Wednesday, 25 May 2016

Etching homebrew PCBs - transfer

There are a few ways you can transfer your design onto your copper clad board. Our preferred way is using the laser cutter; paint the entire board with matt acrylic car paint, then etch away parts of it with the laser cutter, to create a mask.

But most people making PCBs at home aren't going to have easy access to a laser cutter. Quite often, we don't have easy access to ours (it's at the studio about half-a-mile away). So for one-offs and prototypes, we normally use the toner transfer method.

If you can afford it, Press-n-Peel blue paper is far and away the best medium. It's a powder-coated paper and gives great, consistent results.

We use a cheaper Chinese  alternative - it's a starch-based carrier which is water-soluble. It doesn't give quite as nice results as Press-n-Peel blue, and large filled areas are prone to "pitting". But it's a reasonable alternative to PnP.

One thing that a lot of people ignore is the quality of the toner you're using.
The idea with toner transfer is to get the PCB image onto the transfer paper, then - with heat and pressure - force the toner plastic to melt and re-set so it sticks to the copper clad board. The quality of the toner used is critical to getting good results.

  • The best printer we've found is a Xerox Phaser - the toner has a very high plastic content, melts at low temperatures and offers excellent coverage.
  • We gave our Phaser to the cool kids at BuildBrighton (it was getting to be far too big for the unit) so we now use a small desktop Dell laser printer. It's not as good as an HP Deskjet, but it's passable
  • We used to have a Brother laser printer. Don't even bother with these. Their toner is junk.
If your printer allows it, turn the fuser temperature down to as low as it will go - we only want the toner to stay on the paper for as little time as possible; we're not looking to make good, solid, long lasting images on the paper!

To save using up your expensive transfer paper, print the design on a sheet of plain paper. Then cut out a small square of transfer paper, stick it over the printed design with a piece of tape and re-feed the paper into the printer, to print the design again over the original image.

Cut the edges of the transfer paper up close along two of the sides of the design.

Clean the copper board thoroughly. You can use acetone and a tissue to remove greasy fingerprints,  or rub lightly with wire wool (or even a Tommy Walsh sanding block from the pound-shop) until the surface is shiny - but be careful not to make the surface too pitted. Wipe the surface of the copper down with a lint-free paper towel to ensure no dust or debris remains on the surface.

Align up the cut edges of the design with the edges of the board and hold in place using paper-based masking tape. By leaving a little extra transfer paper along the other edges, there's plenty room for the masking tape to get a good grip to stop it slipping.

Run the board, face up, through a laminator set at the hottest possible setting.
Depending on the laminator and its heat settings, you may need to do this five or six times. Don't always put the board through in the centre of the slot - put it in, off to one side, in case the rollers might "sag" towards the middle.

You can use an iron, but we've found it makes things more difficult than they need to be - the constant lifting and moving of the iron can easily cause the image to smudge. With our laminator (an Alphatec found on ebay a few years back) we've never yet had a smudged image.

When the transfer paper appears uniformly translucent on the back, you're done. For speed, we often quench the  board in cold water to speed up the cooling process. You can do this, or leave it for ten minutes or so to cool down.

After wetting thoroughly, carefully roll back the backing paper from the design. Don't lift it off in one go, or let it "snap" as press-n-peel has a tendency to do as this can easily introduce breaks and/or fractures in the traces.

Remove all traces of the tape used to hold the transfer paper down.
Cut the copper clad board to size, trying to get as close to the edges as possible. If there is lots of copper around the outside of the design, colour it in using a Sharpie permanent marker. All the time we want to reduce the total amount of exposed copper.

You're now ready to start etching!