Week 2: Printed Circuit Board Fabrication

Printed Circuit Board
This week we made a printed circuit board using a small CNC mill.

Cicuit Trace

Personalizing the Circuit

Using the basic PCB design, I added a small logo to board using Microsoft Paint. After uploading the image to the fab module I noticed that my logo was too small for the mill to cut out properly. So I went back and edited the logo so that the lines were far enough apart that the 1/64th inch bit could fit between them.

Milling

Milling

MIlling was fairly straightforward. Here are some things to remember:

1. Remember to set the command line in the fab module to ".../ttyUSB0" or nothing will happen.

2. Make sure the dimensions (listed PCB Cutout under "input" on the fab module) of the "traces" and "interior" files match.

3. Make sure that if the clear mill hood is removed you have something to replace the safety switch.

4. Make sure that before you cut out the "traces" you press the "Down" button a few times or the mill will just scratch the copper surface.

Soldering

Soldering was by far the trickiest part of this assignment. It took me a while to get the technique down but I realized that the solder seems to favor the components rather than the copper on the board. In other words, the solder balls up on the board but sticks pretty well to the parts.

To fix this I started heating the copper before placing the components on. This made the solder more willing to wet the copper surfaces which made the soldering much easier.

Another piece of advice is to be very careful with the board when it's hot. The adhesive that attaches the copper to the bottom layer seems to be much weaker when it's hot. I accidentally melted one of the mini-USB-port's pads off by heating it too much (and probably being a little too aggressive).

After you're done soldering, make sure to clean it well with soap and water because the flux seems to tarnish the copper pretty rapidly.
PCB Before Soldering PCB After Soldering Washing the PCB

Programming the Board

There are five main steps to getting your board programmed:

1. Use a multi-meter to check for accidental solder bridges or lack of essential connections.

2. Plug the board into a USB port and measure the voltage accross the power and ground of the 2X3 header component (should be around 5 volts). If you see blue smoke or smell burning electronics at this point something went wrong.

3. Attach the AVR programmer to the 2X3 header. If you see the red light on the programmer turn green then your board is working! If you see a flashing orange light you most likely have the 6-pin ribbon on backwards. If the light stays red something is wrong with the circuit. Check again for solder bridges or unconnected traces.

4. Then follow the programming instructions very carefully.

5. Remove the two jumper resistors.

Mistakes I Made

The biggest mistake I made was being a little to forceful with the copper trace when I was soldering. Thinking that I was keeping the solder in liquid form I tried to adjust the USB port but instead I accidentally peeled the copper pad off the board. It was surprisingly easy to do so I suspect that I had heated the pad so much that I weakened the adhesive holding the pad down.

The other mistake I made was not washing the board thoroughly enough after soldering. Now the board is already pretty tarnished after only a few days.

This final note is less of a mistake and more of bad practice. The first things I soldered to the board were the difficult components (USB port, attiny, etc.). This was a bad idea because I still hadn't gotten the technique down. I thinks it's smarter to start with the easy stuff, like the diodes, then move on to the harder components when you get the hang of things.