We recently sent a 4 layer board to an outside laboratory for Tg
measurement using DSC.  The laboratory reported two peaks on the first
heat scan.  One at 130 C and one at 140 C.  On the second heat there was
only one peak at 141C.  This particular board was fabricted with the
C-stage core from one manufacturer and the B-stage prepreg from another
manufacturer.  The lab is claiming that the 130 C peak is the prepreg
and the 140 C peak is the core.  They are concluding that the lamination
is not fully cured because the second heat had a single peak at 141 C
and the difference between 141 C and 130 C is greater than 5 C.  I sent
a sample from the same board to another lab.  On the first heat they
also obtained two peaks, one at 118C and the other at 138C.  The second
heat also showed two peaks identical to the first heat  within 1/2 C.
The first lab also performed a TMA analysis and obtained 130 C.

I thought I had a good understanding of the physics of the DSC measuring
technique.  I do not understand how one can conclude that the two peaks
on the first heat represents the prepreg and core Tg.  Can anyone fill
me in on this or lead me to literature explaining how one can reach this
conclusion?

Hans Rohr

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