developed to improve antenna performance [31] and an open cavity radiating element was created to relax LTCC fabrication tolerance. In 2008, a grid array antenna and patterned mesh ground plane were devised to enhance reliability and avoid warpage for AiP in LTCC [32]. In 2012, a paper entitled “Dual grid array antennas in a thin‐profile package for flip‐chip interconnection to highly‐integrated 60‐GHz radios” won the IEEE AP‐S Sergei A. Schelkunoff Transactions Prize Paper Award [33]. In 2013, a step‐profiled corrugated horn antenna was realized in LTCC for AiP to operate at 300 GHz [34]. High‐density interconnect (HDI) technologies were developed specifically for the low‐cost production of AiP. In 2012, Samsung developed FR4‐based HDI technology for 60‐GHz radios. Despite the relatively high loss tangent of the FR4, Samsung confirmed that unit loss was comparable with the LTCC‐based AiP at 60 GHz [35]. In 2015, Intel developed liquid crystal polymer (LCP)‐based HDI technology for 60‐GHz radios. By limiting the number of metal layers to four, with the 60‐GHz routing on the same layer as the die pads, Intel demonstrated that AiP achieved an ultra‐thin profile at 60 GHz [36]. Unlike LTCC and HDI, the embedded wafer‐level ball grid array (eWLB) technology eliminates the need for a laminate substrate and replaces it with a copper redistribution layer. It was developed by Infineon in 2006 and proved to be an alternative approach to fabricating AiP in high volume with low cost [37]. However, eWLB technology only produces a single redistribution layer (RDL), which limits the realization of antennas. To overcome this limitation, the Taiwan Semiconductor Manufacturing Company (TSMC) developed the InFO‐AIP technology in 2018, which places the feeding line in the RDL at the bottom of the package, coupled to the patch antenna on the top side of the package. As a result, InFO‐AIP yields a smaller form factor and a higher gain for 5G mmWave system applications [38]. In addition, a major concern with AiP is the risk of electromagnetic interference (EMI). In 2014, Advanced Semiconductor Engineering, Inc. (ASE) developed package‐level conformal and compartment shielding techniques with metal coating to suppress EMI [39].
Accurate characterization of AiP was first made possible with a probe‐based antenna measurement setup built by IBM in 2004 [40]. It used a ground‐signal‐ground (GSG) probe connected to one port of a vector network analyzer with a coaxial cable to feed the AiP. A special waveguide arm with a standard gain born antenna could be rotated around the AiP at a distance to ensure a far‐field condition for radiation pattern measurement. In 2009, Toshiba built a setup to measure the radiation pattern of differential AiP with a ground‐signal‐ground‐signal‐ground (GSGSG) probe [41]. In 2011, Karim et al. introduced a setup to minimize the effect of the probe radiation and to enhance the dynamic range by implementing a backside probing technique [42]. In 2012, Diane et al. demonstrated a setup to measure 3D radiation patterns [43]. In 2015, Reniers et al. developed a bended probe to reduce both the blockage and the interference due to reflections from a conventional probe [44]. In 2018, fast testing of AiP for a production line was proved feasible with over‐the‐air (OTA) contactors [45]. An OTA patch antenna was embedded into the lead backer of a production pick and place handler, which offers a unique and reliable production solution testing an AiP device with 60‐GHz RF signals both radiating out of an antenna array in the lid and connected through the ball grid array. OTA contactors have also been used for testing of 76–81‐GHz AiP automotive radar devices and are being designed for 5G applications at 28 and 39 GHz.
A large number of needs are met by the use of AiP technology. including Internet of Things (IoT) devices at 2.4 GHz, 5G new radio and networked cars at 28 GHz, VR, axial ratio (AR), and gesture radars at 60 GHz, automotive radars at 79 GHz, imagers at 94 GHz, sensors at 122, 145, and 160 GHz, as well as 300‐GHz wireless links. The advantages of AiP technology will continue to generate new applications, for example the adoption of AiP technology in the development of highly integrated micro‐synthetic aperture radar (SAR) for deep‐space exploration [46].
1.5 Concluding Remarks
AiP technology has broken the boundaries between antenna and circuit fields. The methodology and platform to co‐design antenna and circuits is now available. AiP technology has justified developing new materials and processes, which is rare, to the best of my knowledge, and only microstrip patch antennas have received such attention. Testing has to be considered along the whole manufacturing cycle, including test strategies, verification and characterization, production testing, integration and system level testing. Probe‐based measurement setups are suitable for AiP design verification and characterization. OTA antenna measurements are required for production testing, integration and system level testing. In the future, AiP technology will continue to provide direct antenna solutions to highly integrated wireless systems that will operate at even higher mmWave frequencies. It will also provide parasitic or distributive radiator functions to enhance the terahertz antenna performance of AoC technology.
Acknowledgements
The author is grateful to his students Dr. Wang Junjun, Dr. Sun Mei, Dr. Zhang Bing, Dr. Chen Zihao, Mr. Lin Wei, and Mr. Xue Yang, his research staff Dr. Zhang Wenmei, Dr. Sang‐Hyuk Wi, Dr. Tu Zhihong, and Ms. Zhang Lin, and his collaborators Dr. Duixian Liu and Mr. Brian P. Gaucher of the IBM Thomas J. Watson Research Center and Dr Albert Lu, Mr. Chua Kai Meng, and Ms. Wai Lai Lai of the Singapore Institute of Manufacturing Technology for their contribution in the development of AiP technology.
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