Hi Brian,
Indeed, measuring technique has an influence on the Tg, or for that matter,
any other property measurement. Let us also not forget that there are 3
different ways to measure Tg, with the method giving the most important value for
reliability being TMA.
We need to differentiate between laminate and prepreg layers. The prepregs
closer to the platens will have a higher degree of cure and different Tg's than
the layers near the PCB center; on the other hand, laminate layers will be the
same throughout. But the prepreg layers will not only be different in Tg's,
but also resin content, CTE [alpha 1 and alpha 2]; and they will be different
from each other, but even more so from the laminate layers. That is the reason
why barrel fracture invariably will be at barrel locations with prepreg layers
not laminate layers.

Werner

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