Thick Film Conductors and Resistors on Dielectrics for High Reliability Applications

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Abstract
Platinum gold conductors, used as solderable terminations, contain a glass frit which reacts with the minority constituents in debased alumina substrates forming an adhesive bond. Some conductor inks also contain copper and cadmium, in addition to the glass frit, which react directly with the alumina to form a chemical bond. Dielectric inks contain a crystalline filler, such as alumina or zirconia, in a glass matrix. The effect on the physical and electrical properties of platinum golds on various dielectrics was examined in comparison with the behaviour on alumina. Composition and surface structure of the dielectric affects the adhesion strength, solderability and solder leach resistance of the conductor inks. Interaction between the glasses in the dielectrics and conductors was determined by analysis in the SEM. Interdiffusion between the conductor and solder metals occurs and brittle intermetallic compounds are formed. The effects of the intermetallic formation on the adhesion strength and modes of failure, especially after thermal ageing at 150°C, have been examined. Thick film resistors printed and fired onto dielectrics rather than onto alumina substrates generally have different electrical properties. Chosen resistor systems, terminated with a gold conductor, were evaluated on different dielectrics. The values of electrical parameters such as resistance, TCR and noise index were compared with those on alumina. Interactions between the glasses in the resistors and dielectrics were examined as for the conductors. Thermal ageing on various resistor/dielectric combinations was carried out in order to determine the long‐term stability. The activation energies and time dependences of the ageing mechanisms for each combination were found. Corresponding ageing equations were calculated in order to predict the likely behaviour during life.

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