Re: gEDA-user: Any PCB users used PCB Train?

[Dan McMahill <dan@xxxxxxxxxxxx>]

Dylan Smith wrote:
> Hello list,
> 
> I'm a novice when it comes to sending out PCBs to be made, typically, I
> etch my own. But I'm just finishing off the PCB for my densest design yet,
> and I made it 4 layer... while some people have homebrewed their own 4
> layer boards, I've decided I'd rather send it off to PCB Train (especially
> as I want to make half a dozen of the boards).
> 
> Has anyone sent the output from PCB to PCB Train?
> 
> They say they want the drill sizes listed in metric, but looking at the NC
> drill file, they are listed there in mils. I can't seem to persuade PCB to
> emit drill sizes in metric - and I'm not even certain that's the right
> thing to do (perhaps they want a list of drills in the readme file, if the
> Excellon NC drill file only works with inch sizes anyway). A vendor drill
> mapping specified in mm certainly doesn't do the job, at least.

I think currently pcb only outputs in inches.  I'd think it would be 
pretty easy to teach the gerber/drill exporter to do either.  Drill 
files can certainly be in metric even if pcb doesn't currently emit that.

> Anyone got a good guess what they mean by 'flashed pads'? I've asked them,
> but not got an answer yet (My guess are pads that are a different width
> than the trace lines. Their technical document says they use 'flashed
> pads' to recognise the ends of nets for electrical testing).

Yes.  "Flashed" as opposed to "drawn".  In the old days, the photo 
plotters had a big wheel with holes called apertures cut in it.  The 
machine would turn the wheel to pick the correct aperture, position the 
film at the right place relative to the aperture and turn on a light 
that would shine through the aperture and expose the photo sensitive 
film behind.  A "flashed" pad means that the pad shape and size exactly 
matched one of the apertures and to make a pad, you'd position the 
aperture over the pad location and flash the light (like a camera 
flash).  A "drawn" pad by contrast is one where you'd use a small 
aperture, position it, turn on the light and then draw back and forth 
until you drew your pad shape.  Think of this like taking a marker and 
coloring in a pad.

Since these aperture wheels were physical things that had to be made, 
companies would typically have a set aperture list that they'd use for 
everything so they wouldn't have to buy a new aperture wheel every time 
they ordered boards.  This is why the older RS274-D ("gerber") files did 
not include the aperture list but instead there would be a single 
aperture file that got used over and over.

Now the plotters I believe are all laser plotters and the concept of 
flashed vs drawn doesn't have much bearing on exactly what happens 
mechanically during photo plotting.  Also, there is no problem with 
changing aperture lists all the time.  This is part of why the newer 
RS274-X format with embedded aperture definitions exists.  Still, you'll 
see in RS274-X files that some pads are in fact specified as flashed vs 
drawn.

Hope this made some sense.

If any of our wiki people cares to put most of this text in the wiki 
somewhere, it might be useful for others.

-Dan


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