Having decided that I wanted to try the UV lithography approach to making boards, the first thing to do was to get a UV exposure unit. Unfortunately, they seem to cost about £100 – which is more than I was prepared to pay… So how to make my own? I spotted a project on Hackaday where someone had bought UV LEDs from E-bay and used these to expose a board. This is nothing new – here are links to some similar projects: 1 2 3
I ended up buying a pack of 100 UV LEDs and soldering them onto stripboard to make my UV exposure unit.
The circuit is very simple – a 12V linear regulator (7812) controls the voltage, which is passed through many sets of 3 x UV LEDs in series with a current limiting resistor (100 ohm). I used 84 UV LEDS in my unit. Schematic here: [jpg],[ps], [svg]
Once soldered on, you then need a flat surface to put your transparency and PCB on. I used a bit of 5mm thick perspex sheet (e-bay again). Don’t use a material which is opaque to UV (like window glass). This was mounted to the stripboard by PCB nylon spacers (maplin).
The whole thing is mounted in a 120 CD DJ case (maplin)- the rails make good supports for the perspex and the LED board hangs off that.
Power is supplied by a simple wall wart connected by a co-ax power connector attached to the case.
The unit works quite well, and my exposure tests with it indicate that the best exposure time is 90 seconds (which compares well with commercial units).
My only gripe with it is uniformity – the LEDs have lenses in the case, so I’ve effectively got 84 little spots of light across the surface of the perspex sheet. I had tried to mitigate this by mounting the LEDs a decent distance from the imaging plane and arranging the LEDs in a hexagonal pattern (as close to circular packing as you can get).
It isn’t too bad, but I’m realising that uniformity of UV exposure is directly related to PCB yield and the minimum feature size that you can etch. I’m attempting to improve things by putting some light scattering sheets (cut up poly pockets) between the LEDs and the exposure surface. I’ll report back here on how it goes…
The other improvement I’d like to make to the unit is to put in an automatic timer. It would probably be PIC or Arduino based with either a MOSFET or relay to turn the 12V on and off and some sort of screen/7-seg display plus buttons for a user interface. In the meantime, I’ll make do with my watch and the power switch 🙂