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The reliability of palladium-silver crossover systems under 85°C/85-percent relative humidity bias stress has been shown to depend primarily on the porosity of the dielectric. All of the low-porosity dielectrics tested gave consistently good reliability, which met the British Telecom Research Laboratories (BTRL) specification; while all of the high-porosity dielectrics showed considerable variation in reliability and often failed to meet the specification. This variation was only partly explained by constructional features such as the dielectric thickness or the use of an overglaze. The palladium content of the conductors was shown not to influence the reliability of the crossovers. The reliability of all of the copper crossover systems tested was relatively poor, and the best system tested barely met the BTRL specification, many of the remainder failing by a wide margin.