immersion gold, you don't have to worry as it is very difficult to 
reach a thickness greater than 8 micro inches.

Though the cost of gold is very expensive, there are manufacturing 
process differences that may make the cost of a gold coated board 
attractive.  If your vendor uses the nickel/gold as an etch resist, 
then there is no solder plate and solder strip process required.  
There is also no HASL process required.  When removing these process 
steps, you may find the cost of a gold surface coating very close the 
cost of a HASL finish.  It is easy to find the differential in price, 
simply by asking your vendor to quote both ways.  

One ounce of gold will cover a considerable number of boards.   Ask 
your vendor for the name and phone number of their gold supplier.  
The gold supplier will tell you how many square inches of material 
can be covered with 100 micro inches of gold.  You know the square 
inches of circuitry on your board from your CAD data.  You know that 
gold cost $400.00/ounce.  When reducing these numbers down to  a 
requirement of 5 micro inches on your board surface, I think you may 
be pleasantly surprised at the cost relative to what you will achieve 
for assembly colplanarity requirements.

Hope this has helped
Norm Einarson
PRINTED CIRCUIT TECHNOLOGY