Board design considerations like board thickness and layer stacking under a crystal when designing PCB for accurate clock crystals discuss in this app note from Maxim Integrated. Link here
Crystal oscillator is widely used in RTC applications. Clock accuracy, one of the key parameters in real-time clocks, partly depends on the parasitic capacitance of the PCB being employed. Optimizing PCB layout can achieve the desired clock accuracy.
from Dangerous Prototypes https://ift.tt/2VshsRo