Jay Soderberg wrote:
>
> The exact tolerances that can be maintained at etching will depend on
> three things:
>
> 1.  The capability of the board manufacturer.  This will of course
> vary considerably.  It will depend on both the chemistry used, and
> the amount of process control used.
>
> 2.  The copper weight being etched.  The heavier the copper the more
> tolerance that will be needed to make this a manufacturable board.
>
> 3.  Part design.  Parts with many parallel, tightly spaced lines are
> the most difficult to properly etch.  Also, because there is no
> plating to contend with, inner layers are easier to etch than outer
> layers.
>
> Consequently, +/- 20% may be very easy to achieve on a wide line and
> may not be enough tolerance on a narrow line or on heavy copper.  As
> a general guideline, most shops should be able to maintain an etching
> tolerance that is 1.5 - 2.0 times the copper thickness (per side of
> the trace).  If you think about the fact that etching is a chemical
> reaction, and that as it etches it will etch equally in all
> directions, you will realize that the best that could possibly be
> hoped for is a direct 1:1 ratio (or 1.0 times the copper thickness).
>
> Ex:  One ounce copper is nominally .0014 inch thick.  An etching
> tolerance of .0014 X 1.5 = .0021 per side for a total tolerance of
> +/- .004 inch.
>
> > Date:           Thu, 2 Apr 1998 16:05:45 -0700
> > From:          Steve Collins <[log in to unmask]>
> > Subject:       DES: IPC 6012 Trace width tolerance - RF Applications
>
> > This message is pointed to those of you involved in RF circuitry I think.
> >
> > The IPC-6012 specification for trace with tolerance of ±20% is coming under
> > fire here. Typically what can most PCB fabricators hold for trace width
> > tolerance regardless of what IPC says. How do other RF designers specify
> > trace width tolerances in critical circuits.
> >
> > Steve Collins
> > PCB Design Supervisor
> >
> >
> Legal Notice:
> All opinions are my own and do not necessarily reflect the opinion of the Management or Ownership of this station.
>
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> ################################################################From a fabricators point of view where tight track tolerances are required three actions
need to be taken:-

1       The designer Indicates tight tolerance tracks in his original magnetic data(he
        gives them a different "D" code)

2       The fabricator plots these tracks wider in propotion to known etching losses.

3       The fabricator only uses first generation masters.

Roger Hammond

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