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March 2015 • The PCB Magazine 11 naceous layer acts as an insulation layer, pre- venting further decomposition of the material. 2. Reaction in the gas phase—The free radical mechanism of the combustion pro- cess which takes place in the gas phase is in- terrupted. The exothermic processes are thus stopped, the system cools down and the sup- ply of flammable gases is suppressed. In printed circuit board material a number of these mechanisms are employed to achieve flame retardancy by appropriate choice of flame retardant compound. In some cases more than one mechanism is used and synergists can also be added to improve the efficacy of a primary flame retardant. Halogenated flame retardants form the larg- est group of flame retardants used in printed cir- cuit board materials. Halogens comprise five chemically related highly reactive non-metallic elements found in group 17 of the periodic table. They are namely Fluorine, Chlorine, Bromine, Iodine and Asta- tine. The artificially created element 117 falls into group 17 and may also be classed as a halogen. Since Astatine is amongst the rarest ele- ments and occurs on Earth only as the result of the radioactive decay of heavier elements and element 117 is entirely synthetic with a half-life of less than one second these will not be con- sidered further. The other four Halogens, however, find many uses in everyday life (Table 1). Fluorine and Iodine are not used as flame retardants as neither effectively interferes with the combustion process. Chlorine-containing flame retardants release HCl (hydrogen chlo- ride) over a wide temperature range so the flame retardant concentration is reduced and is thus less effective. Bromine is the most effective halogen flame retardant since its bonding to carbon enables it to interfere at a more favourable point in the combustion process. The effective agent, HBr (hydrogen bromide), is liberated over a narrow temperature range so that it is available at a high concentration in the flame zone. The mechanism used by brominated flame retardants (BFRs), is that bromine breaks down to form a bromine radical which then reacts with the hydrocarbon to form HBr. The HBr re- moves the high energy H and OH radicals by reaction. The high energy radicals are replaced with low energy bromine radicals. The HBr con- sumed is then regenerated by reaction with the hydrocarbon (Figure 1). In PCB substrates the bromine source is usu- ally 2,2',6,6'-Tetrabromo-4,4'-isopropylidenedi- Table 1. FIRE RETARDANCy: WHAT, WHy, AND HOW continues Feature