Alkalinity Doesn’t Relate to Chemical Reactivity Anymore!
Over 20 years, cleaning chemistries have evolved from blasts of solvents to intermolecular interactions.
From solvent replacements in the 1990s to modern chemistries, the term “reactivity” has been used synonymously with the level of cleaning performance. In other words, higher pH meant better flux removal, and PH values between 8 and 10 were considered “moderately reactive.” While with traditional surfactants this simplification held true, the emergence of modern cleaning agents began to include other chemical pathways to flux removal. Today’s performance of aqueous cleaning agents hinges on previously unknown reaction pathways that in turn permit the complete removal of alkalinity in the product.
Now, neutral – neither acidic nor alkaline – product technologies offer cleaning chemistries new capabilities. With the implementation of the Montreal Protocol, PCB cleaning changed from a bulk chemistry supply to a specialized, engineered solution proposition. Looking back at the products used, we can reminisce and marvel at the progress made.
Alkalinity has been synonymous with reactivity since the early 1990s. The world has changed! Starting with the first direct solvent-based replacements in the early 1990s, the industry adapted and built a process around these “new” products. Companies developing these technologies were able to collaborate directly with the users. It did not take long, however, for the shortcoming of solvents and their impact on cost and efficiency of the overall process to limit the cleaning step. Assemblers also started to ask for water-based solutions. As a result, the first generation of aqueous products entered the market. They showed high alkalinity and low levels of organic constituents. Although these products relieved some limitations of solvents, they created other process issues. For example, the cleaning performance could not be compared to solvent-based products. They also offered a number of material compatibility issues as the alkalinity affected the quality of the products being cleaned. Due to their limited ability to solubilize contamination, the bath life of these products was very short, and the process became maintenance heavy.
In the late 1990s, the cleaning industry began to introduce “modern biphasic” product technologies. They were revolutionary as solvent- and surfactant-based advantages were combined, without many of the respective disadvantages. Bath life was now the term that changed the face of process costs. These products were able to precipitate contamination from the medium, and a bath life of 6 to 8 months was possible. It meant significant process savings: The cost per cleaned part was reduced to a fraction of what it had been for surfactant- and solvent-based alternatives.
These new emulsion-type products also were able to hold their own with solvents and far exceeded the performance of traditional surfactant-based products. Interestingly, they also showed these elevated cleaning performance levels at concentrations of 10 to 15%. Previously, users had been informed that a 20 to 30% concentration was necessary to ensure full cleaning ability. Lower concentrations only slightly improved the compatibility with PCB assembly materials, such as anodized or blank metal surfaces (as pH is on a logarithmic scale). The new technologies were fully biodegradable and met health and environmental regulations and laws.
New technologies are emerging that further address the remaining material compatibility concerns. The wetted parts, meaning the materials that “see” the cleaning agent during operation, are numerous and interestingly, they do vary in quality from manufacturer to manufacturer.
Recently emerged neutral product technologies are fully compatible with anodized aluminum parts, sensitive metals and other previously challenging surfaces. The main difference now is that there is no alkalinity in the cleaning agent, yet the chemistries can still complete the same job as a “defluxing” agent. This unique physical property also comes into play, as users want to minimize equipment wear-and-tear. The wastewater solution is more cost-effective as well.
Perhaps most important, recent cleaning studies showed that pH-neutral products can remove Pb-free, no-clean residues even more effectively than their alkaline counterparts. With these benefits in hand, a new reactivity standard has emerged. Obviously, the classic acid-base reaction is not included in the chemical removal pathway of the residues anymore. With these new products, the physical chemical pathway rests rather on intermolecular interactions. The physical chemist Peter J. W. Debye was the first scientist to study molecular dipoles extensively. Terms such as induced dipole are becoming relevant, as they cause a brief electrostatic attraction between the two molecules. The electron immediately moves to another point and the electrostatic attraction is broken. Additionally dipole-dipole interactions seem to play a pivotal role in neutral defluxing, as permanent dipoles in molecules are created. Often, molecules even can have dipoles within them, but have no overall dipole moment. This occurs if there is a symmetry within the molecule, causing each of the dipoles to cancel each other out. Tetra-chloromethane is one such molecule. In summary, the new chemical removal pathways hinge on electrostatic interactions between cleaning agent and contamination, a phenomenon that will lay the groundwork for future innovations.
In short, in 20 years, we have moved from ozone-depleting products to solvent and alkaline technologies to more modern products that now seem to offer the overall largest process window, even without alkaline constituents. Has any other electronics manufacturing process seen such rapid and positive advancement?
Harald Wack, Ph.D., is president of Zestron (zestron.com); h.wack@zestron.com. His column appears regularly.
