Hi Tom:
> In 60950, applicable clauses are 2.10.5.3 and 2.10.5.1 (I'm not
> reading genuine IEC/EN 60950, and I'm sorry if I'm wrong) -
> the former states that distance through insulation would be applied
> in such situation, and the latter set minimum thickness of 0.4mm
> for supplementary and reinforced insulation for peak working voltage
> greater than 71V.
Agree.
> However, I think applying minimum distance for pollution degree 1 or
> that for coated PCBs (clause 2.10.6) for inner layers when designing
> PCBs are not bad idea, although both of them wouldn't be applied here.
> The latter set, for example, minimum distance of 1.2mm for reinforced
> insulation for working volgage up to 250V on coated PCBs.
Anyone may, of course, use any distance through insulation
that he chooses, provided the distance is at least 0.4 mm.
However, I would caution against rationalizing the distance
for conductors on inner layers of printed wiring boards by
applying criteria that are not in any way applicable to the
construction.
These requirements (for creepage distances based on
pollution degree and for coated printed wiring boards) are
based on reasonable principles of engineering and physics.
Requirements for pollution degree 1 creepage distances apply
to the interface between solid insulation and air insulation,
not to inner layers of multilayer boards where creepage does
not exist. The inner layer conductors of an ideal multilayer
board are imbedded in solid insulation. The requirements for
creepage distances presume a long-term accumulation of
"pollution," which does not occur to the inner layers of a
multilayer PWB.
Likewise, requirements for coated printed wiring boards apply
to coated PWBs, not to inner layers of multilayer boards.
Thin coatings are not of uniform thickness, and bend around
the surface conductors, both of which contribute to highly
localized voltage stresses on the coating. The requirements
for coatings account for these localized stresses, which do
not occur to the inner layers of a multilayer PWB.
Rationalizing larger distances by applying these requirements
to inner layers of PWBs is contrary to the basis of the
requirements. Such rationalization generates confusion among
designers and among fellow safety professionals.
Indeed, this thread originated because of just such a
confusion!
If the belief is that the 0.4 mm distance is too small for
inner layers of PWBs, then a greater distance should be
specified. But, please don't rationalize the larger
dimension by applying inapplicable requirements.
Best regards,
Rich
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