Tg and Td are important material properties, as they influence the resin system in organic substrates. In lead-free processing,

• Peak assembly temperatures can reach a point where resin decomposition begins.
• Higher temperatures result in increased thermal expansion and moisture absorption and stress on plated holes
• Vapor pressure of the absorbed moisture at lead-free assembly temperatures can lead to blistering/delamination

Glass Transition Temperature (Tg): The temperature at which the resin system changes from a rigid or hard material to a soft or rubber like material.

Issue: Several properties change as the Tg is exceeded, including the rate at which a material expands with temperature. Temperatures above Tg results in,

• Higher moisture permeability
• Higher CTE
• Reduction in modulus

Decomposition Temperature (Td): The temperature at which material weight changes by 5%. This parameter determines the thermal survivability of the resin material.

Issue: Resin decomposition can result in adhesion loss and delamination. The chemical bonds break upon exceeding Td resulting in permanent degradation and damage to the material.

Preferred attributes for Pb-free Assembly:

Decomposition Temperature (Td) >340C

For standard thickness printed wiring boards (1.6mm) a material with a glass transition temperature of greater than 170C is recommended as a minimum requirement

Requirements are primarily driven by peak temperature, which is driven by component and board size / thickness / complexity. With small components and a small board, Tg between 130 and 150C can be sufficient. Larger components and a larger board tend to need Tg closer to 170C to 180C.

The glass transition temperature should be determined using the Thermo-Mechanical Analysis (TMA) method as per IPC-TM-650, 2.4.24C[3]. The TMA method is preferred over the other two methods sometimes used to determine the glass transition temperature, DSC[4] and DMA[5], 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, 2.4.24.6[6] as well as a maximum thermal expansion coefficient in the PCB thickness direction, CTE(z), determined as per IPC-TM-650, 2.4.41[7] 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.