As modern electronic circuit boards evolve toward increased circuit and component densities, thorough cleaning of the boards after solderingt becomes more important. Current industrial processes for soldering electronic components to circuit boards involve coating the entire circuit side of the board with a flux and thereafter passing this coated side of the board over preheaters and through molten solder. The flux cleans the conductive metal parts and promotes adheesion of the solder. Commonly used fluxes consist, for the most part, of rosin used alone or with activating additives such as amine hydrochlorides or oxalic acid derivatives.
After soldering, which thermally degrades part of the rosin, the flux and flux residues are often removed from the board with an organic solvent. The requirements of such a solvent are stringent: a solvent should have a low boiling point, be non-flammable, have low toxicity and exhibit high solvent power so that flux and flux residues can be removed without damage to the substrate being cleaned.
While boiling, flammability and solvent power characteristics can often be adjusted by preparing mixtures of solvents, these mixtures are often unsatisfactory because they fractionate to an undesirable degree during evaporation or boiling. Such mixtures also fractionate during recovery, making it difficult to reuse a solvent mixture with the original composition.
On the other hand, azeotropic mixtures, with their constant boiling and constant composition characteristics, have been found to be very useful. Azeotropic mixtures exhibit either a maximum or minimum boiling point and do not fractionate upon boiling. These characteristics are also important in the use of the solvent compositions to remove solder fluxes and flux residues from printed circuit boards. Preferential evaporation of the more volatile components of the solvent mixtures, which would be the case if they were not azeotropes or azeotrope-like, would result in mixtures with changed compositions having less desirable properties, such as lower solvency for rosin fluxes and less inertness toward the electrical components. Unchanging composition during use is also desirable in vapor degreasing operations where redistilled material is usually used for final rinse-cleaning. Thus, a vapor defluxing and degreasing system acts as a still. Unless the solvent composition exhibits a constant boiling point, i.e., is a pure component, an azeotrope or is azeotrope-like, fractionation will occur and undesirable solvent distribution may act to upset the safety and effectiveness of the cleaning operation.
A number of chlorofluorocarbon-based azeotropic compositions have been discovered and, in some cases, used as solvents for the removal of solder fluxes and flux residues from printed circuit boards and for miscellaneous vapor degreasing applications. Some of these chlorofluorocarbons currently being used for cleaning and other applications have been theoretically linked to the depletion of the ozone layer. As early as the 1970's, with the initial emergence of the ozone theory, it was known that the introduction of the hydrogen moiety into previously fully halogenated chlorofluorocarbons reduced the chemical stability of these compounds. Hence, these now destabalized compounds would be expected to degrade in the atmosphere and not reach the stratospheric ozone layer. What is also needed, therefore, are substitute chlorofluorocarbons which have low theoretical ozone depletion potential.
Unfortunately, as is recognized in the art, it is not possible to predict the formation of azeotropes. This obviously complicates the search for new azeotropic compositions which have application in the field. Nevertheless, there is a constant effort in the art to discover new azeotropes or azeotrope-like compositions which have desirable solvency characteristics and particularly a greater range of solvency power.