@3:37
The Gen3 boards are double-sided.
I started making these boards some months back, and it is possible but I should point out the following:
1. You need to clean your double-sided copper board, thoroughly. There must be absolutely no finger-print marks or any dust on the board. You can hold the board at an angle and watch for clear reflection from the surface. If you can, wear gloves while polishing.
2. I use photo-resist (positive-20), to spray on the copper board. Make sure that you do this in poor lighting condition. Spray from a distance of about 20cm and make sure that the coating is even on the board surface. Note the following:
2.1.If you hold the spray-can too close to the board surface, the some liquid will "bounce off" the board surface, and the chemical application will be uneven.
2.2.If you did not clean the board properly in step 1 above, the photoresist chemical will adhere to impurities and form pot-marks on board surface.
3. After spraying the board, you have two options:
3.1. You can store the board in a dark, dry cupboard for 24 hours at average temperature of 23C
3.2. You can put the board in oven at 80C. If using this method, you will have to experiment to find the right amount of time inside the oven. My test results revealed the following:
3.2.1. Before inserting the board into the heated oven, I had to switch off power in order for the heating element to stop glowing red. I suspect that the light given off by the element was enough to interfere with the photosensitive chemical.
3.2.2. For double-sided board, best oven-time (with oven power off) was 30min per side i.e. put board in while oven temp at 80C, wait 30min, remove board, re-heat oven to 80C, remove oven power, wait for element glow to recede, and insert board again (flipped over)
3.2.3. When working with positive-20, wear gloves, and protective eye-wear.
4.Once the copper board is removed from oven, then it is photo-sensitive, and can now be exposed to UV (approx wavelength 370nm, if I remember correctly). For this purpose, I bought a "black light" from a local lighting store. I rigged a big box, with white A4 paper glued on the inside, and stuck the black-light through the top.
5. You are not ready to use the black-light yet. First you have to make the "book".
5.1. If you have a pdf-format of your PCB layout, then print out top layout and its left-right mirror, on a laser printer on transparency. Print out the bottom-layer board layout, and its left-right mirror, as well. Make sure that the toner density is at least 600dpi.
5.2. Take two top layer printouts, and align such that toner is on "outside" i.e. toner-sides face away from each other. You can determine the toner-side of the transparency by looking for reflection off transparency surface. Toner-side will be dull, and not reflect light. Duck-tape or glue transparencies together.
5.3. Align the bottom layouts, same as above.
5.4. Align top layout (in 5.2) to bottom layout (in 5.3). Duck-tape them together, such that you can slide copper board between.
6. Slide photosensitive copper in-between transparencies, and place between two glass surfaces. If needed, duck-tape glass surfaces together, with transparencies and copper inside.
7. Put UV box (made in 4), over glass surface, and expose for 30min per side.
8. Once exposure is complete, you can develop photo-sensitive copper in solution of sodium-hydroxide (NaOH) and waterglass. You can make the solution in the following way:
8.1. Dissolve a teaspoon of sylicate (chrystal cat-sand) and one teaspoon of NaOH in a cup of hot water. Stir while cooling.
8.2. Dissolve between one and one-and-a-half teaspoons of NaOH (drain cleaner) in 1000ml water.
8.3. Add waterglass made (in 8.1) to NaOH solution.
9. Exposure time may vary. I have had exposure times ranging between 5min and 10min.
10. Once exposure is complete, rinse with water and dry with kitchen-roll paper.
11. At this point, it is better to drill via holes, first. Since tracks will become unstuck while drilling after etching.
12. Also, make sure that all track definitions are well developed i.e. remove all photoresist between tracks with a sharp knife, if necessary.
13. You may also want to increase definition of certain "over-etched" tracks with sharpie or other etch resistant marker.
14. Once all vias have been drilled, it is time to etch the board;
15. Etching can be done in various solutions of your choice eg. FeCl (iron chloride/ferric chloride). I use a prepared solution of CuCl and water. Prepare CuCl solution in following way:
15.1. Add 100ml of HCl (Hydrochloric Acid aka pool acid) to 700ml of water. Add 20ml-40ml of peroxide-40. Add a "throw-away" copper board into the solution - not the board you will develop!!!
15.2. The solution will turn green after a while as the HCl reacts with Cu. If the solution turns dark brown, you have dissolved too much copper, so just add HCl until the green colour returns.
15.3. Remove the "throw-away" copper board. The solution that remains behind, can now be re-used for successive etching i.e. same solution can be stored and re-used to etch many boards.
15.4. The next step is to aerate the CuCl solution by letting a fish-tank pump push air through.
15.5. Add the board that you wish to develop, while the pump is pushing air through the solution.
15.6. Monitor the etching process frequently. The colour of the CuCl solution will proceed from light green to dark green to brown, depending on how much Cu needs to be etched.
15.7. The speed of the etching process can also be increased by heating the CuCl solution.
16. When etching, make sure that you wear protective hand and eye gear at all times. Also make sure that the area where the etching happens, is well ventilated. Use plastic tweezers to remove/ and insert board as needed.
17. Once etching is complete, remove board and wipe with acetone.
These suggestions above was sufficient for me to build a sanguino and V2.3 stepper boards for Mendel. YMMV.
Regards
Marius Botha
Pretoria, South Africa
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