Dennis,
One more question please. When the assembler applies tin/lead paste to the
electroless nickel/immersion gold surface does the gold and nickel both
dissolve into the solder during reflow or does the nickel remain behind as a
barrier over the copper; and then what kind of inter-metallic would be
formed with the tin/lead?
Jim Kittel
L-3 Communications
-----Original Message-----
From: <Dennis Fritz> [SMTP:[log in to unmask]]
Sent: Wednesday, May 03, 2000 12:13 PM
To: [log in to unmask]
Subject: Re: [TN] Electroless/imersion Ni/Au Vs electrolitic
Ni/Au
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
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