Hi Earl,
I can send you lots of pictures of ILS. It occurs on the larger diameter PTHs
of normal MLBs near the surface [layers 2, 3, N-1, N-2]. Higher failure
propabilities: more resin content, higher T-excursions, more imbalance
between barrel wall thickness and inner-layer foil thickness, thicker
core/prepreg layers.
Take a look at IPC-TR-486, Report On Round Robin Study To Correlate
Interconnect Stress Test (IST) With Thermal Stress/Microsectioning
Evaluations For Detecting The Presence Of Inner-Layer Separations.

Werner

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