Transition Temperature, Tg
material property often discussed in semiconductor packaging circles is
the glass transition temperature, or simply Tg. Below are some key
points about Tg:
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
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
solids are also known as
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'.
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.
8) In the
semiconductor industry, knowledge of the Tg's of the various materials
used in packaging (such as die attach materials, molding compounds, and
encapsulating resins) is important not only in optimizing manufacturing
processes, but in understanding the reliability implications of exposure
of the products to thermo-mechanical stresses as well.
Compounds; Die Attach Materials
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