IPC has set up a committee to issue a standard (4552) for electroless
nickel/immersion gold plating.  The standard will specify 2 +/- some standard
deviation thickness in microinches - one microinch equals .025 micron.  While
immersion gold is self limiting in thickness, the standard will be much lower
than the self limit.  Actually, immersion gold is deposited by the
"corrosion" (dissolving) of the surface electroless nickel, so thick
immersion golds may severely damage the underlying nickel.  You are soldering
to the nickel, once the gold dissolves.  No wonder thick immersion gold
sometimes causes "black pad".

If you feel the solder dullness might be due to the dissolved gold, check
your incoming thickness by x-ray fluoresence.  If you can see the gold in a
cross section, it is electroplated, not immersion.

All that glitters is not immersion gold ...........

Denny Fritz
MacDermid, Inc.

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