Folks:
Here is my two cents.
1. Bromine can be introduced as TBBPA, or as other covalent bromine
compounds.
2. Here is my understanding about flame retardant mechanism: Covalent
bromine dissociates upon heating and form bromine free radical. It combines
with thus terminates oxygen free radical or organic oxide free radical, thus
stops oxide free radical from being developed or propagation. This prevents
a catastrophic flame from being developed.
3. If the bromine free radical extract a hydrogen from organic compound, it
forms HBr. Now, it can be a flux.
Ning-Cheng Lee
Indium Corporation of America
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of b_ellis
Sent: Tuesday, August 13, 2002 8:15 AM
To: [log in to unmask]
Subject: [TN] Bromine
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
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