ty 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 observed (Figure 5). This effect highlights the importance of active solvent concentration for effective cleaning.
Organic flux residues deposited onto the surface of flip chip assemblies during reflow soldering operations have been shown, in some cases, to affect the properties of underfill. Formation of metal salts is due to the active ingredients in the flux chemistry. White residues that form as a result of solder/flux interactions are difficult to completely remove. Increased active solvent concentration in cleaning fluids can greatly improve cleaning performance.
The active solvent concentration is an essential driver for cleaning. A proprietary fluorinated material has been identified that azeotropes with the active solvent at higher level. Visual inspection of test boards revealed that increased level of active solvent, solubility of the residue improves.
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MIKE BIXENMAN, chief technology officer, and KYLE DOYEL, president, may be contacted at Kyzen Corp., 430 Harding Industrial Dr., Nashville, TN 37211; 615/831-0800; e-mail: firstname.lastname@example.org and email@example.com.