PCB007 Magazine

PCB-Nov2015

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52 The PCB Magazine • November 2015 Challenges for electronic Substrates Electronic substrates for power electronics applications must support the requirement for a high efficiency of the entire system. There- fore, the losses on this level must be mini- mized. The second requirement is the support for thermal management of the assembly, in which the electronic substrate often plays an impor- tant role. In addition to that, electronic substrates in power electronics must obviously support all functions of electronic substrates in conven- tional electronics. Impact of Materials Losses in Metal Layers Ohmic losses in metal layers that often con- sist of copper or copper alloys play an impor- tant role in power dissipation considerations for electronic substrates. Even though the intrinsic resistance of copper is low, it cannot be ignored if high currents are present, as it causes the con- ductor to heat up. This contributes to heat de- velopment in the entire system and must there- fore be minimized. Properties of Insulators Another important factor in these consider- ations is the thermal conductivity of the sub- strate: The ampacity of a conductor is ultimately limited by the thermal destruction of the con- ductor. The better the heat dissipation of the conductor itself, the more current it can carry. Therefore, factors such as the specific thermal conductivity and thickness of the insulator play a crucial role. Increasing Conductor Cross-sections Increasing the conductor cross-section is an effective method to reduce the ohmic resis- tance. In many cases, there is no alternative to this. However, it must be considered that in- creasing the cross-section leads to additional weight, which is undesired in electric mobility applications as any increase in weight decreases the driving range of the electric vehicle. The conductor cross-section design must in turn satisfy the specifications regarding heat gen- eration. The heat generation depends on the thermal conductivity of the substrate and its connection to a suitable heat sink. The higher the temperature stability of the insulator, the higher the permissible heat generation in the conductor for system design. For these reasons, determining the required conductor cross-section has evolved into a com- plex task. The conventional methods and rules for layout design as described in IPC or FED can no longer be applied in many cases as they do not take these new boundary conditions into account. Temperature Stability of Substrates Another central aspect is the temperature stability of PCB substrates in power electronics. Power semiconductors can usually withstand junction temperatures of 175°C. This tempera- ture range is increased further by new semicon- ductor technologies and is expected to reach 200°C or even 225°C within the next few years. The full exploitation of this temperature range requires substrates that can be used in the mentioned temperature range. Increasing the operating temperature also serves the purpose of minimizing efforts for the cooling system and hence minimizing system costs of the power electronics. Ceramic substrates such as DCB/DBC sub- strates currently have a deep impact on power electronics as they combine excellent electri- cal insulation with high thermal stability. The downsides are the high costs of ceramic sub- strates and the limitation with regard to fine structures and number of substrate layers. ConDUCTInG Very HIGH CUrrenTS THroUGH PCB SUBSTrATeS AT HIGH AMBIenT TeMPerATUreS ArTiCle " electronic substrates for power electronics applications must support the requirement for a high efficiency of the entire system. "

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