WLFO For High-Performance Low-Cost Packaging Of RFMEMS-CMOS

Navigating the trade-offs between performance, size, cost and reliability can be a challenge when considering integrated circuit (IC) packaging and the end-application. The integration of micro-electromechanical systems (MEMS), either monolithic or heterogeneous, introduces yet another level of complexity that has only recently been a major focus of multi-device packaging. Wafer-level fanout (WLFO) technology can enable improvements in several
areas, primarily the reduction in size of parasitic interconnects and, proportionally, a drastic decrease in overall form-factor as compared to more ubiquitous chipscale packaging solutions. Widespread adoption of WLFO packaging has driven implementation costs to a level competitive with traditional fan-in wafer-level packaging.

This study quantifies the benefits of WLFO versus flip-chip land grid array (FCLGA) packaging for a radio frequency (RF) MEMS digital tunable capacitor array integrated with 180nm CMOS technology. RF performance hinges critically upon the ability of the package to transfer signals with minimal impedance, necessitating shortened redistribution layer (RDL) paths and reduced, or removed, solder interconnects. Flip-chip packaging requires a multilayered substrate and an intermediate solder interconnect while chip-first WLFO packaging makes use of direct Cu RDL bond to die pads and a single-level of routing to the ball grid array. Die size in both cases is 1.8×2.2×0.3mm, which enables a direct comparison of form-factor between the two package types. Manufacturability is addressed in this study; a primary challenge of monolithically integrated MEMS/CMOS is the ability to survive typical IC packaging processes wherein thermal, mechanical and electrical overstress may occur.

Daisy chain packaged parts were subjected to board-level mechanical shock. Functional packaged parts were subjected to component-level reliability stresses. RF characterization of functional packaged parts was conducted on printed circuit boards (PCB); the primary figure of merit being self-resonance frequency (SRF) as a result of overall parasitic losses.

By Rameen Hadizadeh, Anssi Laitinen and David Molinero, Ph.D., WiSpry, Inc., and Nelson Pereira and Márcio Pinheiro, Amkor Technology, Inc.

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