Re: gEDA-user: PCB DRC accuracy?

[Russell Dill <Russ.Dill@xxxxxxx>]

On Tue, Jun 21, 2011 at 1:50 PM, Andrew Poelstra <asp11@xxxxxx> wrote:
> On Tue, Jun 21, 2011 at 01:29:02PM -0700, Russell Dill wrote:
>>
>> I've always worried about this too, especially when doing a design in
>> metric and sending it to an imperial board house. My guess is you need
>> to calculate the worst case error and add that to the DRC. The
>> difficult part would be finding all the possible sources of roundoff.
>>
>> Perhaps if you could translate the design to a mil grid with a given
>> precision and *then* run DRC.
>>
>
> We are working on moving pcb toward metric base units -- then
> a mm would be 10^6 nm rather than "about 3937.00787 cmils"
> like we have now.
>
> mils would also be an integer (254) number of base units, so
> hopefully a lot of precision issues will go away.

If one point is on 228600nm and another on 126988nm, it would seem to
make a 4 mil (101600nm) trace/space rule (101612nm). But when
converted to mils:

228600nm = 9 mil
126988nm = ~4.99952756mil

If you output to a X.YYY gerber format, that will round to 5mil and
your features will then only be 4 mil apart.


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