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Cut off some border of the top-layer slide and then simply lie the top- and bottom-layer slides above each other (without the board plate in between which will be inserted lateron) and align them properly. (Don't be surprised to see that your printer was not creating two slides with perfectly identical spacing...) Fix the (smaller) top-layer slide on the bottom slide with one or two stripes of tape so that the resulting mask can be opened like a (note)book. Verify that opening and closing it several times will not destroy your careful alignment (it really works!).

With the mask "opened", put some (1..3) tape rolls (or alternatively tape with two sticky sides) on places where no important layout is located. Such places can be found on nearly any board (unused areas, future drills in the egdes,... If not, you should re-think your design because chances are high that you regret it afterwards.) The purpose is to keep on both sides of the board plate fixed to the mask slides so that everything can be turned upside down after having exposed one of the sides.

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The photoactive layer is most sensitive to UV light, so handling the plate under normal dim light was not yet source of trouble for me. Using a "black light energy saving" (gas discharge) lamp with 20W. (Note that I recommend against using standard UV/"black" light bulbs because their UV efficiency is very low.)

The exposure setup can be seen on the left image: The mask and board are below a glas plate which has weights put on each border to keep the mask slides plane. The top-layer and the bottom-layer are exposed in sequence.

Detailed process parameters:

• "Energy saving" UV lamp with 20W and self-made reflector
• Distance lamp (bottom) to top of 3mm glas plate: 20cm.
• Exposure times: 8 minutes for top layer (first), 6 minutes for bottom layer (last) with 1 minute for turn-around in between. Lamp switched on for exposure only (off before and during turn-around).

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The above mentioned method has some disadvantages: The reflector is very fragile and everything needs to be installed and adjusted each time I want to make a PCB. So, one time (after a longer PCB creation break when I had 2 unsuccessful attempts in creating 10x8cm boards), I discussed the issue with my brother and we finally spent an afternoon building the exposure device which can be seen on the left (which was made out of an old speaker box).

There are no reflectors on the side walls so that most of the light comes from the top (in 90° angle and less light under small angles) so that the edges of the wires on the board are projected sharper. Furthermore, in contrast to the above method, the lamp is installed horicontally.

Here, the detailed process parameters are:

• "Energy saving" UV lamp with 20W and self-made reflector
• Distance lamp (bottom) to top of 2mm glas plate: 16.5cm.
• Exposure times: 5 minutes for one side when starting with a cold lamp.

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Development is rather easy. I am doing it by sight (see left image) and it takes several minutes. As developer, I am using 1g NaOH (solid state) in 100g Water which is enough for a 8x10cm two-sided board with much less than 50% mask coverage. Be sure to mix the developer directly before applying it because it cannot be stored very well in liquid form.

To speed up the process, make it hand-warm and use a pipette to produce some circulation and "blow away" the photo-active layer. Alternatively, a soft brush can be used. I'm using 37°C normally.
Note: Do not make it too warm (e.g. 50°C, but YMMV) because this may also rapidly kill the non-exposed parts of the photo-active layer.

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Corrosion takes longest. I am using 22g Na₂S₂O₈ in 100g water (distilled) which is enough for more than one two-sided 8x10 plate with normal layout. To speed up the process, use 50-60°C and keep the lid closed to keep the water. The Na₂S₂O₈ will get increasingly blue (the color of the sulfur-copper-complex).