Murray,
I suspect you are inter-mingling three kinds of plating - electroplate,
electroless plate, and immersion plate.  All can deposit gold.

Electroplate is driven by an external electrical  voltage source and can
plate any thickness you want - easily 15 microinches if you can afford the
gold cost.   That is the oldest circuit board plating process, and is still
used for the connector contact "fingers" of circuit boards.   However, it
does require all the circuitry to be plated must be in electrical contact
with the voltage source.

Electroless plating is "chemical" plating where two chemicals react and
release electrons to cause dissolved gold salts in the bath to deposit on
surfaces.  Electroless plating comes in two forms, depending on the bath
chemistry.  Catalyized electroless depends on some very thin surface layer
of a catalytic metal - like palladium - to cause the chemical reaction that
releases electrons resulting in metal deposition.  Auto-catalytic
electroless enables the deposited metal to further the chemical reaction
between the reduction chemicals - building potentially thick deposits of
metals.   However, autocatylitic electroless is much slower than
electropate, and also eats up reactant chemicals - much more expensive than
electrical current.

The final plating is "immersion".  This is a chemical replacement reaction
- based on the electromotive (battery) potential. One metal is dissolved
and gives up electrons to chemically reduce another metal in solution.
 That is in the case of ELECTROLESS nickel IMMERSION gold (ENIG), that the
electrolessly deposited nickel is dissolved into a gold solution and gold
deposits on the surface of the nickel.  The reacation stops (thickness is
limited), when the dissolved gold salts in solution cannot find any more
available surface nickel to dissolve.

The dissolution of the electroless nickel is essentially corrosion.  Excess
immersion gold leaves a porous, corroded nickel underlayer - that is
generally called black pad because of its color and corroded appearance.

Hope this helps.
Denny Fritz

On Mon, Jan 6, 2020 at 11:42 AM Campbell, Murray <[log in to unmask]>
wrote:

> My original question came out of curiosity from one of Lee Ritchies fab
> notes stating a max of 15 micro inches of electroplated gold be applied
> over electroplated nickel.  We had historically called for a max of 8 micro
> inches of immersion gold to be applied.  I was curious why either max would
> be called out and why as I would prefer to just call out the min
> requirement as per 4552
>
> -----Original Message-----
> From: TechNet <[log in to unmask]> On Behalf Of Stadem, Richard D
> Sent: Monday, January 6, 2020 7:36 AM
> To: [log in to unmask]
> Subject: Re: [TN] [External] [TN] immersion gold thickness
>
> In addition to the black pad issues Dave mentioned, the 5 uinch limit was
> also to provide sufficient plating thickness to prevent oxidation of the
> underlying nickel, which is what you actually solder to, as well as prevent
> gold embrittlement in the final solder joints.
> At least 2 uinches is required to provide the protection, but you
> certainly do not want anything over 5, and never 15 uinches. I am not even
> sure it is possible to get 15 uinches in an immersion plating process. But
> whatever process that gives you 15 uinches is NOT electroless
> nicker/immersion gold.
> Odin
>
> -----Original Message-----
> From: TechNet <[log in to unmask]> On Behalf Of David Hillman
> Sent: Friday, January 3, 2020 3:44 PM
> To: [log in to unmask]
> Subject: Re: [TN] [External] [TN] immersion gold thickness
>
> Hi Murray - if you can get a true ENIG plating chemistry to put 15 inches
> of gold thickness on a board, you would never want to use the board as the
> gold plating would have severe structure and/or solderability issues. The
> IPC-4552 specification contains the industry consensus gold plating
> thickness range that applies to producing an acceptable and reliable ENIG
> plating layer. Any gold plating thickness outside of that range would be
> questionable at best for quality and reliability unless you are working
> closely with a plating chemistry supplier who is doing a custom plating
> bath composition. One of the early issues of ENIG plating was the industry
> asking the plating chemistry suppliers to provide an immersion gold plating
> thickness that exceeded the acceptable physics of the plating chemistry
> (basically this is how we discovered "black pad"!).
>
> Dave Hillman
> Collins Aerospace
> [log in to unmask]
>
> On Fri, Jan 3, 2020 at 3:30 PM Campbell, Murray <[log in to unmask]>
> wrote:
>
> > Happy New Year All,
> >
> > Is there any good reason to call out a maximum gold thickness / nickel
> > thickness on fab notes or is it sufficient to rely on simply stating
> > that the fabricator shall follow ipc-4552 minimum thickness?  I have
> > seen some companies state that a maximum thickness upwards of 15 µin
> > to be acceptable.  Is there any value in stating an upper range?
> > I know it states in 4552 that an upper range had been debated and not
> > necessary but I still see an upper added to fab notes.
> >
> > Regards,
> >
> > Murray Campbell
> > PCB Design Specialist
> > [log in to unmask]<mailto:[log in to unmask]>
> > [cid:[log in to unmask]]
> >
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-- 
Denny Fritz
Consultant
812 584 2687