Hi all

I have to analyze failures of a subassemblies of our subcontractor. As cause
of problem I suppose cracks/voids in the prepreg (of a16-layer pcb), caused
either by poor siloxane-treatment between glass and epoxide - or - because
crushing of prepreg integrity by microlocal mechanical stress during the pcb
press operation. As cause of this stress I assume following: During the
press operation the prepreg conforms to the bounds formed by the laminate
and their conductors at both sides. Between  the walls/shoulders ot two
buried vias the prepreg will be pressed, the resin will flow away, but the
glass will be squeezed. I think glass fibres squeezing  together can resulte
to resin recession.

But let me tell you some details: The failing subassemblies I've to analyze,
contains transformer windings which are implemented as a 16-layer pcb. 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!
I tend to see any loss of the electric strenght of a laminate/prepreg as
function of the temperature, the hi-temp aging, and material defects.
The occuring pcb temperature of 120°C seems in accordance with IPC-2222, §
4.3. So I'm looking for material defects, especially cracks: if cracks span
a part of
the 65mm (prepreg), the electric strenght will decrease, because the
electric strenght of a cracks (like air) amounts 1kV/mm (instead of 40kV/mm.


Hope to get some engelnotes.

Bernhard

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