Evaluation of thick film materials for high temperature electronics packaging

Abstract

Geothermal well logging requires electronics and packaging working reliably at 300C. Thick film technology has many advantages and has been widely investigated for high temperature applications. Previous studies showed that a commonly used thick film conductor and dielectric with capability of operating at 300C had a leakage current increase and adhesion loss after aging with bias voltage due to the glass modifier, Na, migrating in the dielectric and accumulating on the negative electrode. This dissertation presents the electrical and mechanical reliability testing of alternative thick film materials. Test vehicles were fabricated with thick film materials as capacitor and interdigitated finger structures. For the vehicles with PtPdAu thick film paste, the leakage current with thin multilayer dielectrics increased significantly in a short time after 300C aging with 100 V bias with the positive bias on the top electrode and the negative electrode on the bottom. The leakage current of test vehicles with thick dielectrics remained relatively constant during 300C aging in both capacitor and all interdigitated finger structures. Cross section and EDS analysis showed that Bi, in the PtPdAu paste as the binder, diffused into the dielectrics from both the top and bottom conductors during the firing process in fabrication. The Bi from both sides met in the center of the dielectric and resulting in the increase of leakage current during aging with bias. By using UV-vis spectroscopy analysis, electron traps were found to be 3.2 - 3.5 eV in dielectric with diffused Bi.

The adhesion of the thick film capacitor decreased from 100 hours for test vehicles with thin multilayer dielectrics or from 500 hours for vehicles with thick multilayer dielectrics. The multilayer dielectrics lifted off from the bottom conductor if the conductor was PtPdAu and the 100 V positive bias was applied on the top electrode. With Au conductor or if the 100 V positive bias was applied on the bottom electrode, the adhesion remained relatively constant before and after aging.