Authored By:
Umut Tosun, M.S.Chem.Eng., ZESTRON America
Axel Vargas, ZESTRON America
Manassas, VA USA
Bryan Kim, Ph.D, Pressure Products
Charleston, WV USA
Summary
Historically, the determination of the concentration of cleaning agent in high precision electronic cleaning baths has depended on any one of several possible measurable parameters. Refractive Index (RI) is by far the most common. RI methods are excellent tools for use in simple systems where a single solute dominates the signal. In these situations, it is possible to characterize and calibrate how that solute affects the signal. However, the introduction of flux residues during the wash bath lifetime complicates the bath chemistry/physics to such an extent that RI signals no longer provide the same insight.
The introduction of flux residue has an enormous influence on the Refractive Index. Alternative means of measuring cleaning agent are necessary if cleaning agent concentration is to be known throughout the life of the bath. With a means to accurately measure bath cleaning agent, closed loop automated process control on the cleaning bath is possible; automating this labor intensive step in the production of electronic boards. We have found that acoustic measurements of cleaning bath solution are relatively independent of pH, conductivity, and dissolved solids in some of the most flux loaded baths. Utilizing acoustic sensing technology, field data was gathered from two beta site locations assessing the accuracy of the technology in fresh as well as contaminated wash baths.
Conclusions
In this study, the Flowsensor or acoustic concentration measurement system that was first introduced at SMTAI 2013, was further evaluated at the original beta site detailing 20 days of concentration monitoring data. At this site, the electronic assembly process utilized RMA flux and a cleaning process based on a micro phase cleaning agent.
Additionally, the Flowsensor CLC or closed loop concentration monitoring and control system was installed at a second beta site and evaluated over a 45 day period. At this site, the electronic assembly process utilized both RMA based and water soluble flux and a cleaning process based on a dynamic surfactant cleaning agent.
Upon review of the beta test data, the Flowsensor employed at Beta Site A was found to accurately indicate wash bath concentration while the Flowsensor CLC, employed at Beta Site B, was found to accurately indicate and control wash bath concentration, each throughout the test period. In each case, the Flowsensor concentration data was compared with established measurement methods in order to confirm process accuracy.
With any electronic substrate cleaning system, wash bath contamination accumulates over time, principally in the form of flux residues. The ability to determine accurate concentration measurement in the presence of contamination is critical to any monitoring and control system. Using bath samples collected from Beta Site B and a contamination analysis based on NVR, this study verified that acoustic measurement technology, employed in the form of the Flowsensor CLC within this study, can accurately indicate and control wash bath concentration in the presence of flux residue.
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