So, according to the Wikipedia article on Arduino, the board is Open Hardware, the design is shared under the Creative Commons Attribution Share-Alike license. Alas, Wikipedia provides no direct cite link, and I did not find the board source code repository on GitHub. So, where is the source?
20200723/https://en.wikipedia.org/wiki/Arduino
Searching around quite a bit, I found it in a non-ideal location. First, go to the Arduino store website. Then, click on the product you want the design information for. Go to the Documentation tab, and you’ll see some links to the reference designs. Those links will point to a “downloads” area inside the website merely designated for hosting file storage. No version control, nothing fancy like that. A bummer. Also, the designs are in EAGLE CAD format, no KiCad going on here for the original board designs, but for some of the newer board designs, they are also available in Fritzing format.
20200723/https://store.arduino.cc/usa/arduino/boards-modules
20200723/https://store.arduino.cc/usa/arduino-uno-rev3
20200723/https://store.arduino.cc/usa/nano-every
Searching around quite a bit more on the Arduino website, I found these links to special designs meant to be easily manufacture DIY circuit boards via the laser printer toner transfer photoresist mask method. The first page has broken links, but the links in the second page for the updated version work just fine.
20200723/https://www.arduino.cc/en/Main/ArduinoBoardSerialSingleSided
20200723/https://www.arduino.cc/en/Main/ArduinoBoardSerialSingleSided3
Manually etch a PCB, did I see mentioned? No, they simply referred to the DIY etching method. But, nevertheless, I did search for this topic and found some interesting results. Namely, more information on the laser printer etching process. Once you print your photoresist mask with a laser printer and do to the toner transfer with an iron, you use ferric chloride to do the actual etching. This can come either in a liquid form or a powder form, you can buy the powder form and mix with water to get the liquid. This is also a very dangerous chemical, the fumes will corrode steel. So, not only need you be concerned with protecting yourself by wearing gloves, eye protection, proper ventilation, and respiratory protection, but also your surrounding physical environment, so metal structures in your work room do not get ruined.
The etching process typically happens in 5 to 10 minutes, a hot plate to heat up to 75 degrees Celsius and agitation is required for optimal performance. Then you can wash the board off by soaking in plain water.
How do you dispose of the etching solution when you are done? Apparently, you can dilute it and pour it down the sink. Why? Because typically this chemical occurs in very low concentrations in municipal water: after having been used in the city water filtration and treatment plant, it is left in the water in very low concentrations. That being said, there’s a really interesting trick you can use to get ferric chloride: if it occurs in your city water supply, you can collect and concentrate it from your own tap water. Then you can use it for PCB etching, but it will take significantly longer than using a proper high concentration solution would take. On the other hand, you might prefer the lower concentration solution because it is less hazardous to work with.
Another interesting trick is that you can use a Sharpie permanent marker to draw on your own photoresist, rather than a laser printer. Worthy of note is that solid ink printers also work well, and therefore, thermal transfer printers by extension.
20200723/DuckDuckGo etch pcb by hand
20200723/https://circuits-diy.com/how-to-etch-a-pcb-printed-circuit-board-a-beginners-guide/
This is where the “PCB etching with water” trick is mentioned, on a link to an EEVblog video.
20200723/https://www.ourpcb.com/pcb-etching.html
20200723/https://www.instructables.com/id/Making-a-Hand-Drawn-PCB./
20200723/https://www.instructables.com/id/Making-a-Hand-Drawn-PCB./
Now, I wonder. Can you go and do likewise with an inkjet printer? Maybe. The idea would be that you can substitute pigment ink for toner or wax. But, toner, wax, and permanent marker all contain a plastic or wax material within them, pigment ink is just pigments, and that may not be good enough for a photoresist mask.
20200723/DuckDuckGo print permanent ink inkjet printer
20200723/DuckDuckGo pigment ink inkjet printer
20200723/https://laserinkjetlabels.com/pages/pigment-based-inkjet-cartridges/
20200723/https://en.wikipedia.org/wiki/Permanent_marker