Archive for March, 2006

There are a lot of PCB CAD programs out on the web, often linked to PCB manufacturing companies, but if you are just making a small one off board, these seem like using a sledge hammer as a stapler. Quite possibly a lot of fun, but probably not for all of the right reasons.

The technique I use focuses on simplicity and tools everyone has. The first step of course is to draw the layout, then etch it.

Because I am new at this, I use pen and paper to design the layout of the pcb. One of the most useful guidelines I have seen around the web is to keep all of the IC’s aligned the same way, especially keeping pin 1 in the same place – this one makes it much less likely to screw things up when you are putting it all together. Given the basic layout, it is now time to make the actual PCB mask.

To do this, I load this bmp template into the nearest painting program, such as MS Paint. The template has a regular grid with the same spacing as your components. Note the pads on the right side of the template. These are the pads to use for the components. Just copy and paste them where they need to be. Use the blue grids for your alignment.

A couple of hints.
1/ Use the biggest pads you can, but make sure they do not touch. Bigger pads mean easier soldering.
2/ The square pads at the bottom are for IC’s. The big round pads are too big, and touch when they are placed in adjacent grids.

For the traces, just draw a line between the pads. How big? Bigger is better, just make sure there is a reasonable amount of space between them.

When you have got all of the traces on and are happy that the layout is good, it is time to get rid of the blue grids. If your paint program has a global colour change, then set the blue to be white. If not, just scrub them out. Having a white spot in the center of the pads will make it easier to drill them, as it will act like a punch mark. The grid marks are unlikely to cause too many problems if you don’t scrub them, but will look messy.

Having done this, print it out so that the scale is correct. You might be able to do this by setting the dpi, or you might have a ‘print at x scale’ option, this might take a little messing about to get the size right the first time, but then just remember the settings.

If all has gone right, you will have a sheet with a design which looks like this:

This circuit controls the rear window defog in my MX5 (Miata), I didn’t feel like paying a few hundred dollars for one, but a heated glass window is really nice in the Minnesota winter. If you look carefully, you can see that the IC was put in with round pads rather than the square ones. This worked, but was a pain to solder.

Now all you have to do is etch the board. This is simple enough once you know how that we shouldn’t really post the information on the internet for fear that everyone will learn to do it. All you have to do is print the layout on a laser printer, and then iron it onto your board.

Really, that is it. There are a couple of gotchas, and some explanations, but not much.

1/ Give your pcb a polish first using some fine sandpaper or similar – we don’t want oil from finger marks messing up our work.
2/ Only a laser printer will work, not an inkjet. Laser printers put a plastic layer on the paper, which they then melt onto the paper, so when you iron it, it will melt again and stick to something else, like our pcb blank. Inkjets fire liquid ink onto the paper.
3/ What sort of paper? There is some debate on this on the net, with preferences for brands of expensive photo paper. I use whatever paper is in the printer. This is generally whatever is on sale when it is time to buy new paper. The reason the type of paper is not important is I do not peel the paper off, but now you are getting ahead of yourself.
4/ It doesn’t take long to iron the layout onto the pcb. I have a ceramic tile I use as a plate, and set the iron on full heat. A couple of minutes making sure that there are no cool spots, and you are good to go. Make sure you ask your wife/girlfriend if you can borrow her iron first. If she says no, don’t get caught.
5/ When the paper is stuck to the board, then just drop it in a bowl of water with some soap added and let it soak for a while. I generally give it 20 minutes or so, then just scrub the paper off using an old toothbrush (you do have one of those in your toolbox don’t you? If not, don’t use your wife/girlfriends). You can be pretty aggressive, the toner is stuck pretty solidly to the board. It is good to make sure you get the paper out from the middle of the pads, this will make it easier to drill.

When you scrub the paper off, you will have something which looks like this:

This board did not come out as well as usual, I think I got lazy with the amount of time I spent ironing it on. If you look carefully you might be able to see some gaps where the toner did not stick. No problems, borrow some nail polish (now you see why you didn’t use her toothbrush before) and touch them up:

Now you have a pcb ready for etching. Drop it in the ferric chloride, and wait. Make sure you read the instructions on this, it is pretty nasty stuff – just look what it does to your copper board & ask if you want it to do that to you. Once it comes out, use some fine sandpaper to sand off the toner, and you are ready to drill. It should look like this before and after sanding:

The drills you need are very small and as a result break really easily. You are best to use a drill press at the fastest speed it has and a vice. If you don’t have a drill press, use your dremel, if your drills are not the standard dremel diameter, you can pick up a chuck attachment like this, which will let you use most any size drills. As far as the pcb drills, anything you find at Home Depot is likely to be too big, and as they are very long, they break even more easily. Have a look around, you can buy surplus/refurbished drill bits relatively cheaply. You are likely to break them before they get blunt, especially if you don’t have a press, so I wouldn’t worry much about the quality. Always buy more than you need, because it is bloody annoying to get half way through drilling a board to break your last drill.

Voila, a finished pcb ready for assembly!

This method has a resolution of below 0.5 mm, so should be good enough for most projects.