Making Printed Circuit Boards (PCB’s) is a fairly simple process if you have the correct tools, however by far the most expensive item that is required is the Ultra Violet (UV) exposure unit and even a small basic unit can very expensive to buy. Making your own unit is perfectly feasible and requires only basic components but usually does involve a considerable amount of care, as lethal mains voltages are present in the circuit, but there is an alternative.

Pre-sensitized PCB laminate is available from most good electronics suppliers and consists of standard copper clad board with an additional spray coating of a UV sensitive photo resist layer. This photo resist layer is very sensitive to UV light in the 340nm to 420nm wave lengths.

Traditional UV exposure units use special mains powered UV tubes that emit UV light at a wavelength of around 350nm.

Described here is the basis for a UV expose unit that does not need mains voltages and can be built for around £15 if you do some shopping around for the parts.

To pull of this feat of magic, we are going to use a component that’s been around for a little while but recently become very inexpensive, the UV LED.

These LED’s are now available from several sellers on EBay and for a fraction of the cost from more traditional electronic component outlets; I paid £9.99 including P&P for 100 of these little monsters and they were perfect. (http://myworld.ebay.co.uk/amigoofchina/)

Figure 1. Completed UV Board

Here is a list of the parts you will require:

Figure 2. Blank Stripboard and LEDs


Figure 3. Strip-board layout and cut track locations.

I would assemble the board in the following sequence:

1. Cut the tracks as shown in figure 3, there are 165 cuts to make. A simple way to do this is by hand with a 3.5mm HSS drill bit, or    you can get a special tool but that’s really just a drill bit with a handle.
2. The 5 large links.
3. The 66 smaller links.
4. The 33 resistors.
5. Finally the LED’s. You should make sure that the LED’s are inserted correctly and that they are all set straight and at the same height; this will help produce a steady and evenly distributed light on the target.

REMEMBER THAT FIGURE 3 IS AN X-RAY VIEW FROM THE TOP

Stop and check before you start cutting tracks !!!


Figure 4. Strip-board with tracks cut & track cutting tool.

The initial prototype used 47Ω current limiting resistors and the entire board draws just under 900mA. It also gets VERY hot. I would suggest using 68Ω resistors to cool things down a little bit but the resistor values will really depend on the specification of the LEDs you use.

 
Figure 5. Stripboard with resistors and links all soldered in place.


Figure 6. UV Board being tested after assembly.

You will need to provide the board with 12v DC at around 1 amp. LED’s have a polarity and this should be observed when connecting the board to the power. The negative rail runs along the top edge of the strip board in Figure 6.

If you find any LED’s that are dimmer or brighter than the others they should really be replaced, as you want as even a light source as possible. The packet of 100 x LED’s I purchased from EBay actually contained 102 LED’s and of the 99 installed, none needed to be replaced as they all worked perfectly and gave a good uniform brightness.

Next, you need to install some supports to raise the board to a suitable height. The prototype needed to have the LED board approximately 90mm above the art work; depending on the LED’s you use you may have to adjust this height a little.

With the board powered on, hold it over a piece of white paper and increase / decrease the board height until a fairly even and uniform purple light is covering an area of the paper; this is the ideal board height.

The prototype used 4 legs made from 2 x 45mm metal spacers each, to give the correct height.

The design specified gives a useful UV exposure area of around 120mm x 80mm. You can increase the size of the area by using a bigger piece of strip board and adding more UV LED’s.

Figure 7. UV Board in use and raised up on it's supporting legs.

Figure 7 shows a piece of laminate board face up. On top of that is a transparency containing the art work, and on top of that is a piece of plastic from a CD case to try and force the artwork flat against the board. The plastic wasn’t heavy enough so I had to balance my model knife and de-solder pump at either end of the plastic to give it additional weight. Under the UV light, the laminate actually looks black instead of the yellow copper look.

The first exposure test was 5 minutes and the results were excellent. The next test was 3 minutes and again the results were excellent.

Figure 8.  Small piece of artwork and the result on a scrap of board after UV exposure and development.

Figure 9. Etched PCB.

Figure 9.  Result after the developed board has been through the ferric chloride for 10 minutes.

One simple solution to keeping the UV light out of eye sight is to place an upturned shoebox over the entire assembly.

Improvements can certainly be made to this design. Firstly the exposure area can be increased to an unlimited size if required but for Hobby Electronics use, the current exposure area should be sufficient.

The one area that needs to be thought about carefully is holding the art work in place over the PCB laminate. It’s imperative that the art work is forced down on the laminate and there are no gaps between the two. For my initial testing I used an old piece of plastic from a CD case but this isn’t heavy enough and I had to add some weight. You could probably just stick this down using a couple of pieces of tape but a more flexible solution should be sought if you intend to use this design on a regular basis.

A timer could be constructed so that you don’t have to hang around looking at your watch.