Dear CL Chong:

You have some of the answers already about electroless nickel gold, and you
have some misinformation:

Immersion gold  plating occurs when a small amount of the electroless nickel
is dissolved into the gold solution, and gold is deposited in place of the
nickel (oxidation - reduction reaction).  4-10 microinches are normal deposit
thicknesses, but not much more can be deposited since nickel stops dissolving
through the gold layer.

Yes, electroless nickel is required as a barrier for the gold (as in any
copper/nickel/gold construction since copper and gold migrate rapidly into
each other).  Also, electroless nickel is a convenient souce of nickel ions
to reduce the gold in solution.  Electroless plating coats all the copper
traces - top and sides, without continuous conductor attachement as in
electroplate.  Gold is most commonly electroplated when the original copper
foil gives conductive continuity, or when the circuits are "bussed" together
as in connector tabs.

I am very surprised that 20 microinches of gold gave you embrittlement as
gold is thought to be compatible in solder to 3 or 4%.  That means that you
were putting down less than 600 or 800 microinches of solder paste - true?
Anyway, the thinner immersion gold plate will give you less gold in your
solder joints.

Ohter points:
Most LPI type solder masks are now compatible with electroless
nickel/immersion gold processing.  That is feature most mask suppliers have
been working on.

Yes, phosphorus is co-deposited with the electroless nickel.  But, running
the process by the seller's instructions makes the phosphorus deposit
consistant with bath age, bath loading, etc.   Most currnent thought about
solderability of electroless nickel/immersion gold does not focus on the
phosphorus content of the deposit, but on the corrosion rate of caused by the
immersion gold bath.  Run properly by the manufacturers instructions, there
should be a uniform corrosion of the nickel/phosphorus, and a uniform deposit
of the immersion gold.

4 microinches of immersion gold will have some porosity and lower shelf life.
 Deposits nearer the 10 microinch thickness should store and solder well for
months.

Please contact me off line if you want more specific information -

Dennis Fritz
MacDermid. Inc

##############################################################
TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
##############################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in
the body:
To subscribe:   SUBSCRIBE TECHNET <your full name>
To unsubscribe:   SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information.
If you need assistance - contact Keach Sasamori at [log in to unmask] or
847-509-9700 ext.5315
##############################################################