Development of a Scalable AiP Module for mmWave 5G MIMO Applications Based on a Double Molded FOWLP Approach

In the last years, the increasing demand for ever higher data rates in wireless communication has exposed the limits of current sub-6 GHz communication systems and pushed the technology towards the exploration of the mmWave frequency spectrum. In particular, 3 frequency bands have been licensed at 26 GHz, 28 GHz and 39 GHz for the new generation of mobile communication (5G), each exhibiting as much as 3 GHz of operational bandwidth. However, systems operating at mmWave have to cope with much higher free space losses then in the sub-6 GHz frequency range. Moreover, the physical size of interconnects (chip-to-chip, chip-to-antenna) becomes comparable to the operating wavelength. Therefore, new packaging technologies are required, which allow implementing large antenna arrays for massive MIMO and short interconnects to minimize package losses. Within the funded project IPCEI “mmWave Modules” a scalable and reconfigurable module for mmWave 5G small cell base stations has been developed. In particular, the module measures and exhibits two dual-band patch antenna arrays for operation at 28 GHz and 39 GHz. Moreover, it may be easily upscaled for the deployment in base stations covering larger cells. Packaging technology is based on a double molded Fan-out Wafer Level Package (FOWLP) approach with integrated antenna-in-package (AiP) that has been adapted and evaluated. The Analog-Front-End-IC (AFE IC) has been packaged as a start in an RDLfirst FOWLP on 300 mm wafer size. Redistribution layers were built-up on a glass carrier consequently followed by plating of the through package vias [TPVs] for later front and backside package connection. The AFE IC is then assembled as a flip chip on the RDL and compression molding is used to underfill the flip chip ICs and overmold the wafer carrier in one step. A backgrinding step is applied to open the TPVs for further electrical connection. Direct metallization using seed layer sputtering, copper plating and subtractive etching is used to add a shielding layer on the mold cap. After debonding the wafer is overmolded a second time using compression molding again. The antenna is then directly metallized on the second mold cap connected by laser drilled and metallized vias to TPVs and shielding layer. Finally solder balls are attached and packages are separated by dicing. Focus of the development was the double mold process with the related warpage challenges, direct metallization on mold as well as antenna and via integration and alignment on the second mold layer. Besides the development of the entire package the AiP concept has been evaluated using a GHz/39 GHz dual-band antenna test structure considering the later double mold approach including an inner shielding layer, laser drilled through mold vias and direct metallized antenna on mold. Measured bandwidths of 1 GHz and 700 MHz and gains of 5.3 dBi and 6.2 dBi at 28 GHz and 39 GHz, respectively, prove the applicability of the proposed packaging technology for mmWave 5G applications. In summary the paper will describe the FOWLP package development for a mmWave 5G MIMO application with focus on a double mold approach with antenna-in-package application.