When you design PCB layout, it is necessary "1) to prevent decreasing negative resistance" and "2) to prevent EMI problem".
The length of signal patterns in oscillation circuit should be as short as possible to minimize stray capacitance/inductance Through-holes should not be applied to oscillation circuit because it causes large EMI.
Ground or signal path should not be located in the middle layer of multi-layer circuit board to overlap the oscillation circuit as the stray capacitance between ground and oscillation circuit becomes large.
Large stray capacitance may cause oscillation stop due to insufficient oscillation margin. The signal path located close to the input of C-MOS inverter may generate EMI as a noise in waveform is amplified.
If you place a ground area for electrical shield, place it on the opposite surface of circuit board to the oscillation circuit. A ground pattern in the middle layer may cause oscillation margin as discussed above. The ground pattern around oscillation circuit shall not be too close to the oscillation circuit to prevent large stray capacitance.
For crystal unit mounting, it is recommended to employ machines with optical positioning capability to prevent applying excessive mechanical stress.
Make sure that you have evaluated by using placement machines before going into mass production. Do not use machines with mechanical positioning.
Solder the crystal unit by re-flow soldering.
Refer below table for our recommendable flux, solder and soldering profile.
※Component shall measured on the surface.
Conformal coating or washing cannot be applied to the crystal unit as it is not hermetically sealed.