David

1)  IPC doesn't have anything specifically covering Electroless
Nickel/Immersion Gold (ENIG).  Typical ENIG deposits comply with the IPC
requirements for nickel barrier (50 microinch minimum) and solderable
gold (30 microinch maximum) listed in Table 3-2 of IPC 6012.  The best
way to specify is call out on the print something like  "Surface finish
to be immersion gold over electroless nickel.  Gold thickness to be 6
+/-4 microinches. Nickel thickness to be 100 microinch minimum.

2)  ENIG can be applied before or after soldermask.  There are
advantages/disadvantages to both.

Applying soldermask then gold is typically cheaper since less gold
plating is involved.  The biggest potential drawback is the soldermask
can be attacked by the nickel plating bath.  The extent of the attack is
dependent on the soldermask used, some masks are more susceptible than
others.

Applying gold then soldermask can cause soldermask adhesion problems to
the gold surface.  Soldermask residues can also be left on the gold,
resulting in solderability issues during assembly.

Merix does RF boards with ENIG before and after soldermask, though
soldermask first is predominate.  If you don't care, you should leave it
up to the fabricator.  If you do care, though, by all means make sure it
is called out on the print.

3)  I don't know of any websites off the top of my head.  Meris's
website information on surface finishes is being re-written or I'd steer
you there.

> ----------
> From:         David Schaefer[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.;David Schaefer
> Sent:         Thursday, May 27, 1999 10:49 AM
> To:   [log in to unmask]
> Subject:      [TN] Immersion Gold Fabrication Processes /
> Specifications
>
> Can anyone provide some information on the questions listed below:
>
> 1. - Are there any specifications (IPC or other) which should be
> referenced
> in the PCB fabrication
>       drawing for NiAu finishing?
>
> 2. - At what point during fabrication is NiAu normally applied -
> before or
> after LPI soldermask (or both)?
>       If both, what are pros / cons of each?  Is it important to
> outline
> desired Fab processing in the
>       fabrication drawing?
>
> 3. - What sources of NiAu info are available on the web?
>
>
> Thanks,
>
> Dave Schaefer
> PWB Design Technical Lead
> NORTEL Broadband Wireless Access
> 14 Fultz Boulevard
> Winnipeg, MB   R3Y 1V3
>
> Phone:      (204)631-2298    ESN 771
>
>

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