Homebrew PCBs

I just started to become interested in making some
PCB's and have a very fine step cnc. It has a 8 x 8
inch foot print with steps at over 50,000 per rev.

I wanted to ask for suggestions on a good PCB program
to cut these. This is just a hobby so nothing over the top.
I have bits and micro drills already and a fast spindle.
I just want to lay out and cut some small boards at first.
Thanks,
Mark

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
 




[Non-text portions of this message have been removed]

Wow Paul,
Your fingers must be ready for a vacation after all that.
Thanks, This will take me a little while to digest.
I am looking forward to trying it none the less. I have
been putting this Mach. toghther for a while. I did notice
on another cnc project, that my table is slighty off on the
Y axis, but I have a quick fix for that.
I believe I have Eagle soft ware all ready. It's on my Cnc
computer, so I will check.
Thanks again.
Mark

--- In Homebrew_PCBs@yahoogroups.com, Paul Ward <pward123@...> wrote:

>
> 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
>  
> 
> 
> 
> 
> [Non-text portions of this message have been removed]
>

There is also yahoo group for pcb-gcode. I have tried their software and found that it works well for the board I was working on last summer. I didn't mill it as they were in the group. I made a spring pressured scribe to scratch the surface of the pcb that I had used a Sharpie Magnum Permanent Marker to first paint the copper. The scribe was the fourth design to get one that would work. I had tried several different paper/toners tryingto find the right combo but didn't find one. Staedtler also makes a very good marker that the ink resists the acid for etching pcbs. Look for markers with 313 or 318 ink. The color red works the best. I have seen that they also sell the ink in refill bottles, but have not been able to get it here.

Scribing the board exposes very little copper that needs to be etched away and the acid lasts a very long time. I also can give you a very small isolation distance. I used the acid at room temperature to give me a longer etching time to be on the safe side so that there was less chance of under cutting the copper that was covered with the marker's ink.

I used 1/8 drill rod for the scribe's tip and pipe fittings for the rest of the scribe plus the spring. Two 1/8 hose to 1/8 pipe adapters, a coupling and the compressing ring that was compressed onto the drill rod so that the drill rod was extended about 1" out of the hose end of the hose adapter. Tried the drill rod in the two adapters to find some that had a nice smooth fit with little slop. Some were too tight for the drill rod. And using two I found that I had a lot less side to side runout to keep bit straight while scribing the copper due to the longer support length of the drill rod. I did have to end up using a drop of crazy glue to keep the compression ring from moving on the drill rod. Place the ring on the rod so the a spring will compress about 1/2 to 3/4" when the fittings are joined together and tightened. 

Ground the end of the drill rod to a point and then used a file to smooth the end even more but didn't find it smooth enough for my liking using the milling machine. Ended up buying a few ceramic tiles that didn't have any glazing on them to hone the tip. Worked very well. The tip can always be removed to be touched up on the ceramic tile again. I haven't used it enough to know how long it lasts yet. I used a small piece of pcb material and just compressed the scribe while held in the milling head to scratch the uncoated copper with little resistance and compressing the tip of the scribe about 1/4". It does require more z axis movement while scribing the board. 

The assembly isn't turned while in use and should last a long time as it only scratchs the ink and leaves a nice shiny exposed copper to be attacked by the acid. I also don't clean the copper before painting it with the marker's ink to make it less attractive to the acid.

It took me two weeks vacation time to perfect the scribe and find the ink that would work in the acid. When I did find the ink I went back and bought out the store's on hand stock.

I give all the copper two coating of the ink and allow it to dry overnight. They can be prepared well in advance but just protect them once they are coated to prevent scratching the ink.

You have to be able to have a accurite means of relocating the pcb when you flip it to scribe the second side. I use the top or bottom of the mill table to set the top or bottom of the pcb. I used two homemade clamps maded of pcb. Used the mill to drill two rows of holes that were a tight fit on the bolts that matched the slots in the milling table. Each of the clamps is two pieces of pcb that are overlapped. One of the clamps are not moved while scribing the pcb. It ends up having three rows of bolts that have the side of the pcb clamped on each side. Only the two inside rows of bolts are loosen to remove and flip the pcb material on one side and all three are on the other clamp are loosen in the process of flipping. Using a piece of foam or felt under the assembly would help protect the ink also.  

I use a small Taig cnc milling machine.


I also have several pen plotters that I am thinking of trying this on as they are a lot faster than the milling machine for plotting the drawing. But I have found that they do not have a true 1 to 1 movement so some scaling would have to be done. I have been able to load Eagle Cad drawing saved as cad file into an old dos version of Turbo Cad that has the driver for the plotter so scaling in not the biggest problem.

For prototyping boards I plan to use the milling machine but if I ever plan to make more than a few boards I will look at perfecting the plotter method. 

--- In Homebrew_PCBs@yahoogroups.com, "RL" <rlrc2003@...> wrote:

>
> I just started to become interested in making some
> PCB's and have a very fine step cnc. It has a 8 x 8
> inch foot print with steps at over 50,000 per rev.
> 
> I wanted to ask for suggestions on a good PCB program
> to cut these. This is just a hobby so nothing over the top.
> I have bits and micro drills already and a fast spindle.
> I just want to lay out and cut some small boards at first.
> Thanks,
> Mark
>