My experience of metric-specified parts on an imperial grid has led me to
create the PCB symbols on a 1-mil grid, rounding the *absolute* co-ordinates
of each pad to the nearest mil. (The use of absolute co-ordinates is
critical to avoiding cumulative rounding errors, which can be significant on
even medium-sized parts). The positional variation from this method is
small, and the time taken to create the symbol is more than saved when it
comes to tidying up the autorouter's efforts!
I've used this technique on parts down to 0.4mm lead pitch on a 6/6 rule and
it hasn't caused any problems.
I doubt that 'odd' angles would upset a photoplotter, so for manual routing
snapping to off-grid pads should work fine. The problems I've seen with
pads on a sub-mil grid have been where the autourouter uses a large number
of short track segments to end the connection to the pad, and often uses an
off-centre entry to the pad as well.
Regards
Andy Gulliver
> -----Original Message-----
> From: Tim Hutcheson [mailto:[EMAIL PROTECTED]]
> Sent: 23 June 2001 18:37
> To: Protel EDA Forum
> Subject: [PEDA] Metric footprints and English pcb layout.
>
>
> My board design is in English units, primarily, but I have several small
> outline parts for which the footprint is in Metric units and the pad pitch
> is variable and results in off grid pad centers with respect to
> the English
> grid. My rules are 5/5 mil and the part is a 402 class 4 part resistor
> pack, small enough that the differences in English and Metric
> pitch worries
> me. My question is: what is the proper convention for
> interfacing this part
> into an English unit layout? Does it matter? For example, if I
> define the
> part layout in metric the system accommodates by allowing me to connect
> tracks to the metric center of the pads, which is fine. But do the odd
> angles that result create any problems for the Gerber
> photoplotter creating
> the tracks in tight quarters? Should tracks be laid to the
> nearest English
> pitch inside the pad for example? Any offer of the right way to
> do this, if
> it matters, is appreciated. I have noticed that the "quality" of
> the layout
> begins to be degraded or becomes inconsistent as the layout is
> edited due to
> the different units.
>
> TIA
>
>
> regards,
> Tim Hutcheson
> ICQ #32491889
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