Probably the most indispensable tool for making printed circuit boards, is the UV light box. There are a couple of different methods for transferring of a PCB image. The most common homemade method, seems to be the toner transfer method, judging by the number of sites describing it. However, I use the photo lithography method and I have had great results with it so far, so no need to change.
I am not going describe the process in detail, since there is allot information on it elsewhere on the net. In a nutshell it consists of using a photo resist spray, this is a spray that is sensitive ultra violet light, to coat a copper clad fibreglass board. A transparency, containing the printed circuit image, is then placed on top and the board is then exposed to ultra violet light for a couple of minutes. The UV light degrades the coating in areas where it was exposed to the UV light, making it easily removable in a developer liquid like caustic soda. It leaves behind the coating in a pattern matching the circuit image, and exposes the bare copper in places where there are no tracks. An acid solution is then used to eat away the exposed copper, leaving the neat circuit pattern behind, protected from the acid by the photo resist coating.
The ultra violet light source is key to this process and since a suitable light source is not available as an off the shelf product, I decided to make a light box for this purpose. Now, at this point I should mention that this is my second attempt at building a UV light box. The first one was a monstrosity if a box over a meter long, half a meter wide and half a meter high! Inside was a dual fluorescent light fitting, fitted with two 8 Watt UV tubes. This light box worked well and I used it for a couple of years. But it took up way to much space and was cumbersome to setup and use. Through trial and error, I found that it required an exposure time of 8 minutes, for descent results. The biggest draw back of this box, was that it only allowed me to make single side circuit boards. This was fine then, but as my projects increased in complexity, so did the usefulness of this solution decrease.
I needed something that could illuminate a board from both the top and bottom simultaniously, in order to avoid complex re-allignment of the top and bottom layers in between exposures. Secondly, it need to fit on my desk, since working in the garage is a pain. So I settled on a box built from MDF from the local hardware store and UV leds. The end product lloks as follows:
It contains two sections of 54 LED’s each, one for the top and the other for the bottom. It runs off 12V DC and draws about 1.2A. I found that an exposure time of three minutes gives good results. Keep in mind that UV light is extremely bad for your eyes and you should always take the necessary precautions operating equipment like this. In order to avoid unnecessary exposure to harmful UV rays, I added a micro switch in the lid that will switch the light source off when the lid is lifted.
It is worthwhile mentioning that the dimensions of the box and the spacing of the UV LED’s are key to it’s success. LED’s have a conical light pattern that varies depending on the type and model of LED. Consult the data sheet of your model, before doing anything. The trick is to get as much overlap as possible, between all the neighbouring LED’s to avoid getting ‘shadows’ that will ruin the exposure. Some basic trigonometry will be helpful in determining the spacing between LED’s.
The working area is a normal piece of windows glass, 4mm thick. It is important to get glass that is free of any UV coating that will shield the UV rays. Obviously it has to be spotlessly clean, since any imperfections will show up in the exposure. I use normal methalated spritis to give a good wipe down, every now and then.
The circuit for driving the LED’s is fairly straight forward. The LED’s are grouped in three’s and connected in a current regulator configuration. The schematic for one of the two sections can be found here.
The circuit was constrcuted using vero board. The soldering took about two nights, lots of components. I used two seperate peices of vero board, that I joined using solder joints in a couple of strategic places.
Once the circuit construction was completed it was time to test everything before final assembly. At this point it is worthwhile remembering that UV rays are harmfull to your eyes, take precaution and use common sense.
To hold the vero board circuit in place, inside the box, I used 10mm stainless steel stand-off’s that was glued to the wood. Make sure that the surface is free from dust and grease and only ginger tighten the screws. I found it easier to first drill the holes and screw the stand-off’s to the vero board, before glueing and positioning it in the box. This way there is no hassling with alignment. Very important, do not use allot of force when screwing the stand-off’s to the vero board. The glue will not form a bond strong enough to withstand the torque forces if the screws have to be undone.
Be sure to put some weight ontop of the vero board while the glue sets. The vero board might have a slight bend to it, caused by the heat from soldering the components. Putting a weight on it will assist in getting the glue to dry with the stand-off’s firmly pressed against the wooden surface.
The glass panel was recessed into the bottom half of the box, by cutting it a few millimeter bigger than the actual opening. I used a Dremmel with routing attachment, to mill away a small section of the edge of the box. This ensures that the glass fits in snuggly and flush with the opening. This provides safer working platform with no risk of cutting ot injuring yourself with the sharp edge on the glass.
I hope that you found this article helpfull and even draw some inspiration to build your own UV Light Box. It is a fun project and a very worthwhile investment that makes DIY printed circuit manfacturing just a bit easier and convenient. I might add an automatic timer circuit in future, but it is quite straight forward to to just time it manually.