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Alkalinity, Defluxing, and Materials Compatibility
December 31, 1969 |Estimated reading time: 4 minutes
By Harald Wack, Ph.D., ZESTRON
From solvent replacements in the 1990s to modern chemistries, materials compatibility is the key for cleaned parts to perform better not worse than uncleaned assemblies. Defluxing under low standoff components, non-corrosive cleaning, and other challenges are met by materials compatibility. Now, neutral neither acidic nor alkaline product technologies are bringing cleaning chemistries new capabilities.
Starting with the implementation of the Montreal Protocol, the industry of cleaning printed circuit boards (PCBs) changed from a bulk chemistry supply to a specialized, engineered solution proposition. Looking back at the products used, we are now able to reminisce and marvel at the progress that has been made since then.
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 level of organic constituents. As they relieved some of the 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" 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 68 months was now a reality. It meant significant process savings and users could see the benefits once they started to compare the total process cost for each product technology. The cost per cleaned part was reduced to a fraction of what it had been for surfactant- and solvent-based alternatives.
Compared to the cleaning ability of solvents, these new emulsion-type products were able to hold their own and far exceeded the performance of traditional surfactant-based products. Also interestingly, they showed these elevated cleaning performance levels at concentrations of 1015%. Previously, users had been informed that 2030% concentration was necessary to ensure full cleaning ability. With lower concentrations also came significant improvements for compatibility with PCB assembly materials, such as anodized or blank metal surfaces. These new technologies used much lower pH levels and greatly improved corrosion inhibition, which resulted in fewer complications. The industry was also grateful as these new technologies were fully biodegradable and met all health and environment regulations and laws.
Over the last five years, newer technologies are emerging that further address the remaining material compatibility concerns. The wetted parts, meaning the materials that are "seeing" the cleaning agent during operation, are numerous and interestingly they do vary in quality from manufacturer to manufacturer. For example, Teflon, a product typically considered to be fully compatible with most cleaning agents on the market, can show incompatibility, especially in cases where it is not manufactured to the same quality level as offered by the big chemical companies.
Neutral product technologies are now available that 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 job as a defluxing agent. This unique physical property also comes into play as users want equipment wear-and-tear to remain minimal. This had not been the case in the past, and equipment replacements are costly and can add significantly to the overall process cost. Worker's safety also is improved if operators can avoid working with caustic materials, as is possible via neutral products.
Flux residues and impurities under low standoff components are often hard to remove, since not every water-based cleaning agent is designed to enter this minimal gap between package and board to remove impurities.
Cleaning agent entrapment therefore becomes an issue, i.e. the cleaning agent is not rinsed away properly after cleaning, and impurities together with the remaining cleaning agent are trapped under low standoff components or around solder joints, making the PCB board more vulnerable to creeping, longtime corrosion, which can cause failure, including field failures. Studies are underway to take into account the dissolved flux residues and their respective effect on contributing to corrosion. Generally, the current understanding is that pH-neutral defluxing technologies can limit these risks of corrosion underneath components, while providing higher levels of material compatibility.
ConclusionIt has been fascinating to witness the progression of the cleaning industry over the years. We have moved from ozone-depleting products to solvent and alkaline technologies before discovering more modern products that now seem to offer the overall largest process window, even without any alkaline constituents. The future of cleaning agent innovation therefore seems bright and will hopefully provide applicable solutions to current and future needs as they emerge.
Harald Wack, Ph.D., an SMT Editorial Advisory Board member, is president of ZESTRON America. 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 via e-mail at h.wack@zestron.com. Dr. Wack recently wrote The Importance of Global Technical Support and Material Compatibility Worldwide.