Microwave Engineering Europe - March 2008 - (Page 20)

20 BENEFITS OF BeO Many applications still require unique performance beneﬁts of BeO By John E. Scheatzle, General Manager, Brush Ceramic Products eryllium oxide (BeO) is a familiar material to designers of microwave products, since it has long been employed as an insulating material in highpower RF power transistors, in terminations, and other high-power components. BeO is not the least-expensive insulating material available, so efforts have been made over the years to employ other materials such as alumina (Al2O3) in an effort to reduce cost. However, more than 40 years after it was ﬁrst employed in microwave circuits, BeO’s inherently vastly superior thermal conductivity, and a thermal coefﬁcient of expansion that closely matches that of GaAs and SiGe, uniquely position the material for use in some of the most demanding RF and microwave applications. BeO and beryllium (Be) from which it is obtained are widely used in a remarkable array of largely dissimilar applications, ranging from lasers to medical equipment, semiconductor fabrication, vehicles (where BeO made the ﬁrst electronic ignition possible), aircraft, spacecraft, and even nuclear weapons. (See “What’s New in BeO”, p. 22). However, the primary reason for BeO’s longevity in RF and microwave devices is its inherent superiority to all other viable materials in terms of thermal conductivity (TC), which is about 285 W/ mK at room temperature. Compared to its competitors, only diamond (with its untouchable TC of 1800 W/mK) beats BeO. The next-best material is AlN with a TC of about 185 W/mK, and then alumina (Al2O3) at B Figure 1: A typical stripline-opposedemitter (SOE) package used for UHF-VHF silicon power transistors. (Photo courtesy Zentrix Technologies.) 25 W/mK. Consequently, when thermal management and electrical isolation are key considerations, RF power transistor packages based on BeO such as the one shown in Figure 1 are the optimum choice. BeO also is a very good match to common semiconductor materials in terms of thermal coefﬁcient of expansion (CTE), which at 9.0 ppm/ºC is close to gallium arsenide at 5.8). Its CTE is also an excellent match for use with metal matrix composites (MMCs) such as Cu/W, Cu/Mo, Al/SiC, and most recently E-materials. In addition, the electrical resistivity and dielectric strength of BeO is better than AlN. The material also has a low dielectric constant (6.7) and low loss index (0.0012 at Table 1: Comparison of insulating material properties. 1 MHz), which makes it very attractive for components that operate at high frequencies where power output is difﬁcult and expensive to achieve and losses must be kept to a minimum. BeO also is inherently stable in oxidizing environments, unlike nitrides (of which AlN is one) that will decompose over time to their oxide equivalent. Finally, BeO can be formed into many different shapes in order to meet the needs of speciﬁc applications (Figure 2). Table 1 compares BeO to other insulating materials. BeO (also called beryllia) is an oxide ceramic, so it is extremely stable in environments that contain oxygen and moisture. In military and aerospace systems, in which high strength is essential, ceramic-to-metal joints and metallization coatings have proven to be very reliable. BeO is also about 11 times more resistant to thermal shock than alumina, which provides a signiﬁcant beneﬁt in airborne and spaceﬂight applications (among others) in which rapidly-changing and wide-ranging temperatures are encountered. As a ceramic, BeO’s thermal conductivity decreases as temperature increases. In contrast, the thermal conductivity of metals deteriorates less with increasing temperature, Microwave Engineering Europe ● March 2008 ● www.mwee.com 020_021_022_MWEE.indd 20 21/02/08 11:12:22 http://www.mwee.com

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Microwave Engineering Europe - March 2008 News Contents Comment Wireless Infrastructure: A Direct Conversion I/Q Demodulatordrives Favorable Basestation Cost-performance Metrics Wireless Infrastructure: Mobile World Set to Reshape the Internet RF Amplifiers: Latest Advances in RF Amplifiers Include a CMOS PA Operating at 77 GHz and Significant Advances in PAs for WiMAX and Broadband Applications Many Applications Still Require Unique Performance Benefits of BeO ACE Automated Circuit Extraction Returns to Real Design by Exploring Design Alternatives and Changes in Seconds Exceeding the Standard for Wireless Sensor Networks Products Calendar

Microwave Engineering Europe - March 2008

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