Introduction: Simple PCB Exposure Unit
Sometimes I'm creating small PCBs, and always below Euro-size. For the design of the PCBs I use either Fritzing or Eagle. Since my designs are also fairly simple there is no way to have them produced professionally. That would simply be too expensive. Also a (semi-) professional exposure unit was by far too expensive for me. So I decided to build my own unit, which I present with this i'ble. I'm using this units since several years and it still performs with no issue.
Step 1: Design
You will need just the following things:
- some 3-4 mm plywood
- a lamp fitting
- a switch
- aluminum foil
- wood glue
- a UV lamp
The most expensive part will be the UV lamp. I got mine from ebay those days for something between 10 and 20€. It is typed T5 115 and though I could not find the vendor immediately again I'm sure a little research will show some results (as I notice the zip code on this lamp has 4 digits which was changed to 5 back in 2000 or so; so this was probably some remainder of stock). Those light bulbs are sold for reptile warming in zoo supplies. The UV/A part is important since the PCBs develop at 440 nm light (which is a bit above UV/A towards visible blue).
The casing is built in parabolic form - approximately. It is not necessary to have an exact parabolic form. You will just need 2x2 identical parts on the long side, one top part and two form parts left/right (one with a bore for the lamp fitting). This bore is placed in the focal point of the parabola. The left/right parts are best cut after glueing the printed PDF on a piece of plywood. Test fit the lamp fitting in the one side part. Also check that the lamp will fit the dimensions. If it does not fit you can simply print an enlarged version of the side parts. In that case do not use the measures (in millimeters) for later.
The rectangular side parts can be cut freely on a circular saw with a bit excess. Use sandpaper to fit the miter joints. It must not be exact! But it should also not have holes. If so you should close them with any wood filler you prefer.
Once the parts are glued together paint the inside thin with wood glue or universal glue. Then cover it completely with aluminum foil. Finally mount the lamp fitting and insert the UV lamp.
For my device I used some spare cable and a foot switch (yep, too large and clumsy but it works and I don't care much). I'm sure you are not stupid if you are looking for PCB devices. Anyhow: this deals with high voltage and this is still hazardous.
So, that's all. An exposure unit for less than 20€ all in all.
Step 2: Expose a PCB
I use a cheap small picture frame for the exposure. The cardboard of the frame is used as base for the PCB. On top I place the transparency with the printed PCB design and on top of that I place the glass from the frame. That will create a clean surface for the exposure. Left and right of the frame I place pieces of plastic scrap which are a bit higher than picture frame plus PCB. On these plastic pieces I place the exposure unit so it will not move the glass cover and eventually dis-align the PCB print.
Now I turn on the exposure unit with the switch, wait for 4 minutes and develop/etch the PCB as usual.
You need to keep in mind that the lamp tube covers 2/3 of the case only. The PCB needs to be placed in this area or is will eventually not receive enough UV light.
N.B.: Since my PCBs are always quite tiny I use a Schott beaker put on the stove for etching. Just stoping heat shortly before it starts boiling. Works perfectly ;-)
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