Homebrew PCBs

Oh boy, have you opened a can of worms.  I've yet to perform isolation
routing, but have been doing quite a bit of research about it recently.

Design Software:
It seems like Eagle PCB (http://www.cadsoftusa.com/) is a very popular
application to design board layouts.  I've done some work with it, and I
find it pretty easy to use.  You can generate your schematic and board
layout with it.  The biggest problem I have with Eagle is finding the right
component in the library.  Oh, the freebie hobby version is also limited to
roughly 3"x4" boards.

Generating Gcode from Eagle:
Once you've designed a layout, you'll need to generate gcode from the board.
Eagle has a user expandable scripting feature called ULP.  I've seen quite a
bit of discussion about a ULP script called pcb-gcode.  You can find it at
the Eagle ULP download page
(http://www.cadsoftusa.com/cgi-bin/download.pl?page=/home/cadsoft/html_publi
c/download.htm.en&dir=eagle/userfiles/ulp).

From what I've read, the Gcode that pcb-gcode creates is not very well
optimized.  There's another ULP on that same page that will supposedly
optimize the generated gcode called pcb-gcode-wizard.

Drilling the Board and Vias:
Some of the posts I've seen recommend drilling the board before performing
isolation routing.  If you're using vias, you'll need to worry about plated
through holes.  Some people use through-hole rivets
(http://www.megauk.com/through_hole_rivets.php) but they could come loose
over time and are not dirt cheap.  LPKF has a package
(http://www.lpkf.com/products/rapid-pcb-prototyping/through-hole-plating/che
mical-free/index.htm) that allows you to do through holes, but I think
that's pretty pricey.  Some people just take a copper wire, stick it in the
hole and fill it with solder.

Over at VoodooEngineering.com, they have a pretty good video demonstrating a
DIY electroplating method to deal with vias.  You can find that at
(http://www.voodooengineering.com/index.php/pcbprocess/plating).  Going this
route will require generating a plating mask.  Even though they haven't
posted videos for actual routing, there's a bunch of good video footage here
that may be worth your time.

Fine Pitch SMD Problems:
The biggest problem that you're likely to face if you want to do fine pitch
SMD components is the epoxy layer between the fiberglass and the copper
laminate is not uniform.  Even 1-2 mils of thickness variation can prevent
you from successfully performing isolation of small SMD components.  So,
even if you've milled your spoilboard complete flat relative to the spindle,
the epoxy will vary enough on a single board to cause a ton of headaches.

Over in the PCB Milling forum on CNCZone, you'll find a thread
(http://www.cnczone.com/forums/pcb_milling/82628-cheap_simple_height-probing
.html) where people have been performing a probe pass to measure the
thickness of the board in a grid pattern and then extrapolate the height for
the tool path.  About half way through the thread is mention of a python
script that will perform the probe pass and alter your gcode to reflect the
results.

Another method of dealing with the variation is to use a floating head
design for your Z axis.  The documentation for doing this is pretty sparse
with most of the information I've found being related to plasma cutters.
The best illustration of a floating dead design for a spindle is the January
5, 2006 pictures on this page (http://www.robio.be/cnc/final/index.htm) .

Solder Stencil:
The millpcbs site has quite a bit of information that you may find useful.
Here's one page 
(http://www.millpcbs.com/index.php?option=com_content&view=article&id=28&Ite
mid=49) specifically talking about how to create a solder stencil.

Solder Mask:
Once you manage to perform all of the gyrations above, you'll still need to
worry about a solder mask.  Although Retromaster doesn't use isolation
routing, he has documented a method
(http://retromaster.wordpress.com/pcb-making/) he uses for creating a solder
mask that looks crazy good.
 
Hope this helps and Good Luck!
Paul
 




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