Depending on the project that is chosen, there is a possibility for a lot of hardware. Breadboard circuits work, but at the high speeds that we wish our FPGA projects to run at, breadboard design can easily be flawed. Even tinned wire can make a poor connection if not taken care of properly. This can be a leading cause for ghost problems that can take over a project. Printed circuit boards or PCBs are a wonder way to take care of this headache. Components are grouped together on a rigid fibreglass board and soldered to give a constant high quality electrical connection. Copper foil is bonded to the board and selectively removed during manufacturing to leave user defined "traces" which electrically link all of the components together as required. Some boards have only one side of copper traces and termed single-sided PCBs. Other boards have two or more layers. With two layers, or double-sided boards, a single fibreglass board has copper bonded on both sides. One trace can be connected to another trace on the other side of the board by a hole drilled in the fibreglass which is then plated with copper. This is known as a "via". Since holes must be drilled before vias can be made, the PCB end fabricator must bond the copper in these vias themself. This process takes a large number of specialized chemicals and processes which are impractical to us. Multi-layer boards, (greater than 2 layers), build on this process. For example, a 4 layer board is two double-sided boards which are then sandwiched together with an insulating layer of fibreglass between the two inside layers of copper traces. Traces are then connected similarly with vias. This Application Note assumes that single-sided boards would be built for EE552.
One can easily design their own circuit board on their own computer. Protel Easytrax, a shareware PCD CAD program, allows you the ability to design boards with up to 6 layers. Since we are only interested in one or two layers, this program is more than acceptable. The program runs on IBM compatible machines. The program is available to anyone from Protel's Website. The program comes with its own instruction manual within the compressed set of files. There actually are a number of programs that come with the package. A graphical program sets up the monitor driver for the program. Another allows you to print or created a postrscript file from a file created with the PCB editter.
The only downside to PCB contsruction is the equipment required to produce the boards at home. The process begins with a photo exposing process which requires transparency sheets, special UV light sources, and development chemicals. Then another chemical is required for removing the excess copper from the boards, or etching. The boards must be drilled for any through-hole components. This requires special small scale carbide bits and a drill press to properly drill through the boards. Once the board is done, proper soldering equipment is required to secure your components to the boards. Finally, a sealing chemical called "conformal coat" seals the board from oxidation and improves electrical stability. All of these items can be purchased from your favorite electronics store. Unless you know someone who has at least the hardware required, the unlying costs can be quite large.
The other option is to have a commercial outfit manufacture your boards for you. These companies can do multi-layer boards easily as well as other more professional aspects of design such as silk screened labelling and solder-mask(the green color seen on most boards). There are a number of companies to choose from. The yellow pages lists local companies as well as ones from other cities such as Calgary. Depending on the company, a minimum order may be required.
If you decide to build your own boards at home, then the following is the suggested order of steps to manufacturie your PCBs. After all CAD work is done in the EasyEdit program, open the EasyPlot program. This program allows you to print to a local printer or to a postscript file to use on a postscript printer. Laser printer quality is definitely the lowest resolution required, especially for complicated boards. Other higher resolution methods are available. Under the set-up menu, set the drill guide to 10mils. This gives you a centering mark for drilling pads later. Place an overhead-projector sheet into the printer and print out the circuit board artowrk onto the sheet. Do not continue any further until you can verify that your design is correct.
Now that you have a photo-positive of your artwork, the photo-exposing process can be done. Presensitized Circuit boards are copper clad PCBs with a thin coat of UV sensitive material called photo-resist. The transparency sheet is placed over the this PCB and exposed to UV light for a specified length of time, usually 5 minutes. At this point, the PCB is immersed in a development chemical. Within a minute, the excess photo-resist dissolves off the board. When all of the excess UV material in dissolved, rinse the board in cool water to ensure the process stops. After drying, the board can be placed into an etching solution. This solution eats all of the exposed copper off of the board. All of the copper protected by the UV material will remain in the end to become our traces. Once all of the excess copper is removed, rinse the board again to stop the process. Flus remover is excellent for removing the photo-resist material after the etching process. The etching process doesn't always produce perfect results and you may need to jumper broken traces or use a hobby knife to break trace bridges.
At this point, all holes for mounting the boards and through hole components must be drilled. Carbide bits must be used. Normal steel bits destroy the copper
surrounding the hole. A small motor tool, like a Dremel tool, is perfect for this job. Typical hole sizes for resistors, capacitors and ICs are #64-#66. At
this point the board should be ready for component stuffing. This job requires a good soldering iron as well as solder and even flux. Soldering is a topic
outside of the scope of this Application Note. Once all soldering is complete, use a conformal coating material to seal the board. This will protect it from
normal oxidation as well as accidental shorts.