Electrical – PCB design: how to route through hole connectors with tightly fitted pins

I see a few issues with your design:

1. One of the caps is physcially touching the crystal. Move it just a little bit away

2. Move the crystal up so that it's as close as can be to the PIC18.

3. Make room for the guard ring. From the little I see in the image, you can likely move some things to move it closer.

4. make sure to ground the crystal case itself mechanically (don't forcibly solder it somehow)

5. Change the capacitors for the crystal to ceramic. This will make them smaller and there's no point in electrolytic here.

The reality is that even in its current state, the circuit will work. So it's not a question of whether it will work, but whether you'll get the best performance, cleanest clock, lower EMI, etc.

The following is one app note about how to best layout crystals:

AVR186: Best Practices for the PCB layout of Oscillators

It will probably be fine in your case to via down to the bottom layer and have a small slot in the ground plane. If you were running higher speed signals it could become a problem.

The issue to generally be concerned about with cutting a slot in the ground plane is that return currents will need to go around the slot, which can then act as an antenna and be an EMC problem. Return currents are concentrated on the paths of lowest impedance. For low frequencies, this is the path of lowest resistance. For high frequencies, inductance starts to dominate the impedance, so the return currents will flow in the direction that minimizes the loop area. This means that they will be concentrated directly underneath the signal trace until they reach the slot, then flow around to get back to the signal trace again. This will cause radiated energy and distort nearby signals (or farther nodes if they are high-impedance). With lower frequencies, the return currents will not even care to get close to the slot, though, instead making a diamond shape of lower current density centered on the slot.

That being said, 2-4 MHz is so slow I wouldn't worry about it. If you're running at 500+ MHz with fast edge rates, though, slots in the ground plane should be avoided, especially underneath high speed data lines.