ABSTRACT
Many ceramic materials are available for use in manufacturing of power hybrid circuits. They vary from commonplace to exotic, offering designers the versatility in selection of mechanical and electrical properties. Physical properties of ceramic substrate materials frequently used in power hybrids are listed in Table 3-1. None of the available substrate materials provide designers with an ideal solution. Alumina (AI2O3) has been a substrate of choice for use in hybrid microcircuits for decades. Most thick-film materials were developed and characterized for use on alumina substrates, making them a common material available at low cost for most general applications. However alumina has poor thermal conductivity, when compared with aluminum nitride (A1N) or beryllia (BeO), and is much less suitable for high power applications. The disadvantages of beryllia: higher cost and toxicity of its fumes and powder, are traded for availability of wide selection of compatible thick film materials and superior thermal conductivity. Aluminum nitride is a new ceramic material that offers certain advantages when compared with both AI2O3 and BeO. It is less costly than BeO, has a thermal conductivity much higher than alumina, and matches the thermal expansion of silicon better than both. It is nontoxic and is compatible with selected thick-film pastes. Comparison of variations of thermal conductivity and thermal expansion with temperature, shown in Figure 3-2, helps in selection of substrate material for a specific application. The shaded area defines temperature range relevant to power hybrid applications. For example, thermal conductivity of BeO in Figure 3-2b rapidly decreases approaching A1N at temperatures above 400°C. Despite that, at temperatures below 200°C BeO is an obvious choice when thermal conductivity is a primary concern. On the other hand, if close matching of thermal expansion with silicon die is more critical, then A1N is a better choice. And finally, when power dissipation level is relatively low and the cost is a driving factor, the alumina substrate becomes a prime selection. The thermal expansion and thermal conductivity characteristics of popular materials
0.03-0.1
0.03-0.1
3.25-3.30
3.8-4.4
3.7-3.8
2.0-2.6
%
°C
C
hapter3
Figure 3-2 Ceramic substrate materials, performance variation with temperature: a) CTE vs. temperature, b) thermal conductivity vs. temperature.
