Packaging-Induced Mode Distortions in Wideband Aircoupled PMUT Arrays

This paper investigates how packaging influences the vibrational modes and usable bandwidth of wideband aircoupled piezoelectric micromachined ultrasonic transducer (PMUT) arrays employing frequency compounding. An$\text{Al}_{.7} \text{Sco.}_{3} ~\mathrm{N}$PMUT linear array (10 channels covering$260{-}$390 kHz, plus additional low-frequency devices at$100{-}$180 kHz) was characterized under three packaging configurations: closed back-cavity (no vent), open backcavity with straight through-holes, and open back-cavity with ramped via edges. Results show that the closed-cavity strongly suppresses low-frequency resonances and reduces effective bandwidth; the straight-hole open-cavity restores bandwidth but introduces parasitic cavity-modes and spectral gaps; and the ramped-via geometry mitigates those parasitic modes, yielding continuous wideband response. Finite-element modelling reveals that through-hole edge geometry controls the acoustic impedance of the PCB cavity, which in turn couples into the diaphragm modes. The optimized packaging (ramped vias) achieved$\sim 30 \%$higher acoustic signal-continuity and$\sim 26 \%$broader -3 dB fractional bandwidth relative to the closed-cavity baseline. These findings provide concrete design guidelines for back-cavity venting, PCB thickness selection, and via-edge shaping to preserve wideband performance in frequencycompounded PMUT systems.