Scott,
The last time I looked the positional accuracy of a laser
photoplotter was about 1/4 mil (.00025"). Whatever feature to feature
tolerance you're concerned about will NOT be a function of imaging. What it
will be a function of is copper weight and etching/plating process control.
Copper weight is a factor because as the copper gets thicker it gets
more difficult to etch it extremely accurately. After etching, traces have a
trapezoidal shape. The thicker the copper, the more the difference in width
between the top of the trapezoid and the bottom.
The etching/plating process will vary from chemical batch to
chemical batch (as will the actual thickness of copper on the FR4).
As a rule of thumb, assume your feature widths and spaces will vary
by approximately the thickness of the copper they're etched in. That is, an
8 mil trace may vary between 6.5 - 9.5 mils, and an 8 mil space may vary by
the same amount if etched in 1 oz/sqft (.0014" thick) copper. Most producers
typically do much better, but don't bet the farm on it. Greater accuracy can
be achieved in thinner copper weights or by paying more per piece.
On the other hand, I may be misinterpreting the question. If your
concern is with mechanical design tolerances, that's a different issue. The
positional tolerance of components after they're mounted on the board will
be related to pad shape and size, as well as the positional tolerance of the
component leads. The larger the pad area the more the component can "drift",
but the easier they are to assemble (and vice-versa). I don't think their
are any hard numbers available for that since the number of uncontrollable
factors is so large.
Michael Hiteshew
Lockheed Martin NE&SS Marine Systems
410-682-1259
> ----------
> From: Buscomb, Scott[SMTP:[log in to unmask]]
> Reply To: TechNet E-Mail Forum.;Buscomb, Scott
> Sent: Monday, July 09, 2001 6:07 PM
> To: [log in to unmask]
> Subject: [TN] Pattern to Pattern Tolerance
>
> I am looking for the standard industry locational tolerance of copper
> feature to copper feature on a single sided PCB. Since PCB fabrication is
> a
> photographic process, I am assuming it is a function of gerber plotter and
> artwork imaging.
> TIA,
>
> Scott Buscomb
>
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