Mail Archives: geda-user/2012/04/28/15:04:49
On 04/28/12 19:42, Stefan Salewski wrote:
> 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.
Ah, well my next PCB will be mm everywhere but may use a DIP8 package;
shouldn't cause any issues though, right?
>
> 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.
>
>
>
Not sure what I ment :) My concern was, if I snap to 0.5mm, where the
fab house lists its limits in mil's; is there some problem? I can
imagine that 10, 20 years ago, this could be a huge issue, but in
effect, it's a print/cnc operation and the tolerances are from drill
bits and the photo sensitive operation, not placement of parts? Or is
there some grid size to keep in mind? I know the DRC check keeps in
minds the photosensitive bits and drill size bits, but other then that?
Oliver
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