Performance Optimization of FBAR Filters with Wafer-Level Chip-Scale Package Using Embedded Matching Inductors on Multilayer PCB

This work presents an embedded matching scheme on the multilayer print circuit board (PCB) for film bulk acoustic resonator (FBAR) filters with wafer-level chip-scale package (WL-CSP), which are widely used in front-end modules (FEMs) for mobile communication. The matching inductors connected in parallel to ground with different inductance are designed and integrated into the middle layer of PCB. For the S-band WL-CSP FBAR filter, the simulation and measurement results show that the matching inductance of 7 nH can achieve a good in-band and out-of-band response. According to the test results, the insertion loss is 2.3 dB and the return loss is -16.4 dB at the working center frequency of 2069 MHz, while the out- of-band rejection is more than 40 dB. The embedded matching inductor of 7 nH has a compact size of similar to 2 mm2. This technique has the characteristics of miniaturization, high flexibility and easy integration, which realizes the performance optimization of the WL-CSP FBAR filter.