There's a lot of issues to deal with here, and to simply say water is good or bad is to over-simplify a very complex issue. In fact, in many industrial cleaning applications, water may be the best choice. Let's look at the details:
In general, aqueous cleaning systems are used most often for (a) cleaning parts that are not very complicated in form (e.g., no blind holes or complicated geometry), or (b) the cleaning requirement is not very stringent; i.e., minor surface residue is acceptable, or (c) the parts are very large or the production volume is extremely high.
Simple metal stampings are a great example of a product which can be successfully cleaned in a aqueous system. A typical stamping is reasonably flat, thin and does not have any tight cavities or spaces into which the aqueous solvents could be trapped. A bare circuit board or a circuit board populated with only "through-hole" components would be a similarly suitable application.
So where does aqueous cleaning have problems?
Aqueous cleaning usually is not successful for one or more of the following reasons:
- Size & Spacing: Aqueous solvents typically have difficulty getting into and cleaning in and around parts which are extremely small or have extremely small spaces. This can be somewhat ameliorated by additives in the water or high pressures,but these work-arounds cause problems of their own (residues, damage, higher costs).
- Entrapment: Aqueous cleaners resist coming out of tiny spaces. Imagine the amount of water which could be trapped in a filter, or deep inside a tiny female electrical connector with 100 or more openings. Symptoms of the problem include residues and corrosion. Typical work-arounds include extra processing, such as baking the products in ovens to drive the water out of the traps.
- Spots: Water often leaves unacceptable water spots. Common work-arounds include more additives to reduce the surface tension or enhance drying, more aggressive heating and powerful air knives. Less commonly, a final rinse in a solvent cleaner is often used.
- Compatibility: Water cleaning often is not suitable because some components or products are sensitive to the high pressures of water cleaning, the heat of washing and/or drying, or the minor surface residues mentioned above. There are no common work-arounds which can solve compatibility problems.
- Effectiveness: Many common types of contamination are not soluble in water, so no amount of water, pressure and heat can remove them. Common work-arounds include more additives, higher pressures, and higher temperatures. For more details, review FAQ 62.
- Environmental Problems: Water cleaners require a great deal of water and electricity, and produce a continuous discharge of contaminated water. The most common work-around is an expensive water treatment system to be installed alongside the cleaning system iteself. Aqueous systems require hard-to-get waste water discharge permits which can entail entensive paperwork and government oversight. For more details, see FAQ 61 about water cleaning and the environment.
- Costs: While water is usually cheaper than solvents on a per-pound basis, the costs of buying, installing and operating the machines can be far higher than a solvent-cleaning system. For more details, see FAQ 63 about water cleaning costs.
