Gaby,

Gold thickness requirements are often related as to how you need to use it for
the board fabrication processes and how you solder it.

(1) If you are using the gold plating in conjunction with nickel as an etch
resist you need to apply it about 25-30 microinches thick to withstand the
rigors of etching and still retain solderability; however if you are going to
use it in a reflow process for SMD, you better make sure that you have it hot
enough for enough time in contact with the molten solder so that you do not
leave a layer of gold and layer of a brittle gold/tin intermetallic between
the nickel and the solder.  As Dave Hillman says, make sure that it has time
and temperature to diffuse the gold into the solder and keep the percent gold
in the solder less than 3%.

(2) If you have ability to bus and electroplate conductors with n ickel and
gold after etching the board, 7-10 microinches of gold thickness will provide
solderability and the risk of excess gold concentration in the bulk solder is
very small with the standard peaks at about 217-220 C.  The thinner gold
finishes must be be protected and kept dry.  No steam aging or natural steam
aging that may occur in a shop like in Florida in mid August.  Thin gold
whether immersion or electrolytic is porous and the galvanic reaction between
the gold and the nickel in the presence of ionic carriers, like moisture, will
cause the underlying nickel to oxidize and ruin solderability

(3) Another gold finish process often used is the electroless nickel/
immersion gold process which provides about 4-6 microinches of gold ever
electroless nickel.  Again no steam aging.

(4) If you want a thin electrolytic gold that must serve as en etch resist,
the best choice is an electrolytic nickel with a final plate of about 8
microinches of electrolytic palladium followed by about 8 micro inches of
gold.  This finish can be used as an etch resist and a has good shelf life.  I
have had varied success with some of the electroless palladiums and with a
final overcoat of electroless or immersion gold processes.  The success seems
to be related to the chemistries used and sometimes the shop that applies it.

I always like to be able to test a particular final finish with my process
when doing SMT and then specify type of finish and the thickness ranges that I
know will work especially if I have difficult environments, shock or
vibrations to worry about.

Phil Hinton

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