Hi,
sorry, i thought the EAGLE files are attached. I don´t have them available now. Maybe in 6 hours....
Many years ago i etched may boards too. 18 mils should be OK, but if you make them thicker it is no problem.
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I saw you routed all the GND lines. This is not necessary .. more a waste of time with the use of the GND polygon.
Autorouter.
The autorouter can only be as good as the informations you give...
Here the autorouter doesn´t know about curents and pulses and what device is for VCC decoupling. It doesn´t know about placement of components...
--> so it simply routes all signals for connections. But you could give a lot of informations to the EAGLE autorouter to get better results.
I professionally route industrial layouts for many years now, (the biggest was about 40 x 50 cm with 16 layers and about 40.000 drill holes) and only for a couple of uncritical signals I use the autorouter.
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Instead of using the autorouter I recommend to use some tutorials on how to recognize possible problems with the layout.
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Further improvements...
* Another person will surely do some routings different .... there will be no perfect layout.
* maybe placing the USB connector to the edge, to avoid problems to insert the plug.
* Again, I strongly recommend to use some additional capacitors. At supply and ADC. Read above and my PDFs.
.. but the biggest problems are canceled...
Good luck
Klaus