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54 The PCB Magazine • November 2015 As opposed to that, DCB/DBC ceramics are assembled with bare power semiconductors. The lower coefficient of expansion of the ce- ramic base material leads to a better adjustment of the CTE between substrate and component in this scenario. The bare dies are then connect- ed to the substrate and the terminals using wire bond technology. Due to their ampacity, heavy aluminum wires with a diameter of 500–600 µm are used for this. Failure Mechanisms of Assemblies The classic failure of assembled PCBs is failure of soldered joints in poorly adjusted components due to cyclic thermal loads. In the PCB itself, fail- ure of interconnections has to be mentioned, due to the anisotropy of the CTE in x/y direction as compared to the z direction is common. In ceramic assemblies, however, the classic failure case is bond lift-off from the power semi- conductor caused by the large difference in the CTE between Si (2.7 ppm/K) and Al bond wire (24 ppm/K). Another mechanism is conchoidal fractures of ceramics induced by CTE differenc- es of ceramics (approx. 7 ppm/K) and copper metallization (17 ppm/K). Due to the aforementioned reasons, most power electronics systems use a mixture of ce- ramic substrates and PCBs that are intercon- nected by assembly and interconnection tech- nologies that can be very sophisticated and complex at times. They are implemented by way of wire bonding, plug connectors, solder- ing or welding technologies, and modern power electronics systems often combine all available assembly and interconnection technologies in one single system. optimization of Power electronics Systems Therefore, the essential requirements for op- timized substrates in power electronics systems are as follows: • Increased ampacity • Optimized thermal conductivity • Increased temperature stability • Reduced system complexity • Increased system reliability • Minimized costs Figures 2 and 3: Soldered joint failure of a ceramic chip capacitor. Figure 4: cracks in a heavy wire bond on an IgBt (silicon) surface. (Source: www.powerguru.org) ConDUCTInG Very HIGH CUrrenTS THroUGH PCB SUBSTrATeS AT HIGH AMBIenT TeMPerATUreS ArTiCle