Tuesday, July 28, 2015

3. Drawing the PCB


After having tested the prototype some more without finding any problems, I decided to work a bit to make a suitable PCB for the solding station. For this task I used a very nice freeware program called ExpressPCB. Luckily it works great in Wine and the design process went smoothly. The download links for the PCB files are at the end of the article.

ExpressPCB - free and easy to use


If you want to try it, it can be found here:

http://www.expresspcb.com/free-cad-software/

Each module has its own PCB, this way it is easy to upgrade or fix without throwing away the good parts. For the powerboard, I chose to use through-hole parts because it contains quite a few bulky components: the filtering electrolytic capacitors, the LM7805, LM7809 and LM7909 voltage regulators and the BT137 triac. These need to be secured well and also be able to dissipate a bit of heat (theoretically, in practice the components remained cold in all tests).

The main board uses some SMD parts, with a few exceptions (OP07 op-amp and the ATMEGA microcontroller, the electrolytic capacitors, the 2 multiturn potentiometers, etc.). The current mainboard PCB uses resistors in 1206 package and capacitors in 0805 package. Since we're dealing with digital signals here, the SMD parts will help keep the resistance and inductance lower and also help achieve a smaller board.

Depending on the final design and choice for the case, the four buttons can be mounted on a separate, smaller PCB (this is what I used). An alternative to this is to use pushbutton switches with central mounting which can be attached directly to the case.


Fabricating the PCB

For making the PCB I opted for the photo-positive method. The boards themselves are not expensive at all and the results are very good. My favourite boards are made by a German company, Bungard. They are presensitized so there aren't many things that can go wrong. The steps below lack details, so if you want to learn more about this method, have a look at Nard Awater's excellent tutorial available here: http://www.aplomb.nl/TechStuff/PCB_s/UVsource_PCB.html

In case you decide to do things the same way, here are the steps:

1. Print the PDF files on normal paper using a laser printer. An inkjet printer works just as well, but in my tests, the laser printer made the traces more opaque and it didn't have any artifacts, while the inkjet-printed traces had a bit of noise around them. It could be that the paper I had wasn't very good. In any case, if you have a laser printer, use that one.

Printed pieces

2. Cut the PCB areas out of the printed page then pour a few drops of cooking oil on the back of the cutouts. Then take a piece of paper towel and gently spread the oil drops across the whole surface. Make sure to cover everything. This will make the paper pieces look transparent so that the ultraviolet radiation is able to penetrate them and cure the photoresist layer. After completely covering the pieces, use another paper towel to remove the  excess oil. Make sure not to rub the paper on the printed part because it may ruin the traces. After this operation you should be left with something like this:

Transparent layer

3. Cut the PCB to the needed size. You can use the transparent layers as a guide. After that, remove the protection layer from the PCB and superimpose the transparent layer over it (with the printed part oriented towards the photoresist). The supplied PCB diagrams are already mirrored so you should be able to see the text normally when looking through the transparent layer:

The PCB pieces are cut to size, ready for UV exposure

4. Expose the PCB to UV light. Have a look at the link above to see how to build an ultraviolet exposure box using UV leds.

5. Remove the exposed photoresist layer with Sodium Hydoxide. I use 1-2 teaspoon of NaOH (~ 10 - 15 grams) to 1 liter of water. Make sure to avoid using metal when working with Sodium Hydoxide (glass, wood and plastic are OK) and to avoid touching it with your hand or you might get chemical burns.

6. Rinse the board with cold water, without touching its surface. Just immerse it a few times in water to remove the NaOH solution.

7. Etch the board. I used ferric chloride for this project, but special care must be taken not to touch it or spill its container because it permanently stains anything it comes in contact with. As with the NaOH solution, don't touch metal with it since it's highly corrosive. Make sure to etch the board in a well ventilated area. After the etching is done, dispose of the highly toxic etchant solution by taking it to your local disposal center.

8. Carefully wash the PCB to remove the etching agent then clean it thoroughly with isopropylic alchool to remove the remaining photoresist from the traces.

PCB ready for testing, drilling then soldering


9. Test the traces with the multimeter's continuity test, drill the holes then solder the components.


The links for the PCB files


Power board PCB:
https://drive.google.com/file/d/0B7qGmYL2UHFNYjdFV0FqTmJnU3c/view?usp=sharing

Power board silkscreen:
https://drive.google.com/file/d/0B7qGmYL2UHFNVFhjQzhRaTZPQjQ/view?usp=sharing

Main board PCB:
https://drive.google.com/file/d/0B7qGmYL2UHFNZk4zSGJuX2Y0NW8/view?usp=sharing

Main board silkscreen:
https://drive.google.com/file/d/0B7qGmYL2UHFNdWRjUHUwNDd2VHM/view?usp=sharing

Switch board PCB:
https://drive.google.com/file/d/0B7qGmYL2UHFNLTVwa1pIMzFXdG8/view?usp=sharing

Switch board silkscreen:
https://drive.google.com/file/d/0B7qGmYL2UHFNVEZlUXdzN2cweFk/view?usp=sharing



Next up is the firmware. I'll keep you posted.





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