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In a simplistic sense, the soldering process model is generally treated as solid and liquid metals interfacing with a gas (the soldering ambient). However, realities dictate a modified view. Nearly all metals oxidize when exposed to air and these oxides, whether on the solder itself or on the metals to be joined, always impede soldering. To keep the solder and surfaces to be joined bondable, the oxide must be removed and bonding surfaces must be kept oxide-free until soldering is complete. A chemical fluxing agent is used to this end and is indispensable to the process of joining. Fluxes are tailored to both the metals to be joined and soldering process temperature. Some metal oxides such as aluminum and nickel have chemical-resistant oxides, which make soldering difficult. Even copper oxides can be tough to dissolve. Most fluxes used in printed circuit assembly are acid-based and are formulated to be strong or weak depending on the metal systems to be joined, post-soldering treatments, and reliability needs. Fluxes are typically composed of acids and high molecular weight materials that are slow to dry, slow to evaporate, and slow to decompose. They are often so acidic that they can etch, dissolve, or disrupt oxides and tarnishes on solder and surfaces to be joined. Both the absence and presence of liquid flux have an effect on the extent of solder wetting. Some fluxes can transition from a runny liquid during preheat to a viscous liquid or gel and ultimately to a gummy solid as it dries out, polymerizes, and decomposes during the soldering cycle.
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46.1.1 Flux Delivery Systems Fluxes can be delivered in many different ways. For surface mounting, solder paste is used. The paste is a mixture of pure solder spheres, a fluxing agent, and other materials to adjust rheology for efficient stencil deposition of the solder paste on printed wiring board (PWB) solder lands. In wave soldering, a liquid flux is sprayed or otherwise applied to the secondary side of the board before the preheat and soldering steps. In hand-soldering, the flux can be an integral part of the solder wire, which has lumens within the wire filled with flux (see Fig. 46.1). As the solder melts, the flux is released to do its job. It can also be applied as a flux paste with miniature artist s brush, with a pick, or as a liquid via fine-tipped flux pen. It has been shown that flux can also be delivered as a gas. This technology is rarely used, but will be discussed briefly.
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Selection of the flux type depends on the solder alloy, metal finishes on the board and components, condition of the surfaces to be joined, the type of soldering process selected, required solder-joint attributes, and the intended final use of the assembly. Some fluxes leave residues that are meant to remain on the board. Some fluxes are difficult to clean or may interfere with incircuit test (ICT) probing or may hinder conformal coating or under-fill adhesion. Post-process flux deposits can cause corrosion, especially in the electric field between adjacent conductors on the surface of the printed circuit board (PCB). Fluxes may impart gas bubbles, or voids, which may be frozen into the solidified solder (see Fig. 46.2). The resultant voids may detract from the mechanical strength if they are large enough, are in abundance, intersect bonding surfaces, or are in close proximity to one another. When soldering is conducted in an air environment, once the flux reacts with the oxide layer on the solder and the metals to be joined, the metal surfaces are rendered chemically unstable and are quite vulnerable to reoxidation as they once again strive for equilibrium within the oxygen-rich ambient atmosphere. The flux residue coats oxide denuded, solderable surfaces as well as the solder and retards oxidation. If the solder flux is exhausted or fully evaporated before or even after oxide stripping and before the onset of solder reflow, oxidation would recur and FIGURE 46.1 Solder wire for hand-soldering solder-joint formation would showing flux-filled lumens. be impeded. Solder fluxes are generally formulated with high-boiling-point organic materials to slow evaporation, allowing overcoating of the cleaned metal surfaces. Further, if the flux residues were to become polymerized or charred prior to the solder melting temperature, this too could inhibit the flow of solder. It is therefore necessary to heed manufacturer solder paste or flux time and temperature guidelines from the flux or paste chemistry manufacturer, although some process optimization is recommended. The presence of the hot liquid flux during soldering FIGURE 46.2 A BGA solder joint rife with voids from flux-derived gases helps to promote thermal uniformity on the board. This and improper soldering profile. These is very important in mass reflow and may play an even voids can detract from the reliability of more crucial role in some solder-in-place processes the solder joint. The IPC-A-610 worksuch as hot-bar laser. In some forms of laser soldering, manship standard for printed circuit the flow of hot flux may help to preheat leads and pads, assemblies (PCAs) should be consulted readying them for solder joint formation. In the case of on void acceptability. (Courtesy of Hewlett-Packard.) hot-bar soldering, a pool of liquid flux may help to even out thermal disparities over the length of the heated bar.
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