PCB making - garage lab process

First, the copper must be cleaned throughly. Any traces of oxidation should be scraped  away. More often than not you won't have any noticeable oxidation, but I have a batch of copper clad where oxidation is very evident around the borders. Any grease, paint, old traces from a failed etching attempt - everything but pure copper should be gone. I do that by scrubbing the board with a 3M kitchen scouring pad in warm water with soap. After the surface is clean and smooth I hang the board to dry. Some people recommend wiping the board with acetone, and acetone alone, not alcohol. I've tried acetone, alcohol, and just air-drying, and all methods seem to work just fine. Generally I pre-drill a hole in one corner of the PCB board, or attach a small paperclip so I can hang the board by it.

After cleaning and drying the board I apply ER-71 photoresist. Why do I choose  negative photoresist? Because the chemicals needed for negative photo development seem to be cheaper, at least at my local supply store. Perform this, and the following steps in low light. I use a flashlight pointed to the ceiling, so only reflected light falls on the board. I am sure that I am overly cautious, but it works well. I did not want to wait for each side of the board to dry, so I tried to speed up the process by hanging the board by one corner and spraying both sides at the same time. Bad idea. The photoresist follows the laws of gravity and runs down resulting in an uneven coat. In fact, 99% of the board area ends up having the coat so thin that it is not functional. So, make sure to lay the board flat and spray an even coat. If you see photoresist pool in places, the coat is too heavy. Wipe it off with some photoresist developer and start again. It helps to experiment, so you know what 'too heavy' is. Not having enough photoresist is equally bad, although that can be remedied somewhat by spraying a second coat. After one side has dried, flip the board over and spray the other side. I "bake" the board in the oven at ~120 deg. F. for 5 minutes to speed up the process.

While the resist is drying, print out your design. I use a transparency for laser printers  and copiers, and a HP Laserjet 6L printer. Since the artwork is a negative, you may expect to see large black areas. Not all printers handle that well. In fact, the toner in my printer is getting a little thin, so sometimes black areas have quite a bit of room for improvement. If you have a printer like Laserjet 6L where you have an option of having printouts come out into a vertical tray or flat on the table, choose the latter. You do not want the transparency bend more than necessary. Moreover, the heat seems to distort the transparency a little (it shrinks, and not uniformly). In general this is barely , if at all noticeable, but we'll have to line up the holes in the two printouts - for the top and bottom layers - later, so even minute distortions matter a lot. Make sure that you print only once on each transparency, and preferably place the printout in the same location on each sheet. You may want to mirror the top layer, but not the bottom - that way the toner will be closer to the copper, and you'll get slightly crispier edges on traces. After printing line up the printouts and make sure that the holes on both sides coincide.

Now we have to choose how to proceed - etch the board before drilling, or drill first and etch later. Each method has its advantages. I started out with etching first. This method works well. It is also safer because there are fewer chances to scratch the thin layer of photoresist or expose it to light too much. On the other hand, it is extremely hard to line up both transparency sheets perfectly. If you drill holes first, however, then lining up becomes very easy. So this is my preferred method now.

For exposure I use a 300 watt ER-2000 light bulb in a dark closet. Make sure that it is in  a ceramic socket because it gets hot. it takes less than 5 minutes per side, depending on how far the light bulb is from your artwork (just follow the directions that come with the light bulb).  It helps immensely to have an exposure frame. I did not have one, so I paid a visit to a nearby thrift store, got two picture frames for under a buck, took out their glass sheets and taped them together on one edge. Paper clips hold the other edge shut so the glass firmly presses the transparencies and the board together. Developing the board is straight-forward. Just follow the directions. Etching is straight-forward as well. I warm up the etchant by placing the container

Drilling can be a tedious task. You absolutely need the small carbide drill bits. Also, a drill press is a must - the drill bits are so fragile that hand-drilling is not an option. Use the highest RPM you can - you cannot go too high. However, even with a drill press the results tend to be sub-par. It is extremely hard to drill the holes consistently on the right spots, especially if there are a lot of them. So, after wasting a lot of time and boards I finally started using my taig mill to do the drilling.