Following from the EMPF HelpLine, prepared by Jeff Kukelhan:

There are two types of gold coatings available: electrolytic and 
electroless.
Electrolytic is usually thicker (50 to 100 microinches).  It is often used 
for
mechanical contacts (edge connectors), although I have seen electrolytic 
gold
used for solderable surfaces.

The more common finish for solderable surface mount boards is immersion (or
electroless) gold.  The copper pads are first plated with about 50 
microinches of
Ni.   When immersed in the Au bath, some of the Ni enters solution and 
reduces the
Au.  The Au precipitates out on the Ni surface.  The process is self 
limiting because
eventually all of the Ni is covered with Au.  When there is no Ni left to go 
into solution,
no more Au will percipitate.  The resulting Au coating is about 5 to 15 
microinches
thick.  The amount of Au covering the pads is so small that when covered 
with a
nominal amount of solder paste it will not embrittle the resulting joint.  I 
have copies
of two articles printed within the last two years that deal with immersion 
Au coated
boards, and how to avoid joint embrittlement.  If you are interested in 
obtaining
copies, contact the EMPF HelpLine at 317.226.5616 or 
[log in to unmask] (alt short address 
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From: TechNet-request
To: TechNet
Subject: Gold Flash Coating
Date:  4 Dec 95 2:29PM

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Date: Mon,  4 Dec 95 14:29:57 PST
From: Len Brown <[log in to unmask]>
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Subject: Gold Flash Coating
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        I am looking for information conserning gold flash on pc boards. 
Pros
& cons of gold.
                1)  Does it create a better surface for fine pitch surface
mount parts?
                2)  Is there any possibility of embrittlement of solder
joints?
        Any information will be appreciated   Thanks in advance.
*************************************************************************
*                                                                       *
*       Len Brown                       GRC International, Inc.         *
*       [log in to unmask]                 8310 Guilford Road, Suite A     *
*       410-290-7111 x240               Columbia, MD 21046              *
*       fax 410-290-7122                                                *
*        Date: 12/04/95         Time: 14:29:57                          *
*************************************************************************