The toner transfer method
transfer method works well for printed circuit boards that are too complex, or have physical tolerances too exacting, for hand-drawn boards, and it avoids some of the bother of photoresist methods (or the expense of sending your board to a fab house). It is also a very convenient way to make boards if you've done the board layout on a computer.
The method works by using special paper to transfer the diagram printed by a laser printer (or xerox machine) onto the copper surface. Xerox toner happens to make a reasonably good etch resist, and so you can just dunk the board into etchant at that point. Once it's etched, the toner scrubs off fairly easily.
For this, you'll need plain copper-clad board (not coated), a xerographic
printer or copier (e.g.
a laser printer; inkjets won't do), some toner-transfer paper
, an iron, an etching tank, and the usual tools such as a drill, saw, shear, etc.
The first step is, as always, to lay out the board. There are many programs, free and commercial, for doing this on a computer; in a pinch you could use a decent drawing or diagramming program. The end result is an image you can print that is black where you want the copper to be, and white elsewhere. The artwork has to be mirror-reversed, because of the transfer step.
You print the layout onto toner-transfer paper, which is a somewhat glossy coated paper whose gloss dissolves in water. You have to use a laser printer for this. If you have another sort of printer, or if you decided to draw the layout by hand, you can use a xerox machine to copy the image onto the transfer paper. The point is that the paper works with toner, but not with other kinds of ink.
Transferring the toner to the board is the tricky part. First clean and dry the board as described elsewhere in this node: it should be a bright pink color. Place the board, then the paper (aligned so that the artwork matches up with the board), then a thin towel or a napkin. Take a hot clothes iron — no steam! — and press it against this stack for a while. It will take a small amount of practice to learn how long to do this, how hard to press, how to do it without smudging the transfer, etc. 45 seconds is about right.
Once this is done, dunk the board and the paper (which will now be stuck to the board) into some water and wait. Eventually the paper will curl away from the board, leaving the diagram on the board. (If you've ever made kit models with decals, the process is similar.) Take the board out and dry it gently. Touch up any gaps or pinholes using a resist pen.
From this point, the toner transfer method is the same as the others: etch with ferric chloride or ammonium persulfate, wash thoroughly and scrub the toner off with steel wool, drill and cut if needed, and populate it.
Details and Asides
Cleaning the board can be done with fine steel wool without any solvents
more noxious than isopropyl alcohol
. Clean copper is very light in color, almost salmon pink. It oxidizes in air, so clean it shortly before you use it.
The boards made using this method are good enough to use surface-mount packages such as SOIC or 0604 discretes. (You could probably go even smaller, if you're careful.) For some circuits — especially if you can use surface-mount parts to avoid having to drill holes — it's actually faster to make a board than it is to wire it up using normal prototyping techniques. And the final product is much nicer, smaller, and more rugged.