Silver Migration and the Reliability of Pd/Ag Conductors in Thick-Film Dielectric Crossover Structures

The relative performance characteristics of thick-film Pd/Ag conductors (0-34% Pd) for dielectric crossover structures in hybrid microelectronic circuits are described. The materials were supplied from Du Pont and Engelhard. The experiments involved waterdrop (WD) tests and temperature-humidity-bias (THB) tests at 90°C, 90% RH, and a dc bias of 5-50 V applied between the conductor electrodes for periods up to 1000 h. WD tests showed that the rate of Ag migration decreased by approximately 100 times as the Pd content in the conductor was increased from 10%-19%. However, the results of these tests were considerably scattered for electrodes of high Pd content. In addition, no conclusive results could be reached on the effect of the substrate or various encapsulants on the rate of migration. The rate of migration under THB conditions was considerably lower (10^-4times) than that under the water-drop tests. Surface migration was found to be the dominant mode of failure in dielectric crossover structures. The rate of migration increased with the increasing voltage gradient. When the voltage was increased it caused an abrupt change in the capacitance and isolation resistance. The migration was more pronounced on the surface of bare alumina substrates than on substrates covered with a dielectric layer. Finally, a Du Pont 9137 glass encapsulant was found to be effective in preventing silver migration under THB conditions while an ESL 240 SB encapsulant enhanced the rate of migration.