gEDA-user: Design changs required to mill PCBs?

[Dan McMahill]

Bob Paddock wrote:
> Anyone have experience with PCB milling machines?
>

I used one several times about 10 years ago.  It was a "quick-something" 
I think.  I pretty much always did what I call 1.5 layer board.  By this 
I mean that the back side is pretty much entirely a ground plane and all 
the action was on the top of the board.

> I had the opportunity to use a LPKF milling
> machine to make a four layer board yesterday.

How do you stack things up?  On the machine I used, for a 2 layer board 
there were 2 pins which provided alignment.  Plated holes were a pain, 
you had to insert a small copper tube filled with solder into each hold 
and then wick out the solder.  I found them to be somewhat unreliable.

How do you manage connections to inner layers with the LPFK?

> What I was wondering was if there where any changes
> in PCB layout that would be required, or good to do,
> when milling PCB material, versus the standard chemical
> etch procedures?

I didn't usually change my layouts but I did make one change to the 
milling flow.  With the tool I had, it would first trace around all of 
the traces and then go back and mill away all the remaining copper 
islands.  I often times skipped the step where it milled away the copper 
islands because it took a long time and wore out bits.

> My generally experience with the LPKF machine yesterday
> was they seem to have their process well thought out, but the software
> has usability issues which where minor, and
> any meaningful documentation is nonexistent.

thats funny.  My memory of the software for the tool I used is about the 
same.


Here's a nifty trick. I once was going to do a spinoff of a board where 
most of it was the same but I was going to totally replace about a 2x3 
section of the board with a different circuit.  I used the milling 
machine to make a board that was just the new part and also used the 
milling machine to cutout the part which was going to be replaced on my 
existing board.  Then I scraped off the soldermask on the original board 
right around the cutout and soldered my replacement section in place. 
It worked out fairly well and given the particular set of paperwork and 
prices we were willing to pay it gave me a board in an afternoon instead 
of 3 weeks.

-Dan