Mail Archives: geda-user/2012/04/28/13:43:54.1
On Sat, 2012-04-28 at 16:31 +0200, Oliver Schinagl wrote:
> Hi list,
>
> this time, somewhat unrelated question.
>
> Someone on this list posted a link to Tom Hausherr's blog about PCB
> design and that was an incredible interesting read.
>
Thanks -- I have never heard about that blog, I will look at it when I
have some spare time...
> A few things he mentioned that where extremely interesting was his post
> about metric usage vs imperial usage of parts etc [1].
>
> Here it is mentioned that metric is the way forward in PCB design and
> far it allows for far higher accuracy eventually. I'm sure hobby usage
> won't need/'allow' such high accuracy, most hobby PCB houses don't have
> equipment that does these high tolerances, but even so.
>
> Why is gEDA/PCB by default using the imperial system? It's quite easy to
> change, absolutely, but if it is actually recommended to use
> millimeters, why 'force' a default of mil's? Even if it is somewhat
> autodetected from the system's Language, mm should be the default, no?
Beginners often start with DIP8 packages, that has imperial spacing, so
default to mils should be ok. Advanced users should be smart enough to
select a reasonable grid, based on used parts.
Accuracy should be not worse for imperial units in principle -- it was
limited for pcb to 0.01 mil in the past due to pcb's internal units, but
now internal unit is nm. Should be fine for metric and imperial grid.
>
> Also, he mentions using 0.05 mm routing grid as most component lead pin
> pitches are 0.05mm.
Using a very fine grid is like no grid at all. Indeed size and base
(imperial/metric) is not too important, because ends of traces can snap
to pins. Do you really mean 0.05mm? That is very fine. If I remember
correctly I used parts with 0.5mm pitch for my last layout, and I worked
with 0.25mm grid and 0.2mm width traces.
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