Re: gEDA-user: Metric, Imperial, Rounding, DRC, and board houses

[Stefan Salewski <mail@xxxxxxxxxxxx>]

On Mon, 2011-02-21 at 15:23 -0700, Russell Dill wrote:
> I'm starting a new design and all my components are metric based,
> including a few 1mm pitch BGA components. I'd really like to do the
> layout in metric, but I'm worried about two factors. The first of
> which is that PCB does not yet have the option to store things
> internally in metric (at least from what I understand) so rounding may
> occur on that end. In addition, my board house rounds everything to
> 2.4 format (0.1 mil). I can envision several scenarios where my design
> meets DRC in PCB, but fails when I send it to the board house.
> 
> What is my best option?
> 
> Just use imperial units and cope with weird grids?
> 
> Use metric spacing, but recalculate DRC based on worst case rounding?
> 
> Some other option?
> 

Use metric grid/unit, I guess most of us use that.
Internal resolution is 0.01 mil, which is good. Of course nm would be
better. One problem for me was, that a few 0.01 mil garbage lines were
generated for the layout. Not a big problem. Some not really smart
people have tried to use a very very  fine grid, 0.01 mm or so. That is
like using no grid at all, I call that silly. Try to use a useful basic
grid like 0.25mm grid if most of your parts have 0.5mm pitch -- I think
that was what I did, I am not sure. Important: Enable snap to pads/pins,
so you can make good connections to imperial parts which are not on the
metric grid.
  
>my board house rounds everything to
> 2.4 format (0.1 mil).

Strange -- so pitch of metric parts may vary from pad to pad a bit. That
was a problem in very old days of PCB, when 1mil internal units was
used.




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