Interactions between thick-film resistors and terminations: the role of bismuth

The interactions of thick-film (cermet) resstors with terminations of various metals prepared on alumina substrates has been investigated. The resistivity was studied as a function of resistor length and temperature. The point-to-point voltage drop was measured over the entire length of the resistors. SEM and EDAX were used to investigate the termination material, the termination-resistor interface and the resistor, both lengthwise over the specimen and vertically downward into the substrate. The results show that bismuth, a common component of these termination materials, is primarily responsible of these interactions because bismuth has high diffusivity and high solubility in the matrix of the resistor. Moreover, a strong interaction of bismuth with the alumina substrate was observed.