In general it's bad practice to gold plate SMT solder pads.  The reason is
that proper SMT soldering utilizes a very small amount of solder and the
gold plating
will partially dissolve into the molten solder. Because of the small amount
of
solder, the percentage of gold will be high enough to embrittle the solder
joint,
and it will have a high probability of failure.

Hand soldering can apply a large enough amount of solder that the percentage
of gold in the joint is relatively small and the problem is avoided.

-- Tom, N5EG




On Thu, Feb 22, 2018 at 12:29 PM, Leo Bodnar <l...@leobodnar.com> wrote:

> Here is ENIG fact that is not widely known at the moment but which some
> might find useful.
>
> I could not understand why I get better TDR and insertion loss results
> from solder-mask covered microstrip transmission lines than from otherwise
> identical microstrips on the same substrate with soldermask removed and,
> therefore, covered with ENIG.
>
> Gold can't be bad, right? As it turns out, even gold coin has two sides to
> it.
>
> I have found that Shlepnev and McMorrow conducted extensive research and
> published data, some of which is presented here http://www.simberian.com/
> Presentations/NickelCharacterizationPresentation_emc2011.pdf
>
> In essence, it's not the "G" that is the problem - it is the "N".
> Immersion Gold layer is not thick enough to contain whole of the skin
> effect layer (even towards 100GHz) and as signal frequency increases most
> of the signal ends up travelling through Nickel.
> As Shlepnev commented "Nickel is the most mysterious metal in
> electronics."  It has significant effect on insertion loss and risetime
> degradation.  "Significant effect" is posh for "bad."
>
> Some mass PCB manufacturers have been known to apply ENIG before
> soldermasking.  This causes even more high speed/frequency problems because
> all of the copper on the outside layers will have Nickel over it - exposed
> or not.
>
> Probably not a problem for majority of ENIG users but could cause a
> headache or two for unsuspecting.
>
> Leo
>
> > Date: Thu, 22 Feb 2018 19:02:25 +0000
> > From: Mark Sims <hol...@hotmail.com>
> >
> > Yes, have the board done with ENIG gold.  It typically adds around $15
> per run of boards.  I do all my boards with ENIG gold... if for no other
> reason than the gold color makes it very easy to determine when your solder
> paste properly covers the pads.
> >
> > And, as Charles mentioned,  the quality and thickness of the gold can
> vary depending upon the board house.  I have used gojgo.com for a lot of
> boards.  They do very good, quick work,  are well priced, and they seem to
> have the best gold finish.
> >
> > Hard gold finish is VERY expensive these days.  I've been quoted $250+
> for setup charges and per-board costs of over $25.
>
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