Week 5 -- Printed Circuit Board Design
This week we're copying the echo hello-world board and redesigning the layout from scratch - and, while we're adding it, adding a couple components to the circuit. I decided to add a couple LEDs and button. They're roughly laid out in the shape of eyes and a nose, so when you press the nose - the eyes light up. Copying the circuit over was fairly simple, however, I noticed that Eagle - unlike other circuit schematic programs, won't show little loop hops when two traces grahically cross but aren't electrically connected. This caused some confusion in the double-checking process. The two LEDs I added are hooked up in parallel with 1K resistors in series with each.
For the layout I decided to place my parts on a 1" radius circle to stick with the loose face theme I had with the eyes and nose. I used a minimum trace width of 12 mils, and a minimum clearance distance of 20 mils between wires. I made the mistake of not also defining a minimum pad-to-wire distance and stuck with the default of 8 mils. When I milled the board, three of the pads were connected with a wire and I had to use an exacto knife to separate them manually.
Doing the layout with only a single layer can be really tricky and after a little while of not finding a routing topology that worked, I opted to use a couple 0-ohm resistors to let me hop over existing traces. These simplified much of my layout and made the process a bit nicer.
Mill and Stuffing
The milling and stuffing steps are similar to week3. I made the mistake of making the dimension layer too thick in Eagle when doing the layout and this caused the Modela to try to mill out a thick cut. Rather than a single line width outline, it made a 2 or 3 line width outline. This didn't have any real consequence besides causing the job to take more time.