PCB Layout Tips for Home Etched Boards
By Gary Aylward
- Be conservative! Most CAD programs are aimed at commercial PCB manufacturing. They default to 0.012" (0.3mm) or less tracks and gaps. Whilst it is possible to achieve 0.012" at home, 0.020" is so much easier! Make life easy by using the biggest tracks and gaps you can. If you can get away with 0.1" pitch through-hole parts and 0.050" tracks and gaps it will be a breeze.
- Beef up the pads! Again CAD programs assume some small drill sizes and often minimal pads. Small drills break very easily (so work out expensive) and small pads require very accurate drilling to avoid the drill breaking out the side of the pad. Oval pads can help if you need to get a track between 2 pads.
- Especially beef up your vias! It's OK to use a 0.3mm hole and 0.5mm pad on a professional board, but for home building, stick to a sensible drill size (at least 0.8mm) and a decent pad (2mm is comfortable).
- If your CAD program can do it, put a 0.020" (0.5mm) drill hole in the middle of each pad, and make sure it appears as a hole in the copper on the final print output. You will then have a centre help with the driling.
- Use one side as a power and/or ground plane. It really reduces the number of connections you have to route on the signal layer, and gets rid of the longest net(s). If you use the plane for power and ground, use the CAD program to route both with no links. Then work out a route between the tracks to cut the copper into 2 areas. You don't need to etch the plane side, just cut along your route with a craft knife (Best done after drilling as the holes act as a guide).
- For through-hole, put the tracks on the bottom and the plane on the top. Components that connect to the plane can simply be soldered on the top side.
- For surface-mount the tracks need to be on the top and the plane on the bottom. You will need to make some via holes (use wire links) to connect to the plane.
- For a mixture of through-hole and surface-mount, put through-hole on one side and surface mount on the other. I usually put SMT on the top as that's what I use most of. Make sure you "mirror" the components correctly when putting them on the bottom side.
- If using a power/ground plane, don't forget to mask the plane side when etching the signal side! Ordinary masking tape works fine, make sure it is well stuck down and overlap any joints carefully.
- Put some text on each layer, even if it's outside of the board outline. This way you can easily tell whether you've got the design round the right way! Don't forget text on the bottom layer appears reversed in the CAD program!
If using any of the "iron-on" methods, don't forget the pattern needs to be printed out in reverse. The text comes in handy here. If you are doing the bottom side of a board, you need to print it "straight" from the CAD program as it is already designed in "reverse".
- Add some dimension lines to your design. You can then measure them to check that you really have printed out at 1:1.
- Make sure you use at least a 600dpi laser printer, the old 300dpi ones are no good for fine PCB tracks. Even with 600dpi, don't attempt anything below 0.003" as it might not even print properly.
- Don't use an inkjet printer, it won't work!
© Gary Aylward, 2006