The transition from eutectic tin-lead to lead-free soldering in electronic assembly, mandated by the RoHS legislation, has brought great pressure and challenge to solder material formulation due to the high soldering temperature and high alloy surface tension. Moreover, the demand for halogen-free materials, which should be transparent to process yields, along with the miniaturization trend in the electronic industry, is triggering a revolution in solder flux and paste formulations. The chemical and material interactions related to soldering and assembly processes are many and varied. In general, the chemicals have to be stable during handling at mild temperatures to warrant a process-friendly shelf-life. While at preheat and soldering temperatures, the solder flux or the paste have to provide thermal transfer to the joint area and react with the metallization on the printed circuit board (PCB) and component leads to remove oxide and surface contamination in order to prepare the surface for good metallurgical bonding, prevent re-oxidation with the atmosphere until the solder alloy re-solidifies, and promote wetting to form the joint. Residues created by the flux contain metal salts, as well as organic and inorganic byproducts.
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