The glass transition temperature should be determined using the Thermo-Mechanical Analysis (TMA) method as per IPC-TM-650, 2.4.24C. The TMA method is preferred over the other two methods sometimes used to determine the glass transition temperature, DSC and DMA, because the thermal expansion of the PCB is a critical parameter, which is given by TMA as a function of temperature.
For PCBs subject to soldering processes using the more elevated soldering temperatures required for lead-free solders, specifying a glass transition temperature, Tg, of 140C will not be adequate for PCBs thicker than about 50 mils.
Furthermore, for thicker PCBs, it is recommended that a minimum decomposition temperature, Td, determined as per IPC-TM-650, 220.127.116.11 as well as a maximum thermal expansion coefficient in the PCB thickness direction, CTE(z), determined as per IPC-TM-650, 2.4.41 be specified.
CTE(z) values should be given separately for temperatures below Tg and above Tg; however, frequently the thermal expansion, TE in %, is lumped together from 50 to 260C or even 50 to 288C. Typically, the decomposition temperature is given as Td(5%) to a 5% weight loss; the decomposition temperature, Td(2%), to a 2% weight loss, has been found a very good indicator, but is not as yet widely available.
Frequently, the time to delamination, either T-288 or T-260, are specified either in addition to Td or instead of it. The T-288 delamination time provides a more appropriate level of performance given the process temperature required for LF-soldering. The delamination time is sometimes combined with the requirement that that temperature needs to be survived for 4 to 5 excursions.
The easiest way to specify the three properties critical for the survival of the PCB and the PTH/via interconnect structure - Tg, Td, thermal expansion (TE) - is by specifying a minimum Soldering Temperature Impact Index, STII, which is defined as
STII = Tg/2 + Td/2 - (TE%(50 to 260C) x 10).
For PCBs with thicknesses of 0.06 inches (1.5 mm) or more, an STII-value of 215 or larger is recommended. However, the STII-concept is not widely used as yet.
Thanks to Werner Engelmaierfor all the above information. I had this data since fall of 2006.