Thick film pastes for silicon nitride ceramics

摘要

Silicon nitride ceramic is a potential new substrate material for thick film-/hybrid applications in the field of power electronics or microwave technology. The main advantages are the excellent thermal shock resistance, high fracture toughness, a thermal expansion coefficient close to silicon, high thermal conductivity and good dielectric properties. Nevertheless, the use of Si_3N_4 as chip carries or power electronic package is no achieved due to the lack of suitable connection technology such as thick film pastes. The main challenge for the respective pastes is the adhesion on the substrate. The low thermal expansion of about 2.8 ppm/K of Si_3N_4 and material interactions with the thick film components need to be considered in the paste development. Therefore, new glasses and glass-ceramic composites are required. Reason developments were performed towards thick film pastes for Si_3N_4, consisting of AgPd conductor, resistor and encapsulating paste. Good solder wetting (>95%) and high conductivity (<25 mΩ/sq) of the AgPd conductor film is achieved in presence of 2 percent glass with matching composition and 2 percent of an inorganic additive. A high adhesion of up to 23 N/4mm~2 (wire pull test) of the fired film was obtained. For resistors, RuO_2 was used as conductive material. The influence of the conductive material content on the sheet resistivity and of the TCR modifier was studied. The developed resistor paste has a sheet resistivity of 10 Ω/sq with a temperature coefficient of the resistance (TCR) between -100 and 100 ppm/K. The film structures were analyzed in FESEM studies of cross sections.