Ultrasound Transmission through a Flexible Printed Circuit Board Bonded to the Front Side of a Capacitive Micromachined Ultrasonic Transducer Array: Feasibility Study

Interconnecting the inner elements of a densely populated ultrasonic transducer array with electronics poses a great challenge when pads are located in the inner area such that they are not easily accessible or wire bonding is not a viable solution. To tackle that challenge, we propose the technique of front-side flip-chip bonding capacitive micromachined ultrasonic transducer (CMUT) arrays to flexible printed circuit boards (FPCBs). As the propagation through the flex material can cause signal attenuation, we measured the pressure reduction for a reference transducer and an experimental CMUT and observed that the reference underwent a 19% pressure reduction while the experimental CMUT experienced a 33% pressure reduction after transmission through the flex. We argue that the difference can be partly attributed to inappropriate underfill in the interface between the CMUT and the FPCB. The proposed packaging approach can potentially provide a versatile interconnecting scheme for densely populated small transducers required in applications such as ultrasound neuromodulation.