The Geotechnology Page: Misc: PC Board Transfer Film

PC Board Transfer Film

Carl Moreland

You've found a circuit you would like to build, so what do you do? Assuming you have the schematic, you need to get the parts and you need a PC board. There are plenty of places to get parts, and I have listed a number of good vendors on the Resources page. Likewise, there are plenty of ways to make a PC board, but not all of them are easy. Here is a method that meets my three criteria for making a PC board: easy, fast, and cheap.

The "PC" in PC board means "printed circuit", and that's exactly how this method works. It uses a laser-printable transfer film from Techniks which allow you to create an iron-on transfer of the foil pattern. One of the interesting things about laser printers is that the toner contains polyethylene that resists etching. If you could literally feed a copper clad board into your laser printer then you could print the foil pattern directly and etch. Since we can't do this we need to look at another way of getting the toner onto the board.

Laser printers use heat to fuse the toner onto the paper so we should look to heat to get it off. What we need is a medium that will release the toner, which means that the medium must have less "holding power" than the copper clad. If you try to use regular paper or even overhead transparency film as your iron-on medium you will find that the image does not transfer very well. In the case of paper it is obvious that the paper fibers "grip" the toner better than the copper, and the same is apparently true for overheads.

The Techniks transfer film is a clear plastic sheet with a blue coating on one side. It is the coating that acts as a release agent when heat is applied. So you print the foil image on the coated side of the film and simply iron it on. I used a 600dpi HP LaserJet5; I also did a test print to a 300dpi LaserJet4 and found the image quality to be slightly worse but still acceptable as long as you are not doing aggressively narrow line widths. Here is what the transfer film looks like after printing:

Before ironing this on, you must thoroughly clean the copper board, then clean it again. I used 0000 steel wool to brighten the copper, then soap and water to clean off any oils (including the natural oils in your fingers - don't touch the surface of a cleaned board!) then hot water to rinse, and dried it with a clean lint-free cloth.

When you're ready lay the film, coated side down, onto the copper board and align it. Using an iron on a medium-hot setting (no steam!), press the film gently and evenly to get it started. Don't twist or slide the iron initially as it might move the film. Once you get it started, the film will stick fairly well to the copper and you can move the iron around. I recommend that you use a hobby iron, such as the type used to apply monokote to model airplanes. Work the transfer for several minutes, even for a small board. When you think you're done (it's sort of guesswork) rinse the board under cold water and peel off the film. Here is what you will see:

At this point you may or may not have to do a little bit of cleanup or touch up. The direction sheet says that you can put clear packing tape over the board, then peel it off to remove filled donut holes and the like. In my very first attempt (what you see above) the transfer worked very well with only a few traces that had rough edges where I did not do a good job with the iron. Nothing that needed attention, though, so I went directly for the etch tank. When the etching was done I cleaned the transfer film and toner off with acetone. The result was this:

The two areas that are circled are where the copper edges are a little rough; these are the areas where I did a poor job with the iron. The rest of the board is very sharp, almost no over-etching. Even the text is very readable, although from this scan it is hard to tell. The minimum line width on this board is around 25 mils, so Techniks' claim of 10 mil line widths is possible. The final stage of PC board fabrication is to drill the holes, thoroughly clean the copper, and apply tin coat.

Summary of steps:

Print the layout onto the transfer film.
Iron the image onto a meticulously clean copper board.
Etch the board.
Clean the etched board.
Drill, tincoat, and assemble.

Besides the Technik film I used, there is another PCB transfer paper made by DynaArt Designs, available through Digikey. This appears to be a paper-based product, not a plastic film, as it requires a warm soak to remove the paper after ironing. I have not used this product so I cannot comment on its performance. However, the price is slightly higher (5 sheets for US$14.95) than the Techniks film (5 sheets for US $11.95 at All-Electronics), and directly ordering from Techniks can save you even more on quantity. Hanes also has laser-printable transfer paper for making T-shirt iron-ons; it is possible that this might work for PCBs. There is also a chance that you could come up with some sort of release agent that could be applied to overhead acetate material before printing to it, but this is a little risky as anything you apply could gum up the works of the laser printer.