Strain Sensivity of Thick-Film Resistors

The strain sensitivity measurements of Dupont 1400Biroxr thick-film resistor materials in untrimmed and laser trimmed conditions are described. This was studied to better understand the post-laser trim drift phenomenon and evaluate the effect of packaging/assembly induced stresses on the resistors. The strain coefficient of resistance or the strain sensitivity of the untrimmed 102_ 10
⁶
\Omega
/
\Box$^b$
resistor materials is small, reversible, and independent of resistor geometry or fired thickness (t
f
). This strain sensitivity (?i) is an intrinsic material property which is determined by the conduction mechanism in the material. The strain sensitivity of the laser trimmed · resistors is a sum of the intrinsic sensitivity (?i) and an additional term called the extrinsic strain sensitivity (\gamma
ex
). The additional extrinsic contribution can be large and irreversible and depends strongly on the fired thickness (t
f
of the resistor. A mechanism is proposed to explain these observations. The mismatch of the thermal expansion coefficients of the resistor material and the substrate leads to high internal stresses in the thick resistor. Hence crack formation and propagation is easier which leads to increased strain sensitivity (extrinsic) and post-trim drift. It was experimentally determined that Al
2
O
3
from the substrate dissolved 6-8 µm into the thick-film resistor during resistor firing. For thin resistors the relatively higher amount of dissolved Al
2
O
3
from the substrate in the resistor glass reduces the thermal expansion mismatch between the fired resistor and substrate. Thus its strain sensitivity and drift are much smaller than a thicker fired resistor.