An-all-too-common scenario is reported. A components manufacturer adopts a new, safe, water-based process. No hazardous ingredients are indicated on the MSDS, and the product comes with all manner of environmentally-related certifications. The company safety department has approved the chemistry, the local regulatory agency indicates that it is environmentally acceptable, and the requisite daunting array of qualification and acceptance testing has been performed. Why are some employees complaining of skin irritation?

Use of aqueous systems has gradually increased. Aqueous cleaning processes are an integral part of a number of critical applications such as biomedical devices, wafer fabrication, microelectronics, optics, and navigation systems. Cleaning chemistries and cleaning equipment have increased in sophistication; they are more readily adapted to precision applications.1 In addition, lubricants, photoresists, and other materials that ultimately have to be removed from the product, are increasingly being reformulated for removal with water-based processes. Costs and environmental constraints also contribute to adoption of aqueous cleaning. While most facilities recognize the potential worker safety hazards associated with solvents, aqueous cleaners may present a different set of challenges compared to solvent cleaners, particularly in critical cleaning applications. Just as the aqueous process must be optimized to the expected soil mix and soil loading, materials of construction, component configuration, and component mix, aqueous cleaning processes, like all cleaning processes, have to be managed to minimize employee exposure and to avoid introducing unintended environmental risks.

Aqueous cleaners present a somewhat different set of challenges relative to solvent cleaners. These safety challenges are perhaps best understood in the context of the manner in which aqueous cleaning processes work to remove soils. With solvent cleaning, the emphasis is on solubility of the soil of interest in the solvent; solubility parameters are commonly considered first. Other physical parameters such as boiling point or operating temperature, evaporation rate cleaning force, and time of exposure are also important. However, there are times when immersion alone in ambient temperature solvent results in sufficient efficacy of cleaning.

With aqueous cleaning, immersion in the cleaning solution at ambient temperature is typically not an efficient way to remove soil. Instead, the interaction of the cleaning chemistry with other process parameters is more important in determining process effectiveness. The factors include:

• Chemical composition, concentration
• Temperature
• Physical force or agitation
• Time of exposure