Glass Transition Temperature, Tg

An important material property often discussed in semiconductor packaging circles is the glass transition temperature, or simply Tg.  Below are some key points about Tg:

1) The glass transition temperature (Tg) of a non-crystalline material is the critical temperature at which the material changes its behavior from being 'glassy' to being  'rubbery'.  'Glassy' in this context means hard and brittle (and therefore relatively easy to break),  while 'rubbery' means elastic and flexible. 

2)  Note that the concept of Tg only applies to non-crystalline solids, which are mostly either glasses or rubbers. A glass is defined as a material that has no long-range atomic or molecular order and is below the temperature at which a rearrangement of its atoms or molecules can occur.  On the other hand, a rubber is a non-crystalline solid whose atoms or molecules can undergo rearrangement.

3)  Non-crystalline solids are also known as 'amorphous materials'.  Amorphous materials are materials that do not have their atoms or molecules arranged on a lattice that repeats periodically in space.

4)  At room temperature, hammering a piece of glass will break it, while hammering a piece of rubber won't.  The rubber would simply absorb the energy by momentarily deforming or stretching.  However, if the same piece of rubber is submerged in liquid nitrogen (LN2), it will behave like brittle glass - easy to shatter with a hammer.  This is because LN2-cooled rubber is below its Tg.

5)  For all amorphous solids, whether glasses, organic polymers, or even metals, Tg is the critical temperature that separates their glassy and rubbery behaviors.

6)  If a material is at a temperature below its Tg, large-scale molecular motion is not possible because the material is essentially frozen.  If it is at a temperature above its Tg, molecular motion on the scale of its repeat unit (such as a single mer in a polymer) takes place, allowing it to be 'soft' or 'rubbery'.

7)  Since the definition of Tg involves atomic or molecular motion, time does have an effect on its value, i.e., the mechanical behavior of an amorphous material depends on how fast a load is applied to it.  Simply put, the faster a load is applied to a material at its Tg, the more glass-like its behavior would be because its atoms or molecules are not given enough time to 'move.'  Thus, even if an amorphous material is at its Tg, it can break in a 'glass-like' fashion if the loading rate applied to it is too high.