Learn more about Beryllium Oxide
4.0 BERYLLIA OXIDE CERAMIC SUBSTRATES
Beryllia Oxide Ceramic (beryllium oxide, BeO) belongs to a unique class of electronic materials that combine high electrical insulation resistance with high thermal conductivity. Only a few other materials combine these two divergent properties. Among these are diamond, aluminum nitride, single-crystal boron nitride, and silicon carbide. Generally, good electrical insulators (plastics and ceramics) have very low to moderate thermal conductivities, while electrical conductors (metals) have high thermal conductivity. The thermal conductivity of beryllia approaches that of aluminum metal yet has the electrical insulation resistivity of the best of the plastics. High purity beryllia ceramic has a thermal conductivity approximately 1,200 times greater than that of a typical epoxy plastic, 200 times greater than most glasses, and 6 to 10 times better than alumina ceramic. Figures 7 and 8 compare the thermal conductivities of beryllia with some of the most commonly used electronic materials.
Beryllium Oxide ceramics would therefore make an ideal substrate material were it not for its high cost and the precautions needed in processing it because of its toxicity, but even the toxicity is not a major problem for the hybrid manufacturer who purchases already fired and machined substrates. The toxicity hazard is primarily faced by the manufacturer of the beryllia who must take precautions in controlling dust from machining and vapors from high-temperature processing.
Besides its use as an interconnect substrate for hybrid and microwave circuits, BeO is used as heat sink spacers (heat spreaders) beneath power devices and as a package-construction material for discrete power transistors, integrated circuits, and multichip hybrids.
The electrical insulation resistance of 99.5% BeO is excellent both at room temperature and at the maximum temperatures that hybrid circuits may encounter, for example, about 230°C during solder reflow assembly operations. Values of 1017 to 1018 ohm cm are reported. As with many other ceramics and inorganic and plastic materials, beryllia has a negative temperature coefficient of resistivity; that is, as temperature increases, resistivity decreases.
The dielectric strength of beryllia ceramic (voltage required for electrical breakdown or puncturing) is more than adequate for hybrid applications; it ranges from 600 volts/mil to 800 volts/mil, depending on thickness (Fig. 10). Again, as with plastics, the dielectric strength decreases with the thickness of the sample being tested. The dielectric constant (k) and dissipation factor for beryllia are better (lower) than for alumina ceramic. The k for BeO is 6.7 compared to 9.9 for alumina ceramic of equivalent purity. Both k and loss tangents are quite stable over a wide frequency range of 1 kHz to 10 GHz (Tables 7 and 8). Other physical and electrical properties of beryllia substrates are given in Tables 9 and 10.
