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During reflow, solder may wet to one end of a chip before the other, usually due to one bonding pad heating before the other. As the solder continues to wet around the one end, the surface tension of the molten solder on that wetted end results in a lifting of the chip off the board before the solder paste goes molten and wets the opposite end. This results in an open solder joint. This defect, sometimes referred to as the Manhattan Effect, standing proud, or drawbridging, is most often seen on the smallest passive devices. The defect is exacerbated by excessive oven atmosphere inerting. Oxygen monitors can be used to sense the amount of nitrogen in an oven. Maintaining a reflow oven oxygen concentration level of about 1,000 ppm to 1,500 ppm O2 will benefit reflow and minimize tombstoning. Inerting to this level is also significantly more economical since nitrogen consumption is less than when running at lower O2 concentrations.
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Fluxless Soldering There have been many discussions and investigations into fluxless soldering, but often this term is incorrectly applied. Disruption of the integrity of oxides on the parts to be soldered and on the solder itself is a requirement for solder wetting. If a thermal or thermomechanical method is solely employed (e.g., as by ultrasonics), then it is truly a fluxless method. If a chemical agent is applied for oxide removal, whether it is in solid, liquid, or gaseous phase, it is still a flux-based process. Using cover gases alone does nothing to clean oxidized leads, bonding pads, or solder, but will limit further oxidation.
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Gaseous Fluxing It has already been discussed that flux can exist as a liquid or solid, but it can also be dispensed as a gas. There have been several investigations into the realm of fluxing with various gaseous species, including hydrogen, carbon monoxide, carboxylic acids, and others. Some have been demonstrated to be effective fluxing agents, but either their efficacy or economic impact, or both, is not yet sufficient to displace current liquid flux methods. Investigations continue in this area.
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Hydrogen There has been talk in the industry of hydrogen as a fluxing agent, but at soldering temperatures, hydrogen is predominantly a diatomic, unreactive gas; in fact, even at 1,730 C, it is only 0.33 percent dissociated at atmospheric pressure.16 Therefore, it is relatively stable chemically, serving ostensibly as an expensive cover gas an inerting blanket rather than a reactive fluxing agent. H2 explosively combines with oxygen, so it is typically mixed with nitrogen or another inert gas when used for processing. This mixture, called forming gas, is susceptible to separation. Since hydrogen is lighter than air, it rises and may pocket in the upper recesses of the soldering machine or in the room. Its lower explosive limit in air is only 4.65 percent17 so precautions have to be taken either to keep below this value or to preclude exposure to an ignition source. Its upper explosive limit is 93.9 percent, above which hydrogen is once again considered safe, but this is impractical economically and a gas containment vessel would rule out a reasonable continuous process and affordable equipment.
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Carbon Monoxide Carbon monoxide is an interesting choice. Very dense, it readily displaces air from the soldering environment, but since it is a heavier-than-air toxin with considerable public notoriety, it is unlikely that it will ever be considered an attractive assistive agent for reflow. Also, carbon
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monoxide does not dissociate sufficiently well at soldering temperatures to be useful for effective fluxing, but does make an excellent, weakly reducing cover gas much as hydrogen does.
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Methyl Bromide This material has been shown to be an effective fluxing agent but is toxic. In fact, it is now a banned pesticide.
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Carboxylic Acids The idea of a gaseous fluxing agent has much merit. It could be inexpensive, eliminate postsolder cleaning, and leave no residue to impede inspection or test. Without residues, the prospect of corrosion would be minimized. Carboxylic acids in gaseous phase have been demonstrated to be useful in this regard, but it has yet to undergo large-scale testing in a manufacturing environment. Of the carboxylic acids, formic and acetic have received some attention as fluxing agents. They are very simple materials, weak organic acids that decompose to water and carbon dioxide at temperatures above 160 C.18 Acetic, in its dilute form, is vinegar. Both formic and acetic are widely used industrially and are inexpensive reagents. H. J. Hartmann19, 20 has shown that as formic decomposes, it is a good reducing agent for oxides of solder. The process produces meta-stable tin formate, which further decomposes, yielding elemental tin, water, and carbon dioxide.Acetic acid follows a similar reaction and decomposition cycle, forming meta-stable tin acetate and ultimately elemental tin. Especially attractive is the fact that there is no flux residue on the circuit board after the gaseous carboxylic acid reactive soldering process. R. Iman et al.21 have demonstrated formic and acetic to be especially effective for fluxing in the wave-soldering process in conjunction with a light application of adipic acid, a common food preservative and also a known fluxing agent. However, a water rinse was required to remove the adipic residue. Work by G. Disbon and S. M. Bobbio22 and K. Pickering, et al.23 has demonstrated that a plasma can be used to strip oxides from circuit boards and components, and, if they are maintained in an inert environment, they can be soldered without the use of additional liquid fluxing agents. In these schemes, the plasma serves as the oxide-stripping flux.
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