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November 2015 • The PCB Magazine 53 Organic PCB materials do not have these re- strictions with regard to structure and number of layers, however, the thermal conductivity of the epoxy resins used is significantly lower than in ceramic materials. A big advantage of PCB substrates is their significantly lower cost, which is why the use of ceramic substrates is limited to those power electronics that cannot be implemented with- out the properties of ceramic materials. Assembly and Interconnection Technology for Power electronics Systems The conventional processing technology for PCBs is the fully automatic assembly of SMD components. The through-hole assembly is mostly limited to DC-link capacitors, if these have not yet been replaced by SMD components. In ceramic substrates, the bottom side of bare semiconductors is connected to the sub- strate in the electrical, mechanical and thermal sense by means of conductive adhesives, solder- ing, silver sintering or diffusion soldering. The conventional way of connecting the top side is via aluminum heavy-wire bonding, how- ever, this technique is slowly being replaced by Cu wire bonding. After assembly, the bonds are often stabilized by sealing them with a highly viscous silicone gel. The required logic control and driver elec- tronics are implemented via a separate substrate (usually a PCB) and are often interconnected with the power electronics via press-fit contacts. Most entire systems of power electronics ap- plications have a large number of different as- sembly and interconnection technologies that must meet the required targets regarding reli- ability and costs on their own as well as in com- bination. reliability Aspects of Assemblies Adjustment of the Expansion Coefficients Due to their coefficient of expansion (CTE), organic PCBs are well-adjusted for components in housings such as QFP or DIP. Assembly of bare semiconductor components such as flip chips, however, is critical. Depending on their struc- tural design, ceramic chip capacitors can also be critical as they have significantly lower CTEs. ConDUCTInG Very HIGH CUrrenTS THroUGH PCB SUBSTrATeS AT HIGH AMBIenT TeMPerATUreS ArTiCle Figure 1: IgBt module power electronics based on DcB ceramics with power busbars in plastic hous- ing. (Source: www.enacademic.com)