Hi, Guy
30 uinches for soldering is far too much, and will probably lead to gold
nucleation and embrittlement. This is not ENIG. 
I wonder if your drawing was written with wirebonding intended to be
used on the pads with 30 uinches?
Even 30 uinches of gold will be fully loaded into the solder during
reflow.

ENIG consists of 2-5 uinches of gold over nickel per IPC 4552. It does
seem weird to specify 30 for soldering, unless perhaps they intended for
wirebonding to be done somewhere on the board. In that case, they should
have simply specified ENIG over all of the solderable pads, with 30
uinches of soft gold on the wirebond pads only.

I don't recall a rule of thumb developed in this forum of 10 uinches per
5 mils of stencil thickness. I do know a safe rule of thumb is to not
allow more than 1% Au in the SJ. You may or may not get that with the 10
u"/5 mils, consider;
the metal content in a printed brick of paste in a 5 mil aperture can
vary widely dependent on whether the paste is type 2 or type 6, 88%
metal vs 95% metal content, and I would not want to depend on the
repeatability of any stencil printer on the market to hope that I get
enough paste every time where the fixed amount of plated gold will not
exceed a given percentage.

Gold's rate of nucleation is faster in a 99% tin alloy than it is with
Sn63Pb37, so that situation poses even more of a risk.

I would suggest you talk to the designer. Many design engineers know
little or nothing about surface finishes and their proper selection,
unfortunately. He/she may not even be aware of this, as quite often
fabrication print notes are boilerplated from one design to the next,
and overlooked. Or there may be a good reason for that specification,
and you may need to change your solder volume to compensate for it. The
key is communication. 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: Thursday, October 23, 2008 4:12 AM
To: [log in to unmask]
Subject: [TN] Hard vs. soft gold

Can anyone comment on differences between soldering through hard versus
soft gold. 
 
I saw a drawing with specifications for hard gold on pogo pads with 30
micro inches of soft gold over nickel on solderable surfaces. 
 
Seems like a really weird specification to me. As I recall the rule of
thumb we developed through discussion on this forum was no more than 10
micro inches per 5 mils of stencil thickness. 

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