Yet once again, bromine has raised its head in a couple of recent threads on this list. I'm beginning to question whether brominated fire retardants are as bad as they are made out to be or whether they are just painted bad. Unfortunately, my knowledge of polymer chemistry is insufficient to form a judgement. Here is what I'm thinking: The basic manufacture of epoxy resins is in two stages, the prepolymerisation and the crosslinking. I won't go into more than the basics, as I know that the subject is very complex. The epoxy group is provided by epichlorohydrin which is reacted with bisphenol A (BPA) in the presence of an alkali (NaOH), the latter serving to react with the liberated chlorine atom to form NaCl, during the prepolymerisation. Most of the latter is removed for electrical grade resins, but some remains (hence the high dielectric constant and poor power factors). The epoxy groups are subsequently reacted with cross linking agents for the final polymerisation (curing) of the resin. Theoretically, if all the components are stoichiometric, the resultant cured resin is perfectly homogeneous with uniformly-sized molecules. In practice, this is never perfect. The bromine comes in with FR-4 by using tetrabromobisphenol A (TBBPA) in place of some of the BPA. This has the characteristic of releasing HBr and Br2 when heated to combustion temperature, this, being heavy, blankets the burning resin and excludes oxygen, thereby extinguishing the flame. TBBPA is identical to BPA other than that 4 of the H atoms are replaced by Br atoms, which are therefore covalently bonded to two of the non-cyclic carbon atoms. There is therefore no question of any ionic bromide being present in FR-4. Why then do bromide ions leach out of FR-4, as has been proved time and time again? (Let's discount bromide activated fluxes from this equation.) I have several thoughts on the matter: a) could it be that the TBBPA is supplied with excess free Br atoms or with brominated impurities that are less stable? b) could it be that pressing temperatures already start a breakdown of the TBBPA, allowing free HBr to float around the resin matrix? (This may explain why FR-4 is marginally poorer electrically than G-10) c) could it be that the NaOH is capturing some Br from the TBBPA during prepolymerisation? d) could it be that the TBBPA is astoichiometrically in excess, providing an easily attacked molecule? e) could it be that there are no free bromide ions at all, until our extraction fluid hydrolyses some organobromine compound? (If this is so, then our ion extraction/ion chromatography tests become less meaningful, as providing a means of a reaction!) I think we need the thoughts of a specialist polymer chemist to obtain a much better understanding of what is happening and its influence on the reliability of our FR-4 substrated assemblies. Anyone any ideas???? Brian --------------------------------------------------------------------------------- Technet Mail List provided as a free service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/html/forum.htm for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ---------------------------------------------------------------------------------