Tg
Tg turns up in 2 fields: adhesives, PCBs. Here is what it means to the people in each one.
Tg in adhesives
Glass transition temperature, above which a cured adhesive turns from hard and glassy to soft and rubbery.
Tg marks the temperature range where a polymer's stiffness drops sharply. Structural adhesives lose much of their strength and creep resistance above it, so the service temperature has to stay safely below Tg. It is measured by DSC or DMA and rises with fuller cure and post-cure. Flexible adhesives such as silicones and many urethanes have Tg well below zero and are used above it on purpose.
"Housing sees 90 C in the engine bay and this epoxy's Tg is only 70, that bond will creep."
Tg in PCBs
Glass transition temperature: the point where a board's resin turns from rigid to rubbery and expands much faster.
Tg, the glass transition temperature, is where the resin of a laminate changes from a hard glassy state to a soft rubbery one, measured by DSC, TMA or DMA. Above Tg the board's z-axis expansion rises several fold, straining plated holes and pads during soldering. Standard FR-4 sits around 130 to 140 degrees C and high-Tg materials at 170 degrees C or more, and Tg is usually specified together with decomposition temperature (Td) for lead-free assembly.
"Twelve layers, 3.2 mm thick, two lead-free reflows plus wave, you want high Tg or the barrels will crack."