Rate my first 2-layer board

Question

I would like to hear some comments on my first 2-layer board.

Specificaly if I should go with two grounds (one for 12 V, one for 5 V) or just one big ground plane.

Another thing is the layout of the capacitors for the 7805 [U1] (12 V to 5 V).

Should I add some more vias to the return path after the FET [Q1-3]? And/or generally add more vias? The idea of adding more vias is to have enough more contact between ground and Vcc.

Front Side

Back Side (ground plane)

Back side ("2" grounds)

Schematics

Basicly this uses a Tiny13A to turn some fans on and off.

Answer 1

There's nothing in your circuit that suggests the ground plane should be split at the 5 volt regulator; there are no external interfaces to 12 volt devices that require a really-good solid ground connection AND there is nothing particularly troublesome regards EMI hence, ground the J4 connector on the earth plane side like this and do a bigger fill: -

No need to route any ground on the top layer. The MOSFET drivers are 2N7003 devices and the sort of maximum continuous currents they can switch (115 mA on a very good day) is not going to pose a problem and upset your MCU if it shares the same ground plane.

And, as has been pointed out, you have your N channel MOSFETs upside down: -

Answer 2

Nice job diving in to board design

Specificaly if I should go with two grounds (one for 12 V, one for 5 V) or just one big ground plane.

Typically you think about separating ground planes for high frequency digital vs lower frequency analog, not necessarily based on which supply. I'd suggest one big ground plane. You can do the mental exercise of "where is current flowing in the ground plane?" and see that the 5V and 12 V return take different paths, and may overlap a little bit - but cross talk between supply voltages isn't something to worry about in general (you more worry about "is this a high frequency return path that will have capacitive coupling/cross-talk with other signals or add noise on to the ground plane?"

Another thing is the layout of the capacitors for the 7805 [U1] (12 V to 5 V).

Looks good. Something you can think about is putting a start capacitor at each motor. This way at start-up they can draw the current from that cap rather than through traces since motors draw a lot of current at start up. Probably doesn't matter much in your design, but it's good practice. Some motor datasheets may tell you a suggested start capacitor, or you can estimate it based on peak/starting current draw and how quickly it reaches full speed. Current times time is total charge, and charge divided by voltage is capacitance, so start current times time to reach full speed divided by supplied voltage is a rough estimate for the order of magnitude of the capacitance needed.

Should I add some more vias to the return path after the FET [Q1-3]? And/or generally add more vias? The idea of adding more vias is to have enough more contact between ground and Vcc.

There are online calculators for finding how much current can flow through a via (stuff like: http://circuitcalculator.com/wordpress/2006/03/12/pcb-via-calculator/). You can check to make sure you have more than enough current when starting (peak current). If you have space, it doesn't cost you (much, if anything at all) extra to put more vias in.