6 mil soldermask "traces" (assuming your talking LPISM) are no problem.
We do dams as narrow as 2 mils regularly.

Your requirement for FHS +0.002" could be troublesome.  We consider a
0.002" soldermask clearance as the minimum capability.  In this case,
your adding another tolerance which is the variation of the actual hole
size from the nominal value.  The addition of tolerances here makes it
unlikely that your requirement for no s/m in the hole could be met.

I have a question for you.  Why can't there be soldermask in the hole?
It's only a via, right?

> ----------
> From:         Joy, Stephen C[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.;Joy, Stephen C
> Sent:         Wednesday, February 17, 1999 4:51 PM
> To:   [log in to unmask]
> Subject:      Solder Mask Clearances on dogbone vias
>
> We are looking at the design rules for soldermask clearances on mini
> BGAs. The current rules say the solder mask will be .006" greater than
> the nominal hole size.
> I have 2 questions.
> What is the resolution capability of solder masks? Can solder masks
> reliably resolve a .006" line?
> What is the current capability for solder mask registration? I would
> like to spec nominal FHS+.002"/side as the solder mask opening with no
> s/m in the hole.
> Thanks,
> Steve
>

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