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RF Device Integration on Glass Interposer toward 3D-IPAC Packages
Keywords: RF device integration, Glass interposer, RF passive components
Glass has been proposed as an ideal substrate material for RF applications at the package level by the Georgia Tech Packaging Research Center (GT-PRC), combining the benefits of ceramic, organic and silicon substrates into a 3D IPAC (Integrated Passives and Actives Component) concept [1, 2]. Glass offers high dimensional stability, good CTE match to silicon chip, low electrical loss, and high precision wiring for embedding RF passives. The concept of 3D IPAC involves the use of ultra-thin glass interposers with small through package vias (TPVs) to interconnect either passives or a combination of active and passive components on both sides. The first attempt to integrate band-pass filters in low cost, double-sided glass interposers using coarse design rules was presented by GT-PRC [3]. This paper goes beyond to demonstrate a more complete 3D IPAC which is focused on the RF filter with significant reduction in size using, a) ultra-thin glass, b) novel low loss and thin dry film polymers, and c) excimer laser process to realize small TPVs. This paper will also present design, fabrication, and characterization of the miniaturized low pass filters integrated into the 3D IPAC, along with board level assembly and reliability. Four types of low pass filters, having different cutoff frequencies from 800MHz to 5.5GHz, were designed and fabricated within the dielectric layers of the glass interposer. The size of each filter was only 1 to 2.5 square mm. In the demonstration test vehicle, ultra-thin 100µm thick EN-A1 glass from Asahi Glass Company (AGC) was adopted as the core material, and 60µm diameter TPVs were fabricated using excimer laser. Double-sided re-distribution layers using two layers of 17.5µm thin ZS-100 dielectric build-up film from Zeon Corporation were fabricated to realize the four-metal layers. Using thin dielectric films enabled higher capacitance density leading to miniaturized filter designs. After assembly onto the dedicated test boards with solder BGA connections, each low pass filter was characterized and compared with simulated results. The simulated results showed very low insertion loss (~-1dB) in the pass-band, with high rejection (<-20dB) in the stop-band. The simulated results were corroborated with measurements. For the configuration involving filters on both sides of the interposer, the impact of electrical interference on filter performance was studied and analyzed. To verify the reliability of interconnection between 3D IPAC and the organic board, daisy chain structures, connecting the board, BGA and TPVs in the interposer were fabricated, and thermal cycle testing according to JEDEC JESD22-A104 condition B was conducted. The reliability results will be also discussed. This study for the first time reports the reliability of through vias in small glass BGA packages connected to board by SMT solder interconnections. This paper demonstrates ultra-miniaturized filter devices in the new 3D IPAC concept using thin-glass, small through vias, low-cost and high performance polymer materials and processes to provide unparalleled benefits in performance, cost and form factor for future generations of wireless systems.
Yoichiro Sato,
Asahi Glass Company
Atlanta, Georgia

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