Seno PCB System
The SENO System
The SENO system is a liquid resist that you paint onto a copper clad board. It comes in bottles that resemble the liquid shoe polish applicators. The system includes:
Photo Etch Resist
Developer
Soft Developer
Resist Strip
Flux
Preparing the Board
The board is prepared by cleaning with a polishing block, and then washing any grit away with a solvent cleaner. Then the photo resist is applied. All that you do is press on the foam pad which lets a little of the resist flow. You then coat the board evenly. The board is sensitive to UV light, but you must apply the resist in subdued light. Also the board should be left to dry in the dark. I have a couple of shallow tin boxes (old biscuit tins) that do the job nicely. The resist can form blobs on the surface of the copper, the instructions state that the board should be coated evenly but not too thick, and that the board should be allowed to dry flat.
I have tried several methods to get an even coating. The best method so far is to coat the board while holding it at an angle, finishing the strokes in the same direction. Excess resist will then collect at the bottom edge. Touch this edge against a paper towel, which will wick away the excess, then lay the board flat on the towel and close the box lid. . The resist is quite quick drying, and you can use the board after about 10 minutes if you dry the board in a heated oven. This produces a "soft" resist, and there is a "soft" developer that you can use. Otherwise the boards should be left to dry for at least five hours before use. I coat the boards and leave them to dry overnight. The major problem at the moment is getting good cover on larger boards. Small boards of approx 10x6cm seem to cover OK, but larger boards 10x16cm are a bit more problematic. It's very obvious that it's the coating that is the problem, as some areas of the board are coated fine, and you see the track pattern develop there, but other areas are only very lightly covered. I am beginning to suspect that the instructions to apply the resist " not too thick" may have a different meaning than what I think.
I have discovered one aspect of this system that can cause a problem. During testing I found that some particulate matter was being deposited on the board, making for a poor etch. I traced this to old resist that was left on the sponge. So after coating the boards, use a paper towel and wipe the sponge clean. This ensures that you only use clean resist the next time you coat a board. Once coated the boards seem to have a "shelf life" of several weeks providing they are kept in a light tight box.
