difference between the body and its surroundings.
Therefore, as the temperature of a component
increases and reaches its equilibrium temperature,
the rate of heat loss per second will equate to the
heat produced per second within the component.
Since heat is lost from a component to its sur-
roundings at its surface, the rate of dissipation will
increase with surface area.This is where heat sinks
are used - varying in size and shape, heat sinks can
be designed to offer a significantly increased sur-
face area to maximise heat dissipation. Heat sinks
are often used in LED applications and fix onto the
back of the component. Ideally, these mating sur-
faces should be perfectly smooth to enhance the
efficiency of heat conduction, but this is not usually
possible so air gaps are present at the interface of
the device and the heat-sink, significantly reducing
the efficiency of heat transfer.
There are many ways to improve the thermal
management of LED products and the correct type
of thermally conductive material must be chosen
to ensure the desired results for heat dissipation
are achieved.
Heat transfer compounds will remove air gaps
between mating surfaces and improve the efficien-
cy of heat conduction at the LED junction.They are
designed to fill the gap between the device and the
heat sink and thus reduce the thermal resistance
at the boundary between the two. This leads to
faster heat loss and a lower operating temperature
for the device. Curing products can also be used
as bonding materials. Solid materials such as gap
filling pads and phase changing materials are also
a possibility, where a thin film substrate is used
at the interface. Therefore, an initial consideration
in product selection is whether a curing product
is required to help bond the heat sink in place, or
whether a non-curing thermal interface material is
more appropriate to allow for rework.
Silicone and silicone-free non-curing products
are also available; the silicone products offer a
higher upper temperature limit of 200˚C and a lower
viscosity system, owing to the silicone base oil
used.The use of products based on, or containing,
silicone may not be authorised in certain applica-
tions and as an alternative, a range of non-silicone
products is available for critical applications.
Another option for managing the transfer of heat
away from electronic devices is to use a thermally
conductive encapsulation resin. This offers protec-
tion of the unit from environmental attack while
allowing heat generated within the device to be
dissipated to its surroundings. In this case, the
encapsulation resin becomes the heat sink and
conducts thermal energy away from the device.
Such products can be used to encapsulate the
technology behind and attached to the LED device
and can assist with the reflection of light back from
within the unit, depending on the colour chosen.
The different chemistry options provide a range
of properties and each should be considered de-
pending on the end application requirements. For
example, a polyurethane material offers excellent
flexibility, particularly at low temperatures, a major
advantage over an epoxy system. A silicone resin
can also match this flexibility at low temperatures
and offer superior high temperature performance,
well in excess of the other chemistries available.
Epoxy systems are tough and offer excellent pro-
tection in a variety of harsh environments. They
are rigid materials with low coefficients of thermal
expansion and, in some cases; a degree of flexibility
can be formulated into the product.
Regardless of the type of thermal management
product chosen, there are a number of key proper-
ties that must also be considered. These can be
quite simple parameters, such as the operating
temperatures of the device, the electrical require-
ments or any processing constraints. Other param-
eters are more critical to the device and a value
alone may not be sufficient to specify the correct
product. Thermal conductivity is a prime example
of this. Measured in W/m K, thermal conductivity
represents a material’s ability to conduct heat. Bulk
Table 1: Comparison of various thermal management materials.
Thermal interface
material
Bulk thermal
conductivity
Thermal resistance Material thickness Reworkability
Adhesives
Good
Good
Excellent
Fair
Compounds
or pastes
Good
Excellent
Excellent
Excellent
Encapsulants
Good
Good
Good
Fair/Poor
Thermal pads
Excellent
Fair
Poor
Excellent
Phase change
Excellent
Good
Fair
Good
LiD
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