Handbook of Solvents - George Wypych - ChemTech - Ventech!

916 Martin Hanek, Norbert Löw, Andreas Mühlbauer
with respective cosolvents. The most effective cosolvents for defluxing applications in the
market are based on alcoxypropanols which are specially designed for applications in combination
with HFEs.
Since the boiling points of these two HFEs lie at the upper range covered by the traditional
ODSs such as HCFC-141b, CFC-113 and 1,1,1-trichloroethane, a better cleaning efficiency
is provided due to the increase in solubility of most soils with temperature.
Like ODS solvents, the HFEs are non-flammable, which leads to very simple and thus
inexpensive equipment designs.
The low surface tension of HFEs helps these fluids to easily penetrate tight spaces, especially
between and under components of populated printed circuit boards. In addition,
this is supported by the low viscosity of the HFEs that also allow it to quickly drain from the
parts that have been cleaned.
One useful parameter for assessing the potential performance of a cleaning agent is the
wetting index, which is defined as the ratio of the solvent’s density to its viscosity and surface
tension. The wetting index indicates how well a solvent will wet a surface and penetrate
into tight spaces of complex cleaning substrates. The higher the index, the better the surface
penetration.
Table 14.8.3. Properties of HFEs compared to common CFCs and HCFCs
Property HFE-7100 HFE-7200 HCFC-141b CFC-113
Boiling point, °C 60 73 32 48
Flash point, °C none none none none
Surface tension at 25°C 13.6 13.6 19.3 17.3
Viscosity, Pa s 0.61 0.61 0.43 0.7
Wetting index 181 172 149 133
Since HFEs contain no chlorine nor bromine, they do not have an ozone depletion potential.
In addition, their atmospheric lifetime is short compared to CFCs. Furthermore, the
Global Warming Potential of HFE fluids is significantly lower than other proposed fluorinated
solvent replacements. Practical studies as well as laboratory tests have found that
HFE solvent emissions are 5-10 times lower than the emissions of CFCs or HCFCs. Although,
HFEs are volatile organic compounds (VOC), they are not controlled by related US
directive regulations. Finally, toxicological tests have shown, that these products have extremely
low toxicity which allows for higher worker exposure guidelines, unlike many of
the chlorinated and brominated solvents offered as ODS alternatives.
14.8.4.2.2 Water-free cleaning processes in closed, one-chamber vapor
defluxing systems
Liquid vapor degreasing using solvents has been an accepted method of precision cleaning
for over 50 years. This cleaning process incorporates washing, rinsing, drying and solvent
reclamation in a compact, cost-effective unit. Consequently, this makes it a very attractive
process for many production cleaning applications. Environmental concerns raised in the
last decade have changed both the chemistry and equipment technology for degreasing. Solvents
have been modified or replaced to eliminate ozone depletion potential and other haz-