constant boiling, the compositions do not tend to fractionate to any great extent upon evaporation. Since the mixtures are not easily fractionated, they are useful commercially in standard cleaning apparatuses for cold cleaning or vapor degreasing. The composition of the vapor and liquid phases are considered substantially the same and are either azeotropic or azeotrope-like in their behavior, which is a blend that is suitable for commercial use.
To better understand performance, the soil must be evaluated for suitability with the cleaning technology. Solder flux is an important engineered material that enhances solderability of electronic circuitry. A solder flux needs to perform a number of important functions at the same time. It must promote thermal transfer to the area of the solder joint, enhance wetting of the solder on the base metal, and prevent oxidation of the metal surfaces at soldering temperatures. Among those, the primary task is to remove the tarnish layer form the metal joint that is about to be soldered. The most commonly used fluxes for solder pastes include organic acids, organic bases, organic halogen compounds and organic halide salts.
The ingredients in flux are complicated. In general, fluxes comprise resins, activators, solvents and rheological additives. For certain special systems, additives such as tackifiers, surfactants, or corrosion inhibitors may also be used.
Resin refers to organic materials with medium to high molecular weight. It may include natural products, such as rosin, or synthetic materials, such as polymers. Often it is used to provide fluxing activity, tackiness, and an oxygen barrier. Sometimes it may also serve as a rheological aid. The most commonly used resins are water-white rosin or chemically modified resins. The latter type is sometimes referred to as synthetic rosin or synthetic resin by the soldering industry. These chemically modified materials can undergo polymerization, hydrogenation, or functional group modification. These changes may increase the difficulty of cleaning the residue after soldering.
Although resin may provide certain fluxing activity, the soldering performance of resin alone is rarely good enough for the electronics industry. Often some activator chemicals have to be added to the flux in order to boost fluxing activity. The most commonly used activators include linear dicarboxylic acids, special carboxylic acids, and organic halide salts. Activators with a greater solubility in water, such as glutaric acid and citric acid, generally are more suitable for aqueous cleaning processes. Low residue no-clean activators such as adipic acid, may or may not be cleaned in a solvent system.
Figure 4. (left) Increased active solvent. (right) Lower levels of active solvent.
Visual inspection of test boards revealed that increased levels of the active solvent enhance cleaning. With higher levels of active solvent, solubility of the residue improves. This provides an added benefit of reduced cycle time. Figure 4a is a visual representation of flip chip sites that represent improved cleaning. Figure 4b is a representation of residue when using lower levels of active solvent.
Figure 5. Solvent concentration effect.
Active Solvent Level
One further experiment demonstrates the solubility characteristics of the white residue in the fluorinated family of solvents. Samples of white residue were collected from the boil tank of a vapor degreaser. A small quantity of residue was dissolved in a mixture of 85 percent active solvent and fluorinated molecule. The entire residue was soluble in this mixture. Following this, the mixture was diluted with fluorinated molecule to reduce the active solvent concentration from 85 to 56 percent. The residue remained in solution. Similarly, an equally sized sample of residue was mixed with ACC product (43 percent active solvent). In this case, the residue was not totally soluble in the mixture. Additional active solvent was added to increase the concentration from 43 to 72 percent. During this time, no further dissolution of residue was 