There is a significant push in the electronics industry to produce "halogen-free" products in order to be more environmentally friendly. The industry has begun to adopt the definition of halogen-free as less than 900ppm of Cl (Chlorine) and less then 900ppm of Br (Bromine). If a product contains both Br and Cl, then it must have less than 1500ppm total. In the world of solder pastes and fluxes, the term "halide-free" is typically used. The question has arisen as to whether halogen-free and halide-free are synonymous and what is the difference between a halogen and a halide. I will try to explain using as little chemistry as possible.
The term halogen refers to any element in the Group 17 of the periodic table (see www.webelements.com). The elements in this group include Cl, Br, Fl (Fluorine), I (iodine), and At (Astatine). Therefore, halogen-free should technically mean "does not contain any F, Cl, Br, I, or At." Easy enough so far, but things will get a bit more cloudy as we move to the term halide. Using my trusty dictionary (which is actually www.dictionary.com), a halide is defined as any compound containing a halogen. For example, table salt (NaCl) is a halide. What has happened in the electronics industry is that the use of the term halides is actually referring to "halide ions." A halide in the ionic form, such as Br- or Cl-, reacts with metals in the presence of moisture to cause corrosion and dendritic growth. In terms of electrical reliability, the use of ionic halides in a no-clean flux creates a greater long-term reliability risk.
The differentiation between halogen-free and halide-free has created some confusion in the industry. If you are truly looking to eliminate all Br and Cl, then you cannot only eliminate the ionic halides. Buying a solder paste that is "halide-free by titration" or "halide-free by ion chromatography" confirms that there are no ionic halides in the material, but may not be completely free of all Cl, Br, and the other halogens. If you want a truly halogen-free solder paste or flux, be sure your supplier runs an oxygen bomb combustion followed by ion chromatography on that material. This method is effective at identifying the total halogen content of a flux.