This describes an easy method to make professional looking two-sided printed circuit boards at home.
Step 1: Get Ready
This instructable assumes that you know how to generate a layout file using Eagle PCB or similar layout program. I use the toner transfer method of making PCB's (printed circuit boards) much like many others. The basic idea is to use a glossy paper, print the PCB design on the paper using a laser printer, and to use a hot iron to transfer the toner to the copper. I use the glossy paper that they have behind the counter at Kinko's. Go to Kinko's and ask for some sheets of their glossy laser paper, which is really cheap (about 5 cents a sheet). Some people advocate using glossy inkjet photo paper, but I think this is a waste and the cheap glossy laser paper comes off easier.
Anyway, once you have your design and paper, you will need to print the design. The key here is to mirror the top layer so that it will come out correct once transferred to the copper board. It can also help to include alignment marks (the T shaped things in the pic) beyond the edge of your PCB to help you align the two layers. See below.
Step 2: Align Layers
The next step is to cut out the top layer and bottom layers, and to place a small loop of scotch tape on one corner of the top layer to allow you to tape the top and bottom layers together once they are aligned. You want to cut the top piece smaller than the bottom so that you can stick the top to the bottom. Leave some room around the edge of the design so that no copper will be visible. See pic.
Now, take the two paper pieces with the top and bottom layers printed on them and go to a window or patio door. It needs to be daytime since you will be using the backlight from outside to see through the paper. Place the bottom layer on the glass, and then taking care not to stick the top piece down yet, align the two paper sheets using the alignment marks, your vias, features or other method. When aligned, stick the top to the bottom. Carefully lift another corner of the top piece and add another roll of tape to keep the layers aligned. The key is to make sure the tape will not interfere with the copper board being placed between the sheets. See pic.
Step 3: Iron It On
With the two paper sheets taped together, turn on your iron to the highest setting. Also make sure there is no water in it if you have a steam type iron. Now, take your sheet of copper board and slide it carefully between the two sheets. See pic below. Position the copper clad board as desired, and once the iron is hot, place the iron on the paper and press hard. It takes some practice to get the hang of ironing on the toner, but just press hard and wiggle the iron over the whole board while taking care not to move the paper relative to the board. Once one side is ironed to your satisfaction, then carefully flip the whole thing over and iron the bottom layer. One thing to remember is to clean the board carefully with a cotton ball or old sock soaked in isopropanol (rubbing alcohol) before ironing to remove any finger prints or grease.
Once you are done ironing, cut the paper around the board if desired, and drop the paper and board into a container of water to soak the paper. Let this soak for about 10 minutes. With the cheap glossy laser paper from Kinko's the time is much shorter than the high quality inkjet photo paper. Once the paper is soaked, peel the paper off both sides. This should leave the toner and a thin layer of the paper along with the glossy stuff on the copper board. Using your thumbs or an old toothbrush, carefully rub off the extra paper pulp and junk on the board. See pic, below showing a board with half of the pulp rubbed off.
Once the paper pulp is rubbed off, carefully inspect the traces and features on the board for small imperfections and stuff that will cause problems later. Key areas to pay attention to are closely spaced traces and pads where it is easy for paper or glossy coating to bridge and keep the copper from etching. Also, you can achieve very fine pads for doing TSSOP, QFP, and other fine pitch packages using the toner method if you carefully scrape between pads using an Xacto knife or similar prior to etching. During ironing, the toner tends to smear slightly, so very fine pitch pads tend to mush together. Using the knife, you can scrape between the pads or traces to make sure the copper will etch between. If you are careful, there is no reason you can't get 800 micron or even 500 micron pitch pads.
Step 4: Etch and Clean
When you are satisfied that the critical small features are ready, then put your board in a plastic or glass container and pour enough PCB etchant over it to cover it. I use the Ferric Chloride etchant sold at Radio Shack. Gently swirl the dish with the board and etchant until it is finished. Careful not to spill on anything metal and chromed since the etchant will mess up the finish with a quickness. In most cases, where the bottom layer has a few traces and a ground plane, it is not necessary to flip the PCB during etch. Just make sure to get the board moving around the container so that the bottom etches too. If you have detailed delicate features on the back, you may want to lift the board repeatedly during etching with a plastic fork in order to help the back etch without scratching it. See pic.
When the board is done etching, you need to get the board out and rinse it with lots of water. Dump the used etchant in your toilet and flush it as recommended by the manufacturer on the bottle. As you can see from the pic, you should be able to hold the board up to a light and see through the board at this point to verify the alignment of the top and bottom layers.
Cleaning off the toner is a pain in the butt. The easiest way is to use an aggressive solvent such as brake cleaner or acetone (nail polish remover) and a rag to rub the toner off. Using your preferred method, scrub the toner off the copper and get out your multimeter to test if any of the traces are shorted together. I find that long parallel traces often have small shorts between them if you don't get all the paper off prior to etching. A toothbrush helps. If you find that some traces or pads are shorted, then using an Xacto knife or similar, scrape or cut the copper until the circuit is open. Once all the circuit is verified in this manner, you can start soldering down the parts. I find that putting the fine pitch components down first is key, so that you can verify each pad as you go. Since the toner smooshes the pads of QFPs and TSSOPs and the like together, it is easy to form a solder bridge between pins. Take your time and have your solder wick handy.