Mechanism and Control of Post-Trim Drift of Laser-Trimmed Thick-Film Resistors

Some of the important parameters controlling the postlaser trim drift of 100, l-K, 10-K, and 100-K thick-film resistors are reported. The fired thickness of the resistor plays a major role in the control of post-trim drift. The resistors with fired thickness (tf) 0.5 mil show small ( 0.3 percent) and predictable drift, whereas those with tf> 0.5 mil produce large ( 0.3 percent) and unpredictable drift. A mechanism for the post-trim drift is proposed. Results indicate that the percent trimming for precision resistors should be held within 25 to 30 percent range for minimum drift. The trim geometry such as L-cuts or two P-cuts from the same side of the resistor produces the least amount of drift. The protective overglaze coating on the resistors has a beneficial effect of reducing the drift, especially for the resistors with tf> 0.5 mil. The resistor size (L X W) does not significantly influence the drift as long as the tf< 0.5 mil.