Thursday, 27 October 2011

Creating PCBs - Generating the master artwork

The master artwork is the mask that is used to expose the desired pattern on the photo-resist material bonded to the blank copper board. Typically, it is some kind of thin film material that will block the UV exposure light in some places, while allowing the UV light through in others - resolution and contrast are the important factors.

The simplest way of creating this is to start with some kind of UV transparent film, such as an overhead projector transparency, and print the pattern to be masked with a laser printer. Provided that the laser printer can get enough toner onto the film, you should get a nice high contrast mask. Remember that if you are using negative acting photo-resist, you need to print a negative image; that is, black where the photo-resist is to be removed and subsequently where the copper will be etched from the board. Eagle helps to create the negative image by providing the PS_INVERTED device in the CAM process. The workflow I've come up with is like this:

Use Eagle's CAM processor to generate an negative postscript file. The offset controls on this window can be used to move the design within the page, but it also inverts all the area from the lower left corner, so it's wasteful of toner and ink. We'll fix that in a moment. For now, leave x and y offset both at 0.
Eagle CAM processor setup for inverted PostScript output
Next, edit the generated postscript file, and at this line, after the comment % set the origin:

      LeftMargin add LeftOffset add BotMargin add BotOffset add translate
Add in your desired offsets in inches
      LeftMargin 2 inch add LeftOffset add BotMargin 1 inch add BotOffset add translate

Now print the .ps file to your media. Only the area of the board (usually the "Dimension" layer) will be inverted.

Unfortunately, my laser printer doesn't generate very solid blacks. My inkjet printer (Epson WorkForce 840) generates a much better quality output, but getting the ink to behave requires the right media. Plain paper isn't UV transparent enough. I've tried a number of media, and here are the results:

  • A4 Tracing paper sheets - this is quite inexpensive, but it has a bit of a texture to it. It has areas where the density is different, so I think that may affect the exposure. The ink tended to soak and creep along the fibres of the paper, so the end result, although very black, wasn't as sharp as desired.
  • Rapidraw polyester drafting sheets - this is a very uniform polyester film. It's not clear, but a light grey translucent material. The output contrast was good, but it refused to dry. The other problem was that because the material does not absorb any of the ink at all, there tended to be too much, and it would bleed a little. Changing the printer setting may have helped. Here's a sample (for a sense of scale, the small numerals are 0.8mm high)
    Rapidraw film - too much ink on a base that doesn't absorb it at all
  • Inkjet overhead transparencies - these are the most expensive, and you have to use the ones designed specifically for inkjet printers. They have a coating on one side that assists in "capturing" the ink. It still took a bit of fiddling with the driver settings to get a quality print. This is the "plain paper monochrome" settings output. You can almost see each drop of ink, it needs more...
    Inkjet transparency - plain paper/photo/black settings

    This is the output on "best photo, gloss photo paper" setting...
    Inkjet transparency - Premium photo paper semi gloss / Photo RPM settings

    The only glitch with this is that the ink isn't drying quickly enough, and the paper exit feed rollers are causing that dotted vertical line near the "16". That can be solved.
Even though the transparency was coated, the ink was still "tacky" an hour after printing. This was solved by putting the printed transparency into the oven at around 65°C for 40 minutes to cure the ink better.

The same artwork creation process can be used to create the artwork to use with the UV curable solder mask paints. The only difference is which layers are configured in the CAM processor output, and whether a positive or negative master is required.

Next time, exposing the photo-resist on the PCB.

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