BioSciences - Polysciences, Inc.

328
Electronic Adhesives & Encapsulants
as thermal shock. Most thermal cycling is done for 1000 cycles.
The temperature extremes to which the unit is cycled depends on
the intended application as follows:
The JEDEC definition for Thermal Cycling requires
achievement of the temperature limit within 15 minutes.
Realistic Representative Accelerated
Use Environments Test Condition
Application Low High Low High
Temp Temp Temp Temp
Consumer [ 0°C ] [ +60°C ] [ 0°C] [+100°C ]
Automotive [ -40°C ] [+125°C ] [ -40°C] [+105°C ]
Under Hood [ -55°C ] [+150°C ] [ -65°C] [+150°C ]
Industrial [ -55°C] [ +95°C ] [ -50°C] [+150°C ]
Telecomm [ -40°C ] [ +85°C ] [ -50°C] [+125°C ]
Military [ -55°C] [ +95°C ] [ -65°C] [+150°C ]
THERMAL SHOCK: Refers to rapidly moving a test unit from
a chamber at high temperature to another chamber at low
temperature. The chambers can be air to air, or liquid to liquid.
Heat transfer, and stress on the unit, is much higher when the unit
is dipped into a liquid. The liquid/liquid test is more severe than the
air/air test even if the temperature range is the same.
TGA (Thermogravimetric Analyzer): Measures the weight
loss of a Polymer material experiences as it is exposed to increasing
temperature. There are three characteristics of the material that
are of interest:
1. How much weight loss is there as the material cures (since
this loss is almost always a gas, it causes voids if it is high).
2. After curing, if the material is heated above the cure temperature,
does it continue to lose weight (an indication that the
cure was not complete)?
3. Heat the material to very high temperatures and see
at what temperature it begins to deteriorate (when the
polymer breaks down it will exhibit significant weight loss).
TMA (Thermal Mechanical Analyzer): Instrument that
measures the CTE of a material as a function of temperature. Unlike
most common materials, polymer materials exhibit a relatively
sharp change in their CTE as the temperature is increased. The
temperature at which this change in CTE occurs is referred to as
glass transition temperature.
For more information please call (800) 523-2575 or visit: www.polysciences.com
THERMOSET: Polymer material which, once cured remains a
solid at all temperatures. Thermosetting materials will remain solid
until they char (burn) if the temperature is raised high enough, but
they will never become liquid again.
THIXOTROPIC INDEX: Most liquids exhibit constant resistance
to force regardless of how fast the force is applied, e.g.- if you stir a
glass of water, no matter how fast you stir, the water will resist the
force in the same amount (the viscosity does not change). Some
materials (such as most polymers) show a decrease in resistance to
force if the speed at which the force is applied increases. If you stir
a polymer faster, it gets easier to stir (the viscosity decreases). This
phenomenon is called Thixotropic behavior. In order to compare
materials, a standard test is used which is called the Thixotropic
Index. Viscosity is measured at 0.5 RPM and at 5.0 RPM. The
ratio of these two measurements is the Thixotropic index. The
Thixotropic Index of a material determines how well the material
will perform in several key semiconductor packaging applications.
When dispensing die attach adhesive, it is desirable for the material
to flow easily from the syringe. The material should stay in place
without reflow. This kind of behavior is achieved with a Thixotropic
Index of above 5.0
UNDERFILL: When a die is Flip Chip attached, (turned upside
down and it’s pads are attached with solder or conductive epoxy)
the CTE mismatch of the substrate and die can create stress on
the joints causing them to fail over time or extended thermal
cycling. In order to reduce the stress, a polymer material is flowed
or otherwise placed under the flipped die. This material is called
underfill. Underfill that flows under the die after it is attached it is
called conventional underfill. Underfill that is placed under the die
before the die is attached it is called no-flow underfill.
VISCOSITY: The speed of fluid flow and/or the resistance to shear.
Usually measured in Centipoise (cps). Water is 1 cps, cooking oil
is 200 cps, honey is 50,000 cps peanut butter is 1,000,000 cps.
VOID: Any hole in the encapsulant, die attach adhesive
or underfill is called a void. Voids can exist because air is trapped
during the dispense and bond process (the hole is ‘surrounded’
by polymer and gets trapped) or can form because the polymer
releases gas during curing. Voids reduce thermal conductivity
and, if severe, can reduce adhesion strength. They can also trap
moisture and reduce device reliability.
VOLATILES: Any material that evaporates or boils rapidly at the
temperatures at which it is used or processed is said to be volatile.
Many organic materials which are used as solvents are volatile.
If a polymer requires the use of such solvents (because it won’t
flow without their help) it is said to contain volatile materials.
When volatile materials escape, they can create voids and must be
processed accordingly.