lower overall component
stress as compared
to thermal pads.
The typical effective
modulus of a thermally
conductive pad is in
the rage of ~104 to
106 Pa and, once
applied, is compressed
between 20 percent
and 30 percent. Even
though the pads are
soft, the required
compression puts stress
on the component. By
contrast, the average
modulus of a thermally conductive liquid is ~102 Pa and can be
compressed to the limit of the filler size, which is usually about
~0.10 mm. There is significantly less force required to wet out
the liquid material, which reduces component assembly stress
exponentially. Not only is assembly stress diminished, but thermally
conductive liquid materials also offer protection against vibration,
shock, drop and expansion stress as well. The materials are
dispensed, flow into all of the voids and then cure in place, forming a
solid pad structure to safeguard against mechanical stress. Whereas
conventional thermal grease materials have very low assembly
stress, they have a tendency to migrate or “pump out” over time,
which lowers thermal performance and increases mechanical stress.
PRODUCTION SPEED, SUPPLY CHAIN SIMPLICITY
AND LOWER COSTS
The ability to automate the application of TIMs is a
characteristic of thermally conductive liquid materials that confers
multiple advantages. Most obvious is the ability to increase
throughput and yield through the use of automated dispensing
equipment as opposed to manual application, which is often
the method employed with pad- and film-based materials.
While there are considerations as to dispensing equipment
ruggedness and capability due to the abrasive nature of liquid
TIM fillers, the increases in production speed outweigh any
special dispense head or maintenance requirements. Dispensing
the materials allows for an infinite variety of material deposition
volumes, thicknesses and patterns depending on the application
requirements. A single
TIM can be sourced
for multiple programs,
and vastly simplifying
the supply chain. Pad-
and film-based TIMs,
on the other hand, must
be sourced in various
thicknesses and shapes,
each with its own part
number and qualification
process. A manufacturer
may have two or three
different liquid TIMs
for all of its assembly programs, but the same programs could
require upwards of 100 different pad materials.
Outside of the administrative, tooling, throughput, and yield cost
advantages inherent with liquid dispensed thermal materials, the
procurement cost of these products is also generally less than
that of comparable (thermal conductivity and base chemistry) TIM
pads. As seen in Figure 2, the cost of the 1.8 W/m-K liquid TIM
is 0.75 percent of the 2.0 W/m-K pad TIM.
With gap filling thermally conductive liquid materials, device
designers and assemblers get the best of all TIM scenarios.
Available in a variety of formulations with thermal conductivities
that range from 1.0 W/m-K to 4.0 W/m-K, base chemistries
of silicone, non-volatile and non-silicone, and various cure and
working time profiles, liquid TIMs are exceptionally versatile. They
combine the thermal performance and void filling capabilities of
classic grease materials with the mechanical stability of a pad
TIM, delivering superior thermal transfer and stress protection to
address multiple applications (Figures 3 – 6).
The market trends toward ultra-low profile case sizes, high-density fine-pitch interconnects and increasing power densities
will only continue to accelerate, making next-generation liquid
dispensed TIMs with advanced thermal properties a necessity.
The materials allow adaptability to low-, mid-, or high-volume
production and simplify manufacturing and logistics – all while
providing exceptional performance.
Figure 6: Gap Filler is dispensed in various linear lengths (blue material) on an engine
control unit (ECU).
Figure 5: A self-leveling, thermally conductive
Gap Filler is used to connect the windings with
the chassis in a hybrid electric vehicle motor,
transferring the existing heat efficiently.
Figure 4: Thermally conductive TIM is dispensed at various locations on a PCB to
bond heat sinks on top of components.
Figure 3: A liquid Gap Filler is used to fill the LED
lamp housing before attaching the LED Driver.