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Flux Corrosion Issues in SMT Assembly

16th October 2014

Flux corrosion is one of the most common reasons why a customer may reject an SMT assembly product or why the product does not function at all. The important matter is to understand how flux corrosion occurs and to choose the correct flux products for SMT assembly.

Surface mount technology, or SMT assembly, is a process of securing components to a printed circuit board. Soldering is a method of controlling how one metal dissolves into another to form a firm intermetallic bond. The surface of each metal should be free of contamination — in particular oxides of any of the metals used. Flux functions as a cleaning and flowing agent by reacting with the metal oxides to produce a clean, oxide- and oil-free surface for the metals to bond.

There are four main types of flux: inorganic acids, organic acids, rosin and no-clean fluxes. Highly corrosive inorganic fluxes are rarely used these days in electronics assembly. Organic fluxes are weaker than inorganic fluxes but environmentally better and stronger than rosin fluxes. However, they are not as tacky as the rosin fluxes. Tackiness is necessary for a solder paste to prevent any movement of the component during its placement.

Rosin flux is made from pine resin and a mixture of organic acids. It dissolves thin oxide layers on the metal. No-clean fluxes are the cutting-edge soldering technology and leave no residue after soldering that requires cleaning or any tacky residue that may attract dust.

Flux corrosion occurs in two main processes. It can be a chemical reaction, where a metal reacts into another compound, or an electrolytic process, when one metal in the solder corrodes faster than another when in the presence of an electrolyte.

Even the new no-clean fluxes have corrosion concerns. The most common effect is when too much of the flux is applied and residues from it are not volatized adequately during soldering. Remaining acids may attack any exposed metals such as copper in the printed circuit board and cause short circuits on the board. Sometimes, a weak flux may react in an unexpected manner when placed on the board.

Flux selection should be made on the basis of recognised industry standards. Users should not rely on data sheets that may be many years old. Most importantly, all stages of the soldering process during SMT assembly should be controlled.

Feel free to contact us if you need more information.