Can't say too much about the solder joints issue, but about a decade ago we did have a lot of problems with resonators being damaged by harmonics from Ultrasonic board washing & Ultrasonic welding.
However this mostly manifested itself as cracking in the joining of the internal element to the end termination; the susceptability depended on the vibration mode of the resonator.
The only other major culprit for resonator damage was incorrect Z- height adjustment on the insertion machine.
When you say thickfilm technology do you mean Surface accoustic Wave?
Just a thought, but the board isn't being flexed when it's placed into/or seated in the enclosure is it?
Cheers,
Rob.
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