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66 SMT Magazine • November 2014 than during the reflow process, and is further retarded by storage of the product at tempera- tures below ambient. As the reaction between the fluxing system and the powder progresses, a matrix-type structure is formed between the ac- tive ingredients and the solder powder. This re- sults in a gradual increase in viscosity over time, ultimately leading to a material that would be unsuitable for use in either the print or dispense application in the production environment. Fig- ure 9 details the comparative storage stability of a standard solder material and an equivalent halogen-free system where the level of activity has been increased to compensate. It is clear that the process suitability of the doped halogen-free system has been compromised: the viscosity of the system is seen to increase exponentially over a relatively short period of time. current state-of-the-art Significant resource has been dedicated to developing a new generation of solder materi- als that are completely free of any intentionally added halogen (none detectable under O 2 bomb analysis). Halogen-free materials are permitted to contain up to 900ppm Br, Fl and 1500ppm (Cl+Br). The approach of formulating with zero intended halogen is undertaken to prevent any localised regions of halogen or halide being present in the reflowed residue. The zero hal- ogen formulations have also been designed to exhibit excellent reflow performance without detrimentally affecting the inherent stability of the products. In addition, these materials have also been developed to be compatible with the new Pb-free high-reliability alloy as document- ed within this paper. Figure 10 gives representa- tive examples of the soldering performance of the new halogen-free materials using both stan- dard SAC and high-reliability (90iSC) alloys, re- flowed using the same process conditions used for the previous examples of reflow. It is appar- ent that the reflow quality of the new materials utilising SAC-based alloys is comparable—if not figure 9: Comparative stability of standard products versus high activity (non-stable) test material. HigH-reLiabiLity, Pb-Free, HaLogen-Free soLDer continues arTiCle