Component Linearity Test Improves Reliability Screening Through Measurement of Third Harmonic Index

Environmental testing designed to cull out components which do not conform to typical performance parameters is a time consuming and often destructive method of reliability screening. It does not indicate the component´s normal operational performacne and it cannot be used in incoming inspection or production monitoring. A nominally linear component, however, can be easily checked for its degree of non-linearity by selective measurement of the third harmonic voltage generated in the component when a pure sinusoidal current is applied. This measurement is capable of detecting abnormal conditions in resistors due to faulty welds, film aberrations, resis, tive path inconsistencies, contact instability, and other imperfections that may cause shifts in resistance value. It therefore can identify resistors that have a potential for excess parametric change even though they may never fail in actual operation. The third harmonic voltage is measured by exciting the component under test with a very pure sinusoidal current of 10 Khz frequency and measuring the 30 Khz voltage generated within the component. This requires a specially designed air coil transformer for impedance matching, and associated circuitry which guarantees 150 db suppression of the fundamental signal for minimum residual distortion and optimum instrument sensitivity. Various defects intentionally produced in Bulk Metal Film resistors have been subsequently screened by both the component linearity test and by standard repetitive short time overload tests. Units screened by both methods were subjected to environmental stresses.