John's homebrew pages

Making "optical" printed circuit boards

part populated board

For a long time I had intended to make printed circuit boards "optically", that is from a printed mask rather than a mask painted on to the copper (the earliest technique I used) or drawn on with a permanent marker pen (a later technique) or with cut out vinyl tape. All these techniques had stood me in good stead, even for my first boards for surface mount components (SMD), but for better quality boards, especially for SMD devices with lots of closely spaced legs, I realised that I would have to move to using an optical technique.

The simplest optical technique is to produce a mask at the same scale as the final board, and to make an optical "contact print" onto the photoresist. This was not really a new technique for me, since I have made many a photographic contact print in my time - I started those when I had a Kodak Box Brownie!

Especially as I had started to use the PCB package for drawing out my boards (even for painted or vinyl boards), I realised that making the print for the mask would not be difficult - I could print it direct onto laser printer transparency material. Then at the Magnum Rally in 2010 I came across a UV lamp - it only has a little 4W "black light" tube (150mm / 6 inches), but I thought it would probably be OK for exposing photoresist.

As I am retired I tend to try to do things as cheaply as possible, so had decided to try using "spray on" photoresist as it's a lot cheaper than the ready sensitised boards. However the photoresist can has a relatively short shelf life (a year or so), so I don't yet know if this is a false economy! I looked at many web pages looking for best practice, but couldn't really find consistent advice, so decided to "suck it and see". I'm currently using Electrolube PRP spray-on positive photoresist, and Electrolube PDN photoresist developer.

Before I made the first board, it was essestial to establish the best exposure time under the UV lamp. To do this, I drew a simple PCB pattern with several fine SMD device pad layouts on it, and copied it several times. This was then printed out onto laser transparency material ready for test exposures.

The first board (top board in photo below) was a bit of a disaster. I sprayed on a thin layer of photoresist and left it to dry. Then I used initial exposures from a minute to about ten minutes, mixed up the developer at the right temperature, put the board in and developed it for the recommended time (use the manufacturer's instructions that come with the developer). Great! - it had produced what I expected, an underexposed board at one end and overexposed at the other - so I could work out the best exposure. I hadn't taken a photo of it at this stage. I then put it in the etch solution, again at the right temperature; I reckoned that as that was wet as well there should be no problems. Wrong! The nicely visible pads and lines on the board started to dissolve away, and I pulled it out of the etchant before it was really finished. The result was the odd board at the top in the photo - some of it looked OK, but most wasn't - and I've no idea what the rounded patch is (thumbprint?).

I realised that on all the web pages I had read, instructions included drying the board after development. So, for the second try, that's what I did, using a hot air gun (though no hotter than I could cope with holding the board between my finger and thumb). This came out much better - see the middle board in the photo. I realised though that one end of the board was not being exposed properly, as it was a bit further from the UV tube.

The final test (bottom board in the photo) had the illumination fixed, with exposures up to 20 minutes, and it came out really well. I had also realised by this time that making a spray-on board needs just a single thin coat of the photoresist (only about 2 seconds spraying - 4 seconds is too long) - it's best not to get it too thick.

pcb test strips

So I was now ready for a real board. This is a design that includes a small SMD chip as well as larger SMD chips and even a couple of DIL devices. This photo of the board after development but before etching shows how sharp the result is; it's best, of course, to print the board layour as a mirror image so that the laser toner side of the transparency is in contact wth the sensitised layer during the UV exposure. However, you can see that I have made the coating a bit thick towards the left hand edge of the board.

developed board

This is the same board after etching, and after a first bit of tidying up the stripline on the left side of the board, which the thick resist had prevented from being etched properly. You can see a few other little blemishes (click to enlarge) that need tidying with a burr in the rotary tool, but it's generally pretty good.

etched board

Finally, here's the board touched up and drilled ready for the components. As a first optical board, I was pretty pleased with this.

drilled board

With the basic technique sorted out, I have also tried using pre-sensitised board, which is even easier to do of course, though more expensive. Below is a photo of my first developed and etched board using this stuff - I was a bit disappointed as it wasn't as clean as I'd hoped, needing a bit of work with the burr. However the bits with most detail were good enough so I didn't re-do it.

developed and etched board

After a bit of work with the burr, and the holes drilled, it's fine for use. There's a photo of this board with its components in place near the top of this page.

driled board

So I'm now pretty happy with this process, and can't see me using anything else for the more complex boards I need; though I will still use vinyl tape where the board is very simple. I won't be going back to enamel paint though, I don't think!