Safe Pulse Loading of Thick-Film Chip Resistors

摘要

Resistors employed in such applications as power supplies, motor drivers, lasers, etc. may be required to dissipate significant levels of pulsed power. Existing methods for evaluating the effects of pulse loading on resistors were developed a long time ago for the old-fashioned cylindrical resistor configuration. Today, the dominant style of the contemporary resistors is the thick-film chip resistor. It consists of a flat ceramic substrate of 0.2 - 0.5 mm thickness, covered by a metal glaze resistive film approximately 0.01 mm thick. The objective of this paper is to theoretically evaluate the thermomechanical effects of pulse application to thick-film chip resistor in order to establish the criteria for reliable operation under pulsed conditions. The authors have observed experimentally two catastrophic failure modes in thick-film chip resistors subjected to electrical pulse loading: burnout of the resistive film, and cracking of the resistor substrate. For simulating the conditions leading to such failures, two idealized mathematical models have been developed: (a) a one-dimensional heat conduction model for estimating the temperature distribution through the thickness of the resistor; and (b) a one-dimensional thermoelastic model to evaluate the strain and stress states in the resistor. The input data for the calculations are the physical properties (thermal conductivity, heat capacity, density) of the resistor substrate and the parameters (magnitude, duration and wave shape) of the electrical pulse. Good correlation was observed between the calculated and experimental data.