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1. Introduction
The origins of direct bonded copper (DBC) substrate technology date back to the mid-1980s, when it was developed and patented by the General Electric Company. Combining the high thermal conductivity of copper, and its high current-carrying capacity, with the good mechanical and insulating properties of ceramics, a technology has been developed for manufacturing DBC substrates intended for use primarily in power multi-chip module (MCM) circuits. Moreover, DBC substrates offer an attractive solution for future designs in the field of high-power devices including, inter alia, light-emitting diodes (LEDs) (Dehmel et al., 2007).
It can generally be said that both the scale of circuit integration and operating speeds have increased dramatically, which has resulted in an increase in heat dissipation. This may cause an increase in operating temperature, which can negatively affect the reliability of circuit operation. An alternative is a technological solution that results in a more efficient cooling of circuits. In Figure 1 a diagram of a typical power circuit is shown. Semiconductor dies, usually IGBTs, MOSFETs or diodes, are soldered onto the DBC substrate pattern, which, in turn, is soldered onto the copper base plate. The base plate is coupled with an air-cooled aluminium heat sink by thermally conductive grease. One of the important technological problems related to MCM–DBC modules is the thermal expansion mismatch between the component materials; the CTE of silicon is 3 × 10−6/K; for Cu it is 16.7 × 10−6/K; for PbSn solder, it is 24 × 10−6/K; for Al2O3, 8 × 10−6/K; and for AlN, it is 4.5 × 10−6/K. This results in large mechanical stresses under the influence of temperature within the DBC substrate itself, and also in the solder between the DBC and the base plate below, or silicon dies above the substrate. The DBC substrates themselves have been rapidly damaged under the influence of thermal cycles, for example, substrates cracked after about 15 thermal cycles from −55 to +250°C (Dong et al., 2009). Under high temperature cyclic loading, fatigue cracks ensued at the interface between the copper layer and the ceramic layer of the Al2O3–DBC substrate and split along its width (Dong et al., 2010). The process of soldering Al2O