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Go Greener in Your Cleaner
December 31, 1969 |Estimated reading time: 5 minutes
pH-neutral cleaning can help protect sensitive components while removing aggressive alloy residues. Over the past 20 years, the common notion in this field has been that alkalinity is a must to remove flux residues. pH-neutral defluxing agents are designed to do a superior cleaning job, but still protect extremely sensitive materials and meet stricter rinse water disposal regulations. Harald Wack, Ph.D., ZESTRON, explains the role of pH and electronics components sensitivity.
The concept of pH was first introduced by the Danish chemist Søren Peder Lauritz Sørensen at the Carlsberg Laboratory in 1909. The exact definition of “p” is unknown. Some references suggest the p stands for “power;” others refer to the German word “Potenz” (meaning power in German); still others refer to “potential.” In 2000, Jens Norby published a paper arguing that p is a constant and stands for “negative logarithm,” which has also been used in other works, and that H stands for hydrogen. Sørensen suggested the notation "pH" (power of hydrogen) as a matter of convenience as it abbreviates the logarithm for the concentration of hydrogen ions in solution. Although this definition has been superseded, pH can be measured with a calibrated electrode in a solution of known hydrogen ion concentration. The pH of pure water at 25°C (77°F) is approximately 7.0. Solutions with a pH of less than 7.0 are known as acidic and solutions with a pH greater than 7.0 are basic or alkaline. pH measurements are important in medicine, biology, chemistry, nutrition science, environmental science, oceanography, and many other applications. The other applications include but are not limited to the electronics manufacturing industry. Over the past 20 years, the common notion in this field has been that alkalinity is a must to remove flux residues. pH-neutral defluxing agents are designed to do a superior cleaning job, but still protect extremely sensitive materials and meet stricter rinse water disposal regulations. Previously, commonly used surfactant-based (alkaline) cleaning agents challenged manufacturers of cleaning equipment, stencils, and labels to produce more expensive products that were highly resistant to alkaline cleaners. This was often accomplished by adding corrosion inhibitors, which were supposed to slow the corrosion/oxidation process. This approach is not designed to eliminate the cause of the chemical interaction; it merely covers it up temporarily. Unfortunately, corrosion inhibitors like most chemicals are subject to performance fluctuations. They might produce chemical reactions or lose their effectiveness. Furthermore, solubility aspects (i.e., continuous solubilization) also have to be taken into account. Corrosion inhibitors can work well at first, but subsequently may lead to unwanted performance variations. Recent cleaning studies have clearly shown that pH-neutral products are even more effectively removing the latest lead-free, no-clean residues than their alkaline counterparts. With these benefits in hand, a new reactivity standard has emerged. The classic acid/base reaction is no longer included in the chemical removal pathway of residues. With these new products, the physical and chemical pathway relies on intermolecular interactions. Cleaning under low stand-off components is a current industry trend that coincides with eliminating alkaline constituents in defluxing agents. Cleaning agent and equipment vendors have further developed process windows for companies intending to remove all residues under components with less than 1 mil spacing. At present, lead-free chips seem to be the most challenging. While procedures have been developed to fully remove all residues, companies are now pondering the actual chemicals under low stand-off components. For example, while a user might be able to remove all flux residues from under components, the cleaning agent used to perform this task has an opportunity to remain under certain areas (depending on its characteristics). Due to the chemical environment they create, alkaline products provide a strong potential to induce corrosion. Such redox reactions are a combination of reduction and oxidation. They take place in aqueous environments and are assisted by either alkaline or acidic pH levels. Depending on the pH level, the redox potential, i.e., the likelihood of a redox reaction to take place, varies. For Sn/Sn2+, for example, the redox potential is -0.137εo/V in an acidic solution, whereas its redox potential in an alkaline solution is -0.909εo/V. The pH value of the aqueous solution affects the electrochemical behavior of metals. The higher the redox potential of a metal, the more likely it is to oxidize/corrode. Tin/lead (Sn/Pb) can be used as an example. Tin (Sn/Sn2+) has a redox potential of -0.909εo/V, while lead (Pb/Pb2+) has a redox potential of -0.540εo/V in an alkaline solution. Tin will oxidize, while lead will be chemically reduced, i.e., receive the electrons. On the other hand, a pH-neutral environment would reduce the potential for corrosion/oxidation, as the redox potential difference for Sn and Pb in an acidic solution is almost zero. Residues of chemistry under components can therefore have detrimental effects on the reliability of assemblies. Cleaning agent residues with pH-neutral levels are consequently not as harmful as alkaline products. Completely pH-neutral products (i.e., water) will exclude certain electrochemical reaction pathways. For PCB manufacturers, better cleaning is supplemented by cost savings, which are mostly related to reduced failure analysis, fewer warranty claims, and lower assembly costs. The latter include but are not limited to fewer disposal and material compatibility costs (assemblies and cleaning equipment) and potentially even rework costs. Conclusion
This innovation demonstrates that neutral (neither acidic nor alkaline) product technologies convey new capabilities to the chemical cleaning industry as well as to PCB and cleaning equipment manufacturers. Within the precision cleaning industry, the ability to expand material compatibility, supplemented by increased cost effectiveness and lower part failures, guides the way to greener cleaning processes worldwide. Harald Wack, Ph.D., is the president of ZESTRON and an SMT Editorial Advisory Board member. Dr. Wack has authored and published several scientific articles and has provided technical information for various publications. He received his doctoral degree in organic chemistry from Johns Hopkins University. He may be contacted at (703) 393-9880 or h.wack@zestronusa.com. Dr. Wack recently wrote The Future of Cleaning OA Fluxes, What is Innovation in Chemistry?, Reduce the Cost of Cleaning Processes, and IPA-Water (75/25): Ineffective for Cleanliness Test