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
\Omega//\Box$^b$
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)
\leq
0.5 mil show small (
\DeltaR/R \leq
0.3 percent) and predictable drift, whereas those with t
f
> 0.5 mil produce large (
\DeltaR/R \geq
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 t
f
> 0.5 mil. The resistor size (L X W) does not significantly influence the drift as long as the t
f
< 0.5 mil.