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12 The PCB Magazine • March 2015 aluminium oxide in an endothermic reaction with the release of water vapour. The resulting endotherm cools the polymer so fewer pyroly- sis products are formed. The aluminium oxide, together with the charring products formed on the substrate acts as an insulating protective layer and the water vapour liberated has a dilut- ing effect in the gas phase and forms an oxygen displacing protective layer. It is reported that undesired release of water due to thermal decomposition in multiple sol- dering operations in ATH systems may lead to a loss of the effectiveness of flame retardancy, reduced CAF (conductive anodic filamentation) performance and delamination. Aluminium monohydrate (AlOOH, boehmite) has been proposed as a thermally stable alternative [5] . IV. Halogen-Free? In the context of printed circuit board ma- terials halogen-free can be broadly interpreted phenol (Tetrabromobisphenol-A or TBBPA) and this is incorporated as an additive flame. In the category of non-halogenated flame retardants phosphorous containing compounds are the most popular and of these 9,10-Di- hydro-9-Oxa-10-Phosphaphenantrene-10-Ox- ide (DOPO) is the most widely used. The com- pound is reacted as an additive flame retardant. The flame retardant mechanism is that the phosphorus containing compound is converted by thermal decomposition to phosphoric acid. The phosphoric acid then dehydrates the oxy- gen containing polymer and causes charring (Figure 2). The metal hydroxide alumina trihydrate (ATH, gibbsite) is a widely used synergist in non-halogen flame retardant systems. The flame retardant mechanism of ATH is physical and combines cooling with the forma- tion of a protective layer and gaseous phase di- lution. The ATH breaks down around 200ºC to FIRE RETARDANCy: WHAT, WHy, AND HOW continues Feature figure 1.