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64 SMT Magazine • November 2014 comparison, the equivalent halogen-free mate- rial shows poor coalescence, reduced wetting (spread) and evidence of off-pad solder balling. The wetting speeds of the two materials are also vastly different. The halogen-containing mate- rial exhibited fast wetting with a positive wet- ting force being observed and the halogen-free material showed no positive wetting (and there- fore no evidence of oxide removal). Taking these two performance effects into consideration, the simple approach would be to compensate for the lack of halogen by increas- ing the remaining proportion of metal oxide ac- tive constituents. In principal, this approach to formulation strategy would actually achieve the desired goal of returning the reflow performance of a halogen-free solder material back to the lev- els of traditional halogen-containing products. In practice the solution is not this simple. The level of active materials that can be added to a solder paste is governed by two main contrib- uting factors: (1) ensuring the material is elec- trically reliable as previously mentioned in this paper and (2) guaranteeing the product remains stable for the duration of the storage life. Flux systems are designed to not only remove the ox- ides from the solderable surfaces of the compo- nents and PCBs, but also from the surface of the solder particles within the paste. During reflow, this ensures that the solder alloy will coalesce into a single solder joint. This reaction occurs at room temperature, albeit significantly slower HigH-reLiabiLity, Pb-Free, HaLogen-Free soLDer continues arTiCle Figure 8: Comparative wetting balance curves of halogen-containing and non-halogen flux systems.