Originally posted by mtwieg
High frequency traces (like clock sources and serial data paths, which the ADC will have) are the biggest sources of EMI, so yes, there will some interference. You don't necesarilly need to avoid putting traces under them, though. It depends how vital the stability of the traces are. Analog and other clocked traces are bad to expose to EMI. Same with traces that control mosfets. If the trace powers big, bulky components like LEDs or buzzers or whatever, then a little interference on them probably won't matter.
When doing the layout, I generally design in this order:
1. Power supplies. "Star" layouts are generally good at reducing EMI.
2. High di/dt paths. For you, these are the traces that power the pyros. Keep them straight, short, fat, and very far from vital traces.
3. "Vital" traces. Make sure sensor outputs, FET gate controllers, and high frequency signals are short and direct. High (in your case, clocked data) and low (sensor outputs and mosfet drivers) frequency traces should not be too close, and should not cross. Also, give your oscillator a good ground plane around it.
4. Everything else.
Keep in mind, I have no idea how noisy your parts are. I can afford to bend the rules a lot, since I know my AVRs produce very little noise, even when running traces directly under the MCU with no ground plane. There's a good chance that EMI won't be an issue for you. I can't say, though.
That document I linked you should be better at explaining it than I can.