Statistical Interpretation of Meniscograph Solderability Tests

As part of a thick-film characterization study, a solderability test-program was initiated utilizing the meniscograph. The meniscograph test measures the changes in wetting force as a function of time, and thus affords a description of the kinematics of the wetting phenomenon. Difficulty was encountered in the derivation of unique parameters which provide consistent statistical comparisons. As an alternate to selecting a time interval from the meniscograph trace as a measure of wetting rate or only qualitatively comparing traces, as some investigators have done, we have evaluated the approach of measuring the slope of the trace replotted so as to yield a straight line fit. A plot of the logarithm of a normalized wetting force versus time has generally afforded the best fit. To evaluate the analytical techniques, a large number of meniscograph tests were conducted employing copper, nickel, and Kovar substrates using both tin-lead and lead-indium solders; and rosin flux with varying activity. With the wetting rate estimated on the basis of the logarithmic line fit, the tests have consistently ranked solder systems with regard to substrate solderability and the activity of fluxes. Calculated equilibrium contact angles were found useful in interpreting the meniscograph traces.