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Integration of a K-Band Receiver Front-End Using a Copper Core Printed Circuit Board
Keywords: k-band, MCPCB, wirebond
Abstract— Increasing demand for high bandwidth wireless satellite connections and telecommunications has resulted in interest in steerable antenna arrays in the GHz frequency range. These applications require cost-effective integration technologies for high frequency and high power integrated circuits (ICs) using GaAs, for example. In this paper, an integration platform is proposed, that enables GaAs ICs to be directly placed on a copper core inside cavities of a high frequency laminate for optimal cooling purposes. The platform is used to integrate a K-Band receiver front-end, composed of four GaAs ICs, with linear IF output power for input powers above -40dBm and a temperature of 42°C during operation. I. INTRODUCTION In recent years, various applications, for example VideoOn-Demand and cloud computing, have led to research of high bandwidth satellite communications. High gain antenna arrays with beamsteering capabilities allow users on mobile platforms such as cars or trains to enjoy high speed communications provided by satellite data links. This introduces many challenges regarding the beamsteering capability for terrestrial satellite communications platforms. Electronic beam steering, made of active antenna arrays, is one of the possible options to arrive at highly integrated, robust satellite terminals. In active antenna arrays, each antenna element or group can be controlled in amplitude and phase and, due to scanning requirements, the distance between antenna elements in the array is typically close to lambda/2. Hence, for higher frequencies, the available space for the front-end components decreases, while chip size may not decrease. This leads to significant chip integration challenges. The integration requires a sophisticated packaging technology that offers (1) excellent high-frequency properties, (2) high integration density, and (3) a highly conductive thermal path from the chips to a heat-sink and subsequently excellent heat management. Most examples of K- and Ka-band MMIC integration utilize low temperature co-fired ceramics (LTCC) substrates with wirebond technology. For example, in [1], a single transceiver with a low-noise amplifier (LNA), a power amplifier (PA), switch, phase shifter, and attenuator were packaged in cavities with bondwires in LTCC technology. For [2], Spira integrates four front-ends into an LTCC package for beam steering applications. The integration strategy in [3], also utilizes LTCC. Holzwarth et al. have also proposed a beamsteerable antenna array in LTCC, which addresses significant cooling challenges with a complex water cooling integrated into the carrier. This solution implements digital beam forming, which introduces significant processing complexity in the back-end [4]. In [5], an LTCC package was also used to integrate a Ka-Band for active integrated antennas. Other integration techniques have also been proposed for Ka-Band MMIC packaging besides LTCC. In [6], an LCP package was used to package a two stage LNA in a single receiver front-end. Aihara in [7] also uses LCP to package a single Ka-Band front-end. He also packages a two stage LNA in a receiver front-end. Another approach has been to use a BeO ceramic with a Kovar housing [8]. This approach offers a very credible solution to the heat dissipation problem and retains excellent high-frequency properties. The technique, however, would lead to very high costs, especially where many carrier layers are necessary, and potential fabrication challenges like planarity of the carrier and sizes and tolerances of vertical interconnects and cavities. In [9], the authors integrated GaAs die using MCPCB technology and showed that this technique could be suitable for K and Ka-band applications. This paper will further investigate a K-Band receiver front-end composed of four GaAs ICs that are built in MCPCB technology, where the dies are placed in cavities in high frequency laminates to provide adequate high frequency properties, and directly on the metal core to address thermal challenges of integrating a high density of dies. II. INTEGRATION CONCEPT The PCB material was selected based on processability, dielectric loss tangent and available thicknesses. The die have a thickness of 100um and the die attach material will have an estimated thickness of 50um. Therefore, a laminate and its top side metallisation should come as close to 150um as possible. Ultimately Megtron6™ was selected. With two 60mm prepreqs and a 35um thickness metallisation on the top side, we achieve a very close thickness match of the dielectric stack at approximately 156um. Megtron6™ also has a low dielectric loss tangent, good processability, and wide availability. The goal of the cavity structuring was to keep the wirebonds as small as possible while still allowing automatic die placement inside the cavities. Laser technology was selected, as opposed to milling technology, due to its improved tolerances of +/-50um. Ideally the cavity would be structured as close to the outline of the front end as possible, to facilitate the shortest possible wirebonds. The chips have a length tolerance of +/-50um. The placement tools require additional space of 50um around the edge of the chips. Therefore, the cavities were structured with 100um of additional space surrounding the expected sizes of the GaAs dies. III. 20GHZ RECEIVER FRONT-END MEASUREMENTS Tthe integration of a 20GHz receiver front-end is realized with the amplifier and frequency doubler HMC578, the TGP2615 phase shifter from Qorvo™, the AMMC-6530 single sideband mixer from Avago™, and the low noise amplifier (LNA) CHA3689 from United Monolithic Semiconductor™. The paper then shows the relationship between the IF output power dependent on the LO drive power and the RF input power. Starting at approximately -40dBm input RF power, we see a linear behavior at the IF output. The LO input power shows a maximum output power at approximately 5dBm. A thermal camera was also used to examine the effectiveness of the metal core at spreading the heat generated by the chip. The camera shows that the hot spots on the active chips (HMC578 and CHA3689) are approximately 7°C higher than the temperature otherwise on the chips. The temperature otherwise on the chips show little difference in temperature to the copper core directly near the active chips. The 20GHz receiver front-end is almost 10°C cooler than the 60GHz transmitter front-end because the LNA dissipates less power than the power amplifier. IV. CONCLUSIONS This paper has investigated a K-Band receiver front-end composed of four GaAs ICs that are built in MCPCB technology. The dies are placed in cavities in high frequency laminates directly on a 1mm copper core. This approach is designed to provide adequate high frequency properties, and simultaneously to address thermal challenges of integrating a high density of dies. We have identified this integration approach as a viable concept for the integration of high power active antennas for high frequency satellite communications.
Brian Curran,
Fraunhofer - IZM
Berlin, Berlin

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