Sol Gel Processing of Low Permittivity, Fiber-Reinforced SiO₂-Polyimide Substrates

Low permittivity ceramic substrates with a sandwich structure consisting of a porous, fiber-reinforced SiO₂ core and two thin polymer plates were fabricated by sol-gel processing, and polymer infiltration. The rheological behavior of an aqueous colloidal SiO₂ sol, containing short SiO₂ fibers was studied as a function of particle loading, fiber loading and gelation time. Short SiO₂ fibers were introduced to limit drying shrinkage and thus minimize substrate cracking during drying. After the psuedoplastic sol was tape cast and sintered at 1150°C for 2 h, a polyimide solution was infiltrated into the porous SiO₂ core. Permittivities ranging from 1.81 to 3.08 could be obtained by using 32-42% SiO₂, 5-50% polyimide and 63 to 8% porosity. The substrate strength was increased from 1.93 MPa to 3.83 MPa after polyimide infiltration.