> > I have to analyze failures of a subassemblies of our subcontractor, a
> well-known european hi-tech pcb-manufacturer. The failing subassemblies
> I've to [Wanner Bernhard]    analyze, contains transformer windings which
> are implemented as 14 and 16-layer pcb's . The
> > winding shows dielectric breakdown between different winding layers (and
> > sometimes between different windings on the same layer). This breakdown
> > happens after some hours run-in at 120°C and 500VDC (500VDC as test
> voltage,
> > failures also at lower tension at at AC). Before them, all pcb has
> passed
> > successful a hi-voltage test at ambient temperature.
> > The diel. strength for the closest distance of 65mm prepreg should be
> around 2.5kV, or a little bit less because IPC-2221, § 4.1.2, not 500V!
        [Wanner Bernhard]
> >  [Wanner Bernhard]  May be any loss of the electric strenght of a
> laminate/prepreg  [Wanner Bernhard]  is a
> > function of the temperature, the hi-temp aging, and material defects
> >  [Wanner Bernhard]  ( The occuring pcb temperature of 120°C  [Wanner
> Bernhard]  is  in accordance with IPC-2222, §
> > 4.3. But  [Wanner Bernhard]  this must be in a specified manner, isn't
> it?

        So, my question is: means
> >
>
        [Wanner Bernhard]  As I have reviewed some IPC-standards, there are
some "little" limitations in view  [Wanner Bernhard]  of limitation of
Temprature, electrical strenght, dielectric  [Wanner Bernhard]  thickness,
maximum operating temperature. But none of this comment means we could not
use the electrical strenght at max. allowed operating temperature:

Designstandards

IPC-2221, Generic Standard on Printed Board Design
§ 4.1.2         Material Selection for Electrical Properties, refers to
Table 4-1, Typical Properties of Common Dielectric Materials. Thereafter for
FR-4 the electric strength amounts 39.4 x 103 V/mm2 and notice notice says :
"The stated electrical strength values are commonly evaluated under test
conditions with a 0.125mm core laminate thickness. These values should not
be considered linear for high voltage designs with a minimum dielectric
separation, i.e., less than 0.09mm."
§ 6.3           Electrical Clearance, refers to  Table 6-1. Thereafter, the
minimum spacing for internal conductors (B1) at 301-500V (peak) amounts
0.2mm.
§10.1.2         Electrical Clearance, "Clearances are applicable for all
levels of design complexity (A, B, C) and performances classes (1, 2, 3)
(...)".

IPC-2222, Sectional Design Standard for Rigrid Organic Printed Boards
§ 4.3           Laminate Materials, "The values in Tabele 4-1 are based on
long term aging tests by UL (...) Hot spot temperatures shall not exceed the
temperatures specified in Table 4-1 (...)"Table 4-1: "Clad Laminate Maximum
Operating Temperatures", thereafter for FR-4 and 0.1 mm dielectric thickness
the max. allowed temperature would be 120°C. For FR4 / 0.4mm diel. thickn.
130°C.
§ 4.3           Measurement of Dielectric Thickness, " (...) Thickness by
microsection (...) taken at the closest point between metal cladings."
§ 4.3.2         Dielectric Thickness/Spacing, " (..) If the minimum
dielectric spacing and the number of reinforcing layers are not specified,
the  minimum dielectric spacing is 0.09mm and the number of reinforcing
layers may be selected by the supplier."
§ 4.3.4.3               Glass style, "A variety of glass cloth styles are
available for prepregs (see IPC-EG-140). The glass cloth selection is
dependent upon dielectric thickness and tolerance required, circuit filling
needs, and electrical requirements of the dielectric."
§ 4.3.7         Laminate Material, explains the Material Code Designation
and refers to Figure 4-2 for the recommended material selection process.
>

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