Investigation of properties of alternative substrates and appropriate production methods for application in highpower hybrid microelectronics
The physical and electrical properties of highly thermally conductive materials have long been of interest, especially to power electronics manufacturers for the improvement of device efficiencies by better substrate cooling. This thesis provides a resume of the findings on high thermal conductivity materials and reports in detail on the results of an investigation undertaken to evaluate the suitability of Anodized Aluminium as a high thermal conductivity substrate material. Special anodic film fabrication techniques were developed to produce substrates on which thick film components could be directly printed. These studies were implemented in conjunction with a selection of low temperature Thick Film (TF) conductor and resistor pastes (inks) to achieve the optimum combination of Anodized Aluminium substrate and paste system. Test circuits of these chosen pastes were then printed and fired on Anodized Aluminium, Aluminium Nitride, Beryllia, Porcelain on Steel and Alumina substrates. The circuits were used to investigate the physical and electrical properties of the substrates, such as thermal conductivity, thermal expansion, thermal coefficient of resistance, dielectric constant and adhesion of the pastes to the substrates. The resultant anodized aluminium substrates were shown to:- be insulating to over 500 volts, able to withstand firing temperature of 500°C, and have a thermal conductivity very close to that of aluminium (240Wm-1K"1 at room temperature). Unfortunately electrical measurements have shown the anodic films to be humidity dependent, as the resistivity of the films is greatly reduced by an increase in humidity. The substrates are compatible with Heraeus C180-5554 conductor paste and the ESL 15511 resistor paste series. A low power hybrid device with surface mounted components was employed to validate the substrate/paste combinations. To conclude, this thesis has shown that anodized aluminium is suitable as a high thermal conductivity substrate for high power hybrid microelectronics.