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Use of various flip-chip BGAs in applications such as RRU
Keywords: RRU, FCBGA, FPGA
RRU (remote radio unit) is a huge wireless market. It has become one of the most important subsystems of today's new distributed base stations. RRU contains the base station's RF circuitry plus analog-to-digital/digital-to-analog converters and up/down converters. RRU also has operation and management processing capabilities and a standardized optical interface to connect to the rest of the base station. They are mounted in a tower outdoor, see Fig. below. RRU has a set of distinguished characteristics. High ambient temperature is typical, 55°C is common. For reliability reasons no fan is allowed, therefore natural convection and radiation are the main ways to transfer heat. Another main characteristic is that gap fillers are used as TIM2 (thermal interface material) between the components and the case of the RRU box. Gap fillers are needed to compensate the height difference and tolerance of multiple components. Gap fillers are generally thick, 0.5 to 1.0mm common, hence have high thermal resistance; they are a bottleneck to efficient heat transfer. In addition, most heat has to exit from the top or the case. Because of these reasons, thermal management is a huge challenge. FPGAs and ASICs are used extensively in RRU. For mid to high end applications, these devices use flip-chip BGA (FCBGA) packages. There are several popular FCBGAs, mainly differentiated in lid type, lidded or lidless. In this work, the following factors are investigated: 1. Lidded a. Lid material: Cu lid vs. Al lid b. Lid style: Flat top lid vs. Hat type lid 2. Lidded vs. lidless a. Lidless include molded FCBGA with expose die and bare die FCBGA. Since they both have the same thermal performance, molded FCBGA is used as representative 3. Gap filler thermal impedance a. Gap filler A with high thermal impedance b. Gap filler B with medium thermal impedance c. Gap filler C with low thermal impedance CFD (computational fluid dynamics) models were used to facilitate the study. It is found that there is significant difference in thermal performance between different FCBGAs. The biggest difference exists between lidded and lidless FCBGAs. The gap filler shows great effect on the thermal performance of the devices, not only that, it shows a strong interaction with the FCBGA type; it has significantly more effect on lidless than lidded FCBGA. This study provides valuable information on the thermal performance of various FCBGA in applications such as RRU. It can serve as a guideline on how to properly select different FCBGAs under different application conditions in terms of power and gap filler thermal impedance.
Yuan Li,
Altera Corp
San Jose, CA

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