Hi EK701,
I saw no one commented yet, so let me tune in. Overall you’ve made a good start I think. For example I really like that the bottom layer is so clean with hardly any tracks running through it. I’d gone 4-layer with that MCU already I think, but you prove that that’s not really necessary. At the same time I think you might have missed certain things, so sit back, take a coffee for here it goes
• The first thing I noticed was the isolated plane just above the MCU. THE biggest rule when isolating part of a plane is: “Absolutely NO trace crosses the gap”
So in your case: every trace that has nothing to do there must be routed around. In addition, I see that that isolated section is connected to some MCU pins. In that case extend the isolated plane to include those pins as well. Anything else getting in or out should be power or GND only and must cross the gap by way of a ferrite bead. (note that sometimes signals do enter a completely isolated plane, but then trough ferrites or common mode chokes. Preferably signals enter directly into the plane from an IC pin or connector)
• Crystals especially require a ground plane directly underneath if possible. That way high frequency noise currents are coupled into the plane and have a fast way back to their source. So really do put a ground fill under them. If possible that can be done on the top layer too, with a few vias to the bottom ground fill or plane.
• While on the topic of ground plane, is there a specific reason to remove the plane under the 9-DOF sensor? If not just put it there. A general rule of thumb is that a plane is better than no plane. (note the ‘in general’ and ‘rule of thumb’).
• Any ground fills on your top layer are best connected to your bottom layer. At least in their corners. Sometimes flood filling can lead to floating fills, and they’d better be removed if they can’t be stitched with one or more vias.
• C9 to C12, are they the power supply decoupling caps for the MCU? If they are, and I remember the Freescale Kinetis correctly, they should each be close to a pair of VCC/GND pins on the MCU. I usually place them first. If routing of nearby traces requires it, I move them a bit further out, but only as far as really needed to route adjacent traces. Also, use at least one via to your ground plane per cap. Even better is to place two vias on each side of the pad.
• My personal preference is to place all components as such as to minimize the need for routing them partly on another layer. That becomes harder if connector locations are predetermined of course. Looking at the bottom layer you did a good job trying to minimize crossing the ground plane.
In the case of the Display connector you have 4 signals that need to cross each other to get to the MCU. I trick I use in such a case is to try an route them right at the connector pads in such a way they leave in the order as they need to arrive at the MCU. For example you could try to route the trace from pin 2 upwards first, then right, and down between pins 4 and 5. Leave enough room and with the same trick route the trace from pin 4 upwards, then left, and then down between pads 2 and 3. If I’ve seen it correct your four signals now don’t have to cross anymore. Same trick could be done U3 if you wish: route the two signals to the left first, then down and let them leave at the bottom right corner. Now their order is reversed and crossing is not needed anymore.
• Sharp angles in copper are (can be) a problem when manufacturing the PCB. Try to make sure every possible corner is at least 90 degrees. For example on R3, top pad the trace leaves at a quite sharp angle. Consider routing the trace “due north” a bit first before bending. The same applied to the bottom left pad of the GPS chip, and to some extend to the bottom pad of R3.
• On to USB. USB can be considered a high speed signal. You want those traces running on one side, and one side only. Best is to route those signals first. If possible in Eagle (if I interpret the EDA looks correctly), try to route them as differential pair with a differential impedance of 90 Ohms. (bit hard on two layer, but if you’re in the ballpark for a hobby project you could be fine) R6 and R7 must be placed as close to the MCU as possible, as they terminate the high speed transmission line.
• Just below Y2, is that a cap? If so, consider routing the larger trace directly to its pad, and then from the pad to the MCU. This minimizes trace resistance and inductance between the signal/power line and the cap. Same for C14 and 3v3 the lines.
• Finally, just a matter of personal preference. In your power supply you could work with ‘areas’ instead of traces. Again, this is probably just personal preference: you could for example draw a fill/polygon between the fuse (?) D3 and inductor 07HVP.
So there you have it (phew
). I really hope this helps you. As I said in the beginning, I might reconsider two-layer for such a board as you show that it's fine to do so. But once you’ve done four layer, and can just stitch ground and power to their respective planes directly… …let’s just say I’d really miss that. But we’ve been spoiled: in the old days it was two layer only and they made it work and for very complex boards too…
Good luck,
Christean