Homebrew PCBs

Hi Jerry! I've had great success plotting boards using the 'scratch' method
with my 7475A. I love it! Such fun to watch.

If you want to see a plot of the boards generated, go to groups.yahoo.com,
into the Homebrew_PCB group, click on the "Photos" link to the left, then on
"Eagle Scratch" folder. You'll see two pictures (front and back) of boards I
made with my plotter scratching away the resist.

You can see from the bottom side that my holes didn't line up quite right -
the board that I photographed was one that I goofed on. I did several of
that board that turned out just fine.

The method I use to align the two layers is as follows.

First, I put a piece of paper into the plotter. I then have the plotter draw
a vertical line about two inches from the left hand side of the paper. Then
I take a spare piece of PCB material, maybe an inch wide by 8 inches long
that has one very straight edge. I line the straight edge up with the
plotted line on the paper (which is what the plotter thinks is vertical) and
tape it down. All of the above steps I just do once for all my boards.

Then when I want to plot a board, I will use my software to move the board
plot such that it sits about 1/8" to the right of the vertical PCB that's
taped on the paper. I have the plotter plot the bounding box of the plot
file on the paper, so I know approximately where to put the blank PCB. Then,
before I plot the top side, I draw a thick black line on the vertical PCB,
about in the middle (vertically) with black sharpie. I then tape the blank
PCB down over the outline that was plotted on the paper, pushing the board
up against the vertical PCB tight. Then I tape it down. Then, when the
plotter scratches the top side of the board, it also scratches one
horizontal line on the vertical PCB exactly in the middle of the top and
bottom limits of the board. It also plots a small horizontal line on the
blank PCB at the same vertical location, but in the 1/8" of blank space
between the vertical PCB and where the scratching outline starts on the
blank PCB.

Once the top side is scratched, I have a very sharp horizontal line on the
vertical PCB exactly in the (vertically) middle of the plot, and I have a
corresponding line on my PCB. I take the PCB off the paper (untape it), then
drill a very small hole through the 'reference' scratch. Then, I flip the
board vertically, and line up the hole with the scratch on the vertical PCB.
I again push the PCB up against the vertical (flat) reference PCB, and then
tape it down. When you plot the bottom, it will line up. 

Is it close enough to do tiny vias? No. Is it close enough to have .100"
headers line up on both sides of the board - no problem.

The plotter has a resolution of .001", and the head seems accurate to almost
that tight of a tolerance. The problem I currently have is that my
scratching tip is not very securely held by the head, and so drags behind
the head as it's being pulled across the copper and resist. So my lines
aren't always exactly the width that I want them to be. But it works fine
for me - I haven't had any electrical problems with the boards I've made.
(The good ones anyway.)

I hope this helps-

*Brian

> Hello, I was wondering if there is a technique to plot
> double sided pcb's with a plotter. I have an HP 7475A
> plotter that will never plot in alignment. Any
> suggestions would be appreciated! Thanks, Genaro M.