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.
Also heard as
- glass transition temperature
Related terms
FR-4
Flame-retardant glass-epoxy laminate grade that is the default base material for rigid circuit boards.
Laminate
Sheet of reinforced resin, usually clad with copper foil, that rigid printed circuit boards are built from.
Delamination
Separation between layers of a board's laminate or between laminate and copper, usually from heat or moisture.
Reflow
Soldering surface mount parts by melting printed solder paste in an oven that follows a set temperature profile.