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TABLE 43.4 Flux Constituents and Activity Levels Flux materials of composition Rosin Flux activity level (% Halide) Low (0%) Low (<0.5%) Moderate (0%) Moderate (0.5 2.0%) High (0%) High (>2.0%) Low (0%) Low (<0.5%) Moderate (0%) Moderate (0.5 2.0%) High (0%) High (>2.0%) Low (0%) Low (<0.5%) Moderate (0%) Moderate (0.5 2.0%) High (0%) High (>2.0%) Low (0%) Low (<0.5%) Moderate (0%) Moderate (0.5 2.0%) High (0%) High (>2.0%)
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Flux type L0 L1 M0 M1 H0 H1 L0 L1 M0 M1 H0 H1 L0 L1 M0 M1 H0 H1 L0 L1 M0 M1 H0 H1
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WATER-SOLUBLE FLUX
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Water-soluble fluxes have also been called organic acid fluxes. This name is misleading since all fluxes used for electronic soldering contain organic ingredients and many contain organic acid activators. The term organic acid flux probably originated from the designation of water-soluble fluxes as organic and those activated with organic acid activators as organic acid. Other activators for these fluxes include halide-containing salts and amines. Although the correct name for this category of fluxes is water-soluble, it should also be noted that the flux solvent is normally not water, but alcohols or glycols. As the name implies, water-soluble fluxes are soluble in water and their soldering residues are also expected to be water-soluble. These fluxes are much more active than rosin fluxes, have a wider process window, and give a higher soldering yield with reduced defects. This means that the final assembly will require less touchup or repair. On the down side, watersoluble fluxes contain corrosive residues that, if not properly removed, will cause corrosion in the field and long-term reliability problems. As indicated earlier, water-soluble fluxes usually contain glycols, polyglycols, polyglycol surfactants, polyethylene oxide, glycerine, or other water-soluble organic compounds as the primary vehicle. These provide good solubility for the activators, which are usually the more corrosive amines and halide activators. With the onset of highly efficient cleaning equipment, this type of flux became popular for computer and telecommunication applications. F. M. Zado7 raised concern in the late 1970s that water-soluble fluxes affect the electrical characteristics of the epoxy-glass laminate by reducing the insulation resistance.This reduction
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FLUXES AND CLEANING
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was due to the dissolution of the polyglycols of the flux formulation into the epoxy substrate during the soldering process. Later work by J. Brous8 indicated that some polyglycols were much more deleterious than others. In general, polyglycol containing fluxes (and fusing fluids used by board manufacturers to fuse tin-lead plating) can cause an increase in moisture absorbance of the epoxy-glass substrate. The user must consider several factors when determining whether to use water-soluble fluxes in a given application. One important factor is the operating environment. If the assembly will experience extremes of temperature while under power, it is possible that localized condensation will occur and dendrites will form, shorting out some of the circuit elements. Assemblies of this nature should be conformally coated. A second critical factor is the use of a board design and cleaning process, which ensures that corrosive residues are removed. A third consideration is the voltage gradient in the electrical design of the circuit. Within reduced lines and spacings, a failure mechanism called conductive anodic filament (CAF)9 formation has been associated with high humidity and high voltage gradients. This will be described in detail in a later chapter.
LOW SOLIDS FLUX
Until the mid-1980s, liquid soldering fluxes were formulated in 25 to 35 percent (weight percent) solids or nonvolatile liquid. Then flux chemistries changed and new formulations that were lower in total solids content came on the scene. These fluxes are composed principally of weak organic acids, often with a small amount of resin or rosin. Early formulations had 5 to 8 percent solids, but today s low solids fluxes are 1 to 2 percent solids composition. For lead-free soldering, weak organic acids with higher molecular weight are being used. Nomenclature of these fluxes moved from low solids flux to low residue flux to noclean flux. The thinking was that the amount of residue left by these fluxes after soldering was so minimal that it did not need to be removed. This is only true if the residues are noncorrosive. One challenge of low solids fluxes is the processing window. Unlike water-soluble fluxes that give very low defect levels and have a wide processing window, the soldering process for low solids fluxes must be carefully designed. To begin with, the recommended preheat temperatures are different from those for rosin flux and the preferred solder wave temperature is lower than that for rosin. Additionally, the solderability of incoming boards and components must be ensured. Although water-soluble flux can cut through heavy metal oxide layers, the amount of fluxing ingredients in low solids fluxes is not sufficient to accomplish the task. Lower cost manufacturing can be achieved with low residue fluxes if the cleaning step can be eliminated. This assumes that the incoming components and boards are clean, and that operators handling the boards are careful not to introduce contamination. This requires a flux with noncorrosive residues that will not impede or contaminate the electrical bed-ofnails test probes. A new category of low solids fluxes was introduced in the early 1990s to meet the needs of localities where volatile organic compounds (VOCs) are regulated.These fluxes are marketed as VOC-free or low-VOC fluxes. The solvent in this case is 100 percent water or at least greater than 50 percent water. Use of these fluxes requires special care in the preheat step where the water (solvent) must evaporate before the assembly reaches the solder wave. Failure to do this will result in excessive solder ball formation. Solder fluxes and pastes have gone through significant evolution since the early 1980s. As of 2006, in North America about 70 percent of the fluxes were not cleaned, 25 percent were water-soluble, and 5 percent were rosin-based for military applications.10 However, in the lead-free soldering world, more cleaning is required.
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