Re: gEDA-user: PCB question

["Mike Jarabek" <mjarabek@istop.com>]

Patrick Bernaud <b-patrick@wanadoo.fr> said:

> Hi,
> 
> Chris Ellec writes:
>  > I need to make a new Pcb with a QFP that has a 0.5 millimeter pin 
>  > spacing. (a little under 20 mil)
> 
> I have just came across a similar problem. The devices is a QFP64 with
> same spacing.
> 
> I have created the symbol with the help of a little script in guile
> (former goal of this script was to become familiar with goops).

The m4 interface in PCB is not too bad, once you get used to it.  I try to do
calculations in scaled units, i.e. 1 mill = 100 `units', then divide by 100 on
the way out.  We calculated the accumulated error on a reasonable size device
using this approach and the numbers come out under the tolerances for the
board manufacturing process.

> 
> The problem I encountered is that it violates the DRC of the PCB house
> : 8 mil is the minimum track spacing for them. And the rounding
> sometimes reduces the free space between two pads to less that 8.
> 
> So I am going to redesign this symbol in a way that this rule is no
> more violated.
> 
> 
>  > The current PCB has a minimum grid of 5 mil, wich is not good enough for 
>  > that chip, since each side has 50+ pins and the error adds up fast.
> 
> If you do not have the same problem, you can still design your device
> and round mm to nearest mil.

This is what we have been doing here, and is working out allright.  Our board
has not gone to be built yet.  But the initial DRC that was done came out
okay.
 
> 
> Grid spacing is not really a problem: I do not know how to decrease it
> under 5 mil through the interface but if you edit your pcb file you
> can decrease it as low as 1mil (I do not remember the line but file
> format is documented).

You can decrease it to 1 mil by adding a new hotkey definition to the system
app defaults file. ( /usr/X11R6/lib/X11/Pcb )

Add lines such as:

  Shift<Key>t:        SetValue(Grid, -1) \n\
  <Key>t:          SetValue(Grid, +1) \n\

to the pcb.masterForm*output.baseTranslations X property.  Now pressing the
`t' or shift-`t' keys will adjust grid spacing by 1 mil at a time.

> 
> Though it is somewhat painful to deal with 1mil grid spacing for a
> complete board. So you can place your lines connected to device with
> 1mil grid spacing and then switch back to 5mil for other lines.
> 
> 
>  > Has anybody here been doing that kind of design, or know how to make Pcb 
>  > do metrics ?
> 
> I do not know how to make Pcb metrics, but would be very
> interested. Though it is too late for my current board.

	PCB can be set to `scale' the output by 1/2.54, then inches become
centimeters, but _all_ the footprints have to be re-done. Additionally, the
maximum board size becomes about 32x32cm. 

-- 
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                              Mike Jarabek
                                FPGA/ASIC Designer
   http://www.doncaster.on.ca/~mjarabek
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