gEDA-user: PCB question

[Patrick Bernaud <b-patrick@wanadoo.fr>]

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 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. 

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).

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.



Patrick