Hi Amol,
The rationale behind the STII is that the three properties,Tg, Td & TE
(thermal expansion), are independent of each other, but all three affect the
reliability potential of a PWB. Td, and to a lesser extent Tg, affects the capability
to withstand the soldering temperatures without thermal degradation and
lamination; Tg and TE (a combination of alpha 1 and alpha 2) directly affect the
capability of PTHs and PTVs as well as micro-vias to survice excursions to
soldering temperatures.
The relationship of these three properties within the STII is simply based on
fitting the STII to my experience.

Regards,
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 386-437-8747, Fax: 386-437-8737, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com

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