There is no sich animal as halogen-free FR-4, nor is it likely there
ever will be. FR-4 is based on an epoxy system, the prepolymer for which
is based on four components: sodium hydroxide, epiCHLOROhydrin,
Bisphenol A (BPA) and tetraBROMObisphenol A (TBBPA). There may be others
as additives. The BPA and TBBPA are morphologically similar molecules,
except that the latter has four of the C-H covalent bonds replaced by
C-Br bonds. They can therefore be used in whatever proportion suits,
from 0% TBBPA (which would then become non-flame-retadant G-10 laminate)
to theoretically 100%. In practice, the TBBPA, which is more expensive
than BPA, is dosed to give the required flame-retardant properties,
according to the standards. A number of other additives can be added to
the G-10 prepolymer mix to render it flame retardant, but they are not
chemically part of tha true epoxy resin and they therefore change the
mechanical and electrical properties. One (rather unsuccessful) approach
has been adding such large quantities of inert fillers such as titanium
dioxide that there are insufficient organics to propagate a flame.
Another approach is to add a substance, such as phosphorus, that has
such an affinity for oxygen that it starves the area burning of oxygen
and autoextinguishes. A third approach is to add nitrogen compounds that
tend to smother the burning area with released nitrogen. Although all of
these are free of TBBPA (or may have reduced quantities), they all have
disadvantages. With one or the other (or possibly a combination), it is
possible to eliminate the bromine, at both technical and financial
costs. Whether such FR-4-substitute laminates can be called FR-4 or not
is a moot point. Semantically, I would say it would be better not to,
otherwise there will always be a confusion. I would suggest that the IPC
could persuade the powers that be to call them FR-41, FR-42... according
to the added chemistry and keep FR-4 for the bog-standard TBBPA-retarded
stuff.
But what about the chlorine in the epichlorohydrin? That is also a
halogen. It is an essential part of the process, because it provides the
true epoxy bond (the Cl atoms are replaced by oxygen atoms, providing
two bonds back into the molecule to two adjacent carbons. The oxygen is
provided mainly by the NaOH: 2Cl + 2NaOH > 2NaCl + H2O + O). This epoxy
bond is subsequently an essential part of the cross-linking process
during curing. For electrical grade resins, the sodium chloride is
largely extracted by ion exchange, but certainly not completely. The
residual ionic sodium chloride is the reason why the dielectric constant
is relatively high (>~4.5, as opposed to ~2-2.5 for linear polymers) and
the power factor is not exactly outstanding. Furthermore, if the
reaction is not perfectly stoichiometric, there could still be some raw
epichlorohydrin or by-products with the C-Cl bond still intact.
Stoichiometry is an impossible ideal in polymerisation reactions,
because the molecule size is finite and the reaction always stops before
absolute completion. Whatever, you always have some Cl, both covalent
and ionic, in an epoxy resin, whether you like it or not.
Halogen-free FR-4 is therefore an oxymoronic term.
That having been said, let's say, for the sake of argument, that you
obtain a halogen-free FR-4. Will the components you add be halogen-free
(the same arguments apply to those with epoxy resins and
polycarbonates)? Will all your insulants be halogen free, including wire
insulation, right up to fluorocarbon ones? Will you guarantee solder
masks (temporary or permanent) to be exempt from halogens? Are your
soldering fluxes and pastes entirely halogen-free? Are your cleaning
solvents, including water, halogen-free? Are you sure??????
So, what's the point?
Brian
Eddie Rocha wrote:
> I've asked two top laminate mfg'rs on their status with halogen free
> FR4 and both only manufacture this in Europe. I believe the
> European directives require FR4 to be halogen / bromine free. Am I
> mistaken or should we all be concerned with laminate also?
>
> -- Thanks,
> Eddie Rocha
> (408) 978-8992 x 282
>
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