Matthew,
        This is a very common practice as the movement to reduce lead
continues to grow. It also is more "Friendly " to the board itself . Yes
your Solder will build up gold over time with 1 um of gold thickness is
will be low however it should be monitored with the normal alloy checks
done on the pot. There is an upper limit on the amount and I am not in
my office to check a reference work , However if you can get your hands
on a copy of a paper done by Joe Keller there is quite a lot of
information about the maximum levels of Au in the Solder before it
becomes excessively brittle. I want to say 2% as the maximum level  of
Au in the SnPb before embrittlement becomes a problem. As for
solderability if the gold is clean and not heavily organic laden then
you will see an improvement in your wet out during soldering. Long term
you should see no difference in the performance of the product. AS a
side note HP has used gold only surface for quite a number of years as
have MANY  large volume OEMS such as Motorola . Just keep your
solderability test in your incoming inspection to assure that the
supplier is providing you a goods surface to solder to and you should
not see many problems.
        The down side is you have to provide enough solder to form a
fillet on the component either through paste or wave.
        The best upside feature is the surface is virtually Planar and
makes placement of Components more accurate.

 ----------
From: Matthew Park
To: [log in to unmask]
Subject: [TN] gold over entire board
Date: Thursday, February 19, 1998 7:51PM

One of my pcb vendors (oversea) wants to fabricate boards
using  electrolytically deposited gold (1um of Au over 5um of Ni)
over entire board surface.  That includes hole walls, soldering
pads, conductive traces and ground planes.  SMOBC is out the
window and silkscreen/LPISM are last steps in board fabration
process.

This is first time that I was informed that a board vendor
fabricates boards by depositing gold/nickel over entire board
surface.   My questions are:

What negative effects will it have for smt and th board assembly
manufacturing?
Are smt/th process parameters  req to be readjusted such as
reflow profile?
Will it increase diffusion of gold in solderpot?
Will it increase solder joint brittleness?
Are there long-term reliability issues?
Is there any other negative issue that need to be addressed?

There's no qualm about price.  It is better than SMOBC boards
with electroless Au/Ni immersion on soldering pads.

regards
Matthew Park
NII Norsat International Inc.

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