Mechanism for downward trimming of polymer resistors

The effect of infrared radiation on the resistivity of polymer-based thick film resistors was studied. it was found that the resistivity decreases with both the density and duration of the infrared and with the number of exposures. The change in resistivity was attributed to the viscous flow of the polymer at the temperature attained by the film when exposed to infrared radiation. A model has been proposed through which the percentage change in resistance can be estimated in terms of the number of exposures, duration of exposures etc. Based on these studies it was possible to suggest a downward trimming method for polymer film resistors in which the polymer film is exposed to intermittant infrared radiation pulses of the duration of the order of a few seconds. This method of trimming appears to be a clean process, with the resistors being free from hot spots.