Electronic – arduino – PCB Layout – Design Critique Requested

1. Yes. The traces are very short.
2. For some reason, you decided to place the cap near the V_SSA pin and then run the track to V_DDA. I would suggest that you place the cap near V_DDA and connect it to the pin with a short trace. The ground pad of the cap should go to the plane through a via. V_SSA should be connected to ground with a via. Keep the traces to GND vias as short as you can.
3. Apart from the above, it looks fine. C717 routing is good.

Extra comments:

• The polygon clearance on the top layer looks quite small. You may want to double check that.
• Your 100nF and 10uF caps are the same size. Are the bigger caps tantalum?

The main issue I see here is that the U1-U2 GND trace infringes on the isolation barrier in the design. I would shift the P1/P2/R1/R2 complex closer to U1/U2 -- this leaves room to reroute the ground trace to U1 between P2 and the edge of the board, then have it pop out between the pins at the bottom of P1/P2, so it doesn't compromise the isolation barrier U1 and U2 provide.

The other issue with this design is an annoying mechanical matter -- the BTA12-600CW has an isolated tab, but it's only rated to 2500V isolation, which isn't sufficient for a reinforced barrier in 240V land AFAIK, and the heatsink infringes on the isolation barrier otherwise. I'd change it to wrap around the side in an L-shape, with the fins exiting the board envelope altogether -- this increases the mechanical envelope of the design, but keeps the isolation area clear and also allows you to increase creepage and clearance distances from the HS to the TO-220's pins. Either that or find a TO-220(iso) triac with a 5kV isolation rating on its tab and suitable specs otherwise (600V/min 10A). Of course, if you're happy with 2.5kV of isolation here, you simply should make sure the HS has sufficient clearance/creepage to the TO-220 pins.

Another option, depending on how much of an isolation stickler you are, would be to shift the optoisolators left by a couple hundred mils. This would place the existing heatsink firmly on the mains side of the isolation barrier. You may wish to swap the optos for 400mil packaged ones while you're at it, too, as these provide improved creepage, to the full 8mm needed for 240VAC reinforced insulation (surge rating of 5kV).