PCB Fabrication

TESTING

The primary components of Sense-It are surface mount and require the fabrication of printed circuit boards with headers to in order to prototype in the lab. We chose an FeCl3 etching method to create our boards. For many standard sized components, PCB's can be purchased through many online retailers. For Example, we found a board for our 28 pin SSOP, PIC32 module on Amazon. Our BM77 bluetooth module, on the other hand, we needed to create ourselves. The process will be detailed below.

PIC32 and headers soldered to prefabricated PCB from Amazon.

Photoresist

A photoresist must be applied to the copper clad FR-4 board and allowed to rest for 48+ hours.

Mask

Using EAGLE, we created a mask for our board. In this process, we will need a negative so that once printed to a transparency, our paths can be seen through the print.

U.V. Exposure

In our lab we have a set of U.V. bulbs in a machine resembling a flatbed scanner. Once the mask has been place over the photoresist covered board, it can be loaded into the U.V. bed. Be careful that the mask has been put on right-side up. Once the board has been exposed to the U.V. light you cannot go back. You must also be careful not to expose the board for any longer than 30 or 40 seconds. We typically have the best success if we keep the board exposed for 20 to 30 seconds.

Sodium Soak

After the board leaves the U.V. bed, it can then be thrown into a sodium bicarbonate bath. Here, the board should soak for approximately ten minutes with regular agitation. While the board sits in the bath, the photoresist that was not exposed to the U.V. light should slowly wash off revealing the underlying copper. Be careful not to over agitate the board or you could wash away the hardened photoresist. However, if small amounts of the copper paths are left exposed, once the board is removed and dried off, you can touch up the paths with a permanant marker. We have had success using Sharpie brand markers.

FeCl3 Bath

In this next step, you will be working with a strong acid that can eat through cloths, fingers and eyes so it is important to wear appropriate protective devices, such as gloves and goggles. You may also want to wear clothes you don't mind discoloring. Once the board has been dried off and touched up, you will need to submerge it in the FeCl3 solution. Once submerged, the board will require constant agitation. We used a circular agitator. We have had varied results with this part of the process. Some boards are ready within 30 minutes. Other boards have taken up to an hour or more. If the board takes an hour, there is a good chance that you will weaken the adhesive between the copper and the FR-4 surface. A board is ready when the exposed copper is gone leaving only FR-4 and photoresist visible. The remaining photoresist can be washed away with water and a mildly abrasive cloth.

Soldering

The board is now ready for soldering. For larger components, a standard iron and some solder works very well and is how we installed our headers. For smaller components, we have found that solder paste is a much safer route. For our BM77 module we used solder paste and a hot plate at 220 degrees Celcius.

Finished Board

With some patience and some luck, your board should be ready to be plugged in.