An Ionic Contamination Detection System with Improved Performance for Quantizing Residual Ionic Species

Mildly activated rosin flux is commonly used to clean mating metallic surfaces during soldering operations. This practice is generally recognized as necessary and usually promotes a good soldered connection. However, the active constituents in fluxes are often ionic species, which, if not adequately removed, might lead to electrical surface leakage and unacceptably low circuit reliability. Therefore, both an effective flux removal procedure and some technique for quantizing residual ionic surface contamination as a function of the cleaning procedure are needed to insure acceptable surface cleanliness. Some innovations employed in designing and building a sensitive ionic contamination detection system (ICDS) with improved performance are described. The completed system is capable of automatically detecting ionic contamination at a level as low as the calibrated equivalent of 0.35 &microg NaCl with a reproducibility within 2 percent. Manual override can be used to lower the detectable limit to the calibrated equivalent of ~0.10-t&microg NaCl. The basic system includes a recirculating alcohol-water loop, a conductivity cell, an ion exchange column, a sample reservoir with provision for an argon blanket over the fluid, and an analog-to-digital converter to interface with an HP 2114B minicomputer for accurate rapid data reduction. This system has recently been used to detect ionic residues on printed wiring board circuits. It has also been used to determine the effectiveness of several cleaning procedures for flux removal on both thin- and thick-film circuits that have been subjected to external terminal attachment by soldering.