> "FR4 is essentially glass - epoxy resin laminate.with bromides
>
> incorporated to provide self extinguishing properties.
> Usually routed rather than punched.
>
> CEM3 (more Japan and some other Asia) is glass-epoxy but instead of a
> woven fabric as in FR4 uses random non woven glass fibres. Instantly
> distinguishable by its white colour. Equally as good as FR4."
>
> So under what conditions might a person choose CEM3 over FR4? Is it
> suitable for SMT?

I can only tell you what I've read (but it sounds OK :-) )

The two sound largely interchangeable and it may be geographic ; and
availability factors may predominate.  CEM3 is said to be wholly
equivalent to FR4 for practical use.
It's widely used in Japan and much less so in US and Asia. I note that
a Hong Kong supplier offers both FR4 and CE3. I've not seen CEM3
offered here but it may well be available. The key difference appears
to be that CEM3 uses random oriented fibres (probably equivalent to
saying CSM or chopped strand mat) whereas FR4 uses woven glass mat.
CEM3 is an opaqish white colour. The random orientation is said to
provide a material with isotropic mechanical properties. - suggesting
that FR4 may have somewhat different mechanical properties in various
axes - not something you'd expect to be  a major issue in PCB use
where glass PCB is usually substantially "stronger" than needed to
prevent mechanical failure.


Note that this is NOT true of phenolic paper PCB material and maybe
marginally true of epoxy-paper material. These material will happily
fracture and crack propagate under mechanical stress. Note the recent
vehicle recall mentioned  where a PCB fracture was causing problems in
some controllers made by only some sub contractors. No reasons were
given but one could guess that a change of PCB material may be the
cause.

Relatively (even more) uninformed comment: I FR4 is not nice to punch.
Routing is usually used when slots etc are required. I'm not sure of
the limiting factors but the combination of glass mat and general
strength is probably hard on tools and the mat also tends to stop the
holes being clean. Some material I have read implies that CEM3 may be
better but that may be due to other factors - ie AFAIR CEM3 was being
used in very thin form and being punched for watch use. The thinness
may have been the main factor.

This wide ranging PCB materials reference contradicts several things
that I have said above :-) - and looks extremely useful.
It notes:           CEM-3 is similarly impregnated with epoxy resin
and has woven glass cloth surfaces, but its core of non-woven matte
fibreglass is more compatible than CEM-1 with through-hole plating.
CEM-3 is much more suitable than FR-4 for punching and scoring, and
its smoother surface gives better fine-line capability.

        http://www.ami.ac.uk/courses/topics/0233_abm/index.html

PCB maker - happened upon with above search - looks at least interesting
Claim a VERY wide range of capabilities and products.

                                http://bit.ly/PCB_Viafine

They say here (DOC file)

                         http://bit.ly/zzzPCBViafineCapabilities

5. CEM-3
 	Very low Z-axis thermal expansion coefficient.
 	Electrical, thermal & chemical characteristics similar to FR-4.
 	Like CEM-1, it punches cleaner than FR-4.
 	CEM-3 is excellent for plated-thru-hole applications.
 	CEM-3 as an alternative to FR-4 for both single & double-sided
product in the U.S.
 	Very good flatness, thermal resistance and dimensional stability.
 	Outstanding thermal conductivity.

6. FR-4
        Material has the best flexural strength and dimensional stability.
 	FR-4 is very abrasive and greatly reduces die life as compared to CEM-1.
 	Punched-hole quality (excessive fibers) is not nearly as good as
CEM-1and CEM-3.
 	FR-4 has been the standard for plated-thru-hole applications,


_________

And another

            http://www.npc.com.tw/emd-new/di1/c/2009pdf/Data%20Sheets/71~73=
-CEM-3-92%20_UV%20BLOCK%20CEM-3-92.pdf
--=20
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