Wednesday, 19 October 2011

Creating PCBs - from design to finished board

Over the next few posts, I'll be looking at the processes involved in creating a finished printed circuit board using photoresist film.

I'll be using the negative photoresist film, as available from a number of vendors on eBay, but much of the process would apply equally to the positive acting materials available from places like Kinsten. I got the blank PCB, photo-resist film, developer and copper etchant from eBay.

The film is designed to be exposed with ultra-violet light. Typically, it's UV in the 365nm wavelength region. I'll be using a single 9W UV nail gel coating curing lamp, also from eBay. Seeing a pattern here? The lamp has a flat area of about 140mm x 75mm, which is large enough for the small boards I intend to make. For larger boards, there are 4 lamp versions of these available. The lamps used in these units are all pretty much the same, and are supposed to have their peak output at 365nm, so they seem ideal for this purpose. It should also be suitable for the UV cured solder masks that I see available on eBay - without knowing too much about manicure products, I suspect that the UV cured solder mask paint is very similar in chemistry to the nail gel coatings that are meant to be used with this lamp.

I've created a test PCB pattern in Eagle that I'll be using to see what kind of resolution I can achieve, and for exposure calibration. The pattern consists of 3 sets of:
  • 2 x SSOP 28 packages, which are far smaller than anything I expect to actually use
  • a 0.1 inch through hole header, for a sense of scale of a standard DIL package
  • some 608 SMD resistor pads
  • a series of lines from 10 to 50 mil (one mil is 0.001 inch or 0.0254 mm)
  • Vector text in two sizes: 32 and 50 mil (0.8 and 1.3 mm)
  • Some traces at 12 and 16 mil (0.3 and 0.4 mm), which seemed like reasonable widths for most interconnects. Perhaps a little narrow, but this is just a test to see how far we can push the process.

The first step will be printing this artwork in negative on suitable material to use as the exposure mask for exposing the photo-resist material. That's next post...

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