Design and implementation of a rigid-flex RF front-end system-in-package

Abstract

This paper presents a kind of 3D stacked system-in-package (SiP) for the minimization of the RF front-end system used in a micro base station. The RF SiP, based on a rigid-flex substrate, retains a small volume of 5 cm × 5.25 cm × 0.8 cm, which is almost 95 % reduced from the prototype board. The detail design process is presented including system-in-package design, substrate design and thermal management evaluation. Simulation is performed for the electrical characterization of the substrate and the thermal management evaluation of the package. The simulation results reveal that the insertion loss of the RF signal going through the rigid-flex substrate is controlled under 0.38 dB and the eye diagrams of the differential clock signals meet the requirements. Power distribution network is also well designed since the biggest DC voltage drop is less than 6 mV and the highest current density is 3370 A/cm2, which is much lower than the maximum current-carrying capacity of the conductors with 20 °C temperature rise according to the IPC standard. Also, thermal simulation demonstrates the reliability of this RF SiP because the highest junction temperature reaches 79.3 °C for the normal air convection environment (20 °C) and 91.1 °C for the extreme condition (60 °C) with the forced air cooling method. Junction temperatures of both conditions are below 95 °C, which indicates the proper functioning of active chips in this RF SiP.