Load Life of Thick-Film Resistors

When a potential is applied to a thick-film resistor, a number of effects can act to produce permanent changes: 1.Joule heating raises the temperature of the resistor and encourages diffusion and chemical reaction of constituents. This may be more pronounced in regions where the current density is increased by trimming or other constrictions.2.Direct current flow may cause drift of some of the component materials, e.g. electromigration at the contacts.3.Especially in high resisitivity materials, high voltage gradients may exist under dc conditions, causing temporary or permanent electrical breakdown effects in the less conducting parts of the matrix.4.In circuits containing a number of resistors or active components, the thermal conductivity of the substrate leads to the operating temperature of each resistor being dependent on the heat dissipated in its neighbours. This is in addition to any background temperature provided by the ambient conditions.