In the manufacture of circuit card assemblies (CCAs) such as multi-layer ceramic cards, the ceramic or other such substrates are populated with a variety of devices, including leadless chip carriers having typical board to device clearances of, for example, 0.006 inches and generally a minimum stand-off of 0.004 inches. Inasmuch as solder and solder flux mountings and connections are most commonly used in mounting and connecting the various devices to the substrate, the extremely close spacing between device and board traps flux residues that must be removed to avoid problems in subsequent manufacturing steps, such as when the CCAs are conformally coated. The removal of flux residues has been generally accomplished through the use of vapor degreasing equipment.
Typical of such prior art degreasing equipment is a dual sump assembly in which the first sump contains a degreasing solvent which is heated and the second sump also contains the same solvent. The first sump functions as a wash bath and the second sump functions as a rinse bath. A suitable conveyor means introduces a basket, for example, containing one or more CCAs into the heated solvent of the first sump. After a washing period, which may be of any suitable duration, the basket is removed from the liquid of the first sump and lifted into a vapor area above the sump through which it is passed to the second sump, where it is rinsed.
In some cleaning applications, it is highly desirable, if not mandatory, that the contaminant level after cleaning not exceed ten micrograms per square inch on the circuit board. More particularly, the ionic contamination of the surface of the board, which generally comprises a ceramic substrate populated with various devices and is, for example, six inches square, must be lower than ten micrograms (NaCl equivalent) per square inch; ion chromatography requirements by type must be less than one microgram per square inch for each of the ions chloride, bromide, and sulfate; and there should be no visible residue left on the surface under 7.times. magnification. Heretofore, excellent cleaning results which met the foregoing criteria had been achieved with 1,1,1 trichlorethane in both of the sumps of the previously described vapor degreasing equipment. However, that particular solvent is a Class 1 ozone depleting substance (ODS-1) CFC covered under the Montreal Protocol, the use of which is restricted by U.S. Environmental Protection Agency regulations, and international treaties.
As a consequence of the foregoing, it is extremely important that there be a cleaning system for circuit boards that uses environmentally "friendly" materials while meeting or exceeding the aforementioned cleanliness criteria.