The overwhelming technology used in post-reflow cleaning of electronic assemblies is aqueous. Prior to the 1990's, solvents and vapor degreasers dominated the cleaning industry. After the popular solvents were banned via a United Nations treaty, alternative solvents attempted to take their place. Unfortunately, no alternative solvent worked as well as the banned solvents. The industry en-masse switched to aqueous-based cleaning technologies. Today, aqueous-based cleaning technologies has become the conventional wisdom. While there are specific circumstances where aqueous-based technology can not be used (primarily due to non-sealed components), they are relatively rare and there are "work-arounds".
Concern about assembly moisture is valid. Moisture can cause many reliability issues. Switching to a solvent-based cleaning method does not, on its own, prevent moisture. Moisture pick-up can occur as soon as the laminants are bonded together. Moisture pick-up can begin during board fabrication and continue through storage, assembly, and in-use. Regardless of the cleaning method, assemblies would benefit from an extended drying or bake-out process after assembly and cleaning. Most solvent-based cleaning systems lack a vigorous drying process (and, in many cases lack any drying process). This is because they rely on the rapid evaporation characteristics of solvents.
Aqueous-based cleaning technologies can feature a powerful drying system what will not only remove cleaning related moisture but also moisture gathered over time prior to cleaning. An aqueous-based cleaning process, backed up by real-time cleanliness verification, ending in a vigorous drying process represents best practice in cleaning.
Feel free to download a free book "The Reasons for Cleaning". It covers in more details many of this topics in this response. The book may be dowloaded here:
http://www.aqueoustech.com/book-1-download
I hope this information is helpful.
Mike Konrad
Aqueous Technologies
konrad@aqueoustech.com
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