The wetting of a surface depends on characteristics of both the fluid and the surface. The Wetting Index (WI) was developed by Bill Kenyon in the 1980s (perhaps even earlier) as a means of comparing the ability of various cleaning agents to wet a surface and to access tightly-spaced components.
WI = 1000 x Density / (Viscosity x Surface Tension)
WI is based on the physical characteristics of a solvent or cleaning agent. It was originally designed as a teaching tool to help electronics assemblers choose an effective cleaning fluid. Over the years, based on empirical observations, it has proven to have much broader utility.
The WI is largely intuitive. For a fluid to wet a surface effectively, it must flow easily and get into any tight spaces. The lower the viscosity of a fluid, the easier it flows. If the surface tension is high, it is hard for the fluid to penetrate a small space. Therefore both these parameters appear in the denominator of the formula. Explaining the idea that wettability is directly proportional to the density is a bit more difficult. Because we were having trouble expressing the rationale in one phrase, we contacted a colleague, a very experienced formulator1 who explained that density appears in the numerator because denser fluids exert a higher pressure on the surface, making it easier to drive a liquid into tight spaces. In a sense, density is acting as a molecular version of macro methods of increasing fluid pressure, such as agitation and spray.
Table 12 contains a number of common cleaning fluids with the parameters that contribute to the Wetting Index. Molecules with large polar and/or hydrogen bonding forces,3 such as water, tend to have higher viscosity and surface tension, thus resulting in a low WI. Surfactants are frequently added to water based cleaners to reduce the surface tension and thus increase the wettability. However, water with surfactant is not as effective at wetting as many organic solvents, which have little or no polar bonding. There is also the issue of rinsing, of removing the cleaning agent additives that may themselves become contaminants. Pure water has a lower wetting index and therefore will not be as effective as the aqueous cleaning agent in accessing blind holes.
For a given fluid, one parameter that can be adjusted to improve wettability is temperature. The viscosity of liquids decreases with increased temperature. For example, water is 3.6 times less viscous at 100°C as at 20°C.
Wetting Index eludicates part of the story but not the whole story, because it describes only the properties of the wetting fluid. The WI is a measure of the upper limit of wettability. The nature of the surface being wetted is not part of the WI equation; the molecular forces between the surface and the wetting fluid play a strong role in how effective the wetting of the surface will be. The most familiar examples of surface effects in wetting are illustrated by the Water Drop test and Contact Angle measurement to describe how well water wets a surface. If the surface forces are primarily non-polar, as in the case of an oil coating, the surface does not attract the polar water molecules. Water will bead into drops and exhibit a high contact angle, rather than flow or wet the surface effectively.
While successful residue removal requires a cleaning agent with appropriate wettability, it is not sufficient. Designing the cleaning process requires understanding of multiple factors as well as experience. This means that assigning development of critical cleaning processes to someone who is, in fact, “wet behind the ears,” may not be the wisest move.
Getting the fluid to the destination is crucial, but the real question of importance is, does it clean? This is where such issues as agitation, impact force or pressure, temperature, and solubility enter the picture. For example, while many hydrofluoroethers (HFEs) and hydrofluorocarbons (HFCs) have a very high wetting index, the solvency characteristics are not favorable for removing most industrial soils of interest. To fully characterize cleaning is a complex mixture of the properties of the cleaning agent, the method of application of the cleaning agent, the soils and the substrate surface. Parameters such as Wetting Index can help in narrowing the choices, but there can be no substitute for actually testing the efficacy of cleaning.
- K. Dishart, personal communication.
- B. Kanegsberg and E. Kanegsberg, “Defluxing for New Assembly Requirements,” On-Board Technology, Nov 2011.
- J. Burke, “Chapter 4: Solvents and Solubility,” Handbook for Critical Cleaning: Cleaning Agents and Systems, Second Edition,” CRC Press, B. Kanegsberg and E. Kanegsberg, Editors (2011).