|
Thermal
conductivity and thermal interface materials – A quick overview
Thermal interface materials act as an essential part of an
efficient thermal management system by transferring heat between two or more
solid surfaces. Thermal adhesives, a category of thermal interface materials,
are a specialized type of glue that handle heat transfer while holding
components together. They are available as curing fluids, tapes or sheets.
Thermal conductivity is a material’s intrinsic ability to
transfer or conduct heat. It is also defined as the amount of heat per unit
time per unit area that can be conducted through a plate of unit thickness of a
given material, with the faces of the plate differing by one unit of
temperature.
As molecular conductivity occurs due to molecular
agitation and collision, it does not result in the movement of the solid. Heat
moves from an area of high temperature and high molecular energy to an area of
lower temperature and lower molecular energy. This process continues until
thermal equilibrium is reached.
A material’s thermal conductivity depends on a number of
factors, including the properties of the material, temperature gradient, and
the path length followed by the heat. Air has a low conductivity 0.024 W/mK at
0°C while copper is a highly conductive metal (385 W/mK).
The temperature of a material also affects thermal
conductivity. At higher temperatures, molecules move more quickly, and heat is
transferred through the material at a higher rate. It implies that an increase
or decrease in temperature can change the thermal conductivity of the material
quite significantly.
An understanding of the effect of temperature on thermal
conduction is important to ensure that products retain their function and
performance when they undergo thermal stress. This is especially important when
working with electronics and developing heat- or fire-protection materials.
The structure of the material is also a factor when it
comes to thermal conductivity. The thermal conductivity values of some
materials differ depending on the direction of heat travel. Based on how heat
transfer occurs, materials can be divided into gases, metallic solids and
non-metallic solids.
The thermal conductivity of gases is lower than that of
solids. As the molecules in non-metallic solids are tightly packed, they have a
higher thermal conductivity than gases. But thermal conductivities can vary
across different non-metallic solids.
Metals have the highest thermal conductivities of all
materials except graphene. They have both thermal and electric conductivity,
which correlate positively, that is, materials with higher electrical
conductivity/low electrical resistance exhibit higher thermal conductivity.
The thermal conductivity of materials determines their
application. Materials with low thermal conductivities do not allow heat to
pass easily through them, and are therefore ideal for insulating homes and
commercial establishments. Those with high thermal conductivity are utilized
where heat must be moved quickly and efficiently from one area to another, such
as in cooling systems of electronic devices.
Why use thermal
silicone?
Silicones are a diverse family of high-performing
materials. They are extremely environmentally friendly, solvent free,
non-volatile and do not contribute to ecosystem degradation. Cured silicone
rubbers are not biodegradable or affected by weathering, ozone and exposure to
UV. Cured silicone rubber is resistant to extreme temperature changes (–60°C to
+250°C). It also offers protection against moisture, and withstands shocks,
vibrations and chemical attacks.
At low temperatures (-75°C), they remain highly elastic,
besides offering excellent temperature stability when exposed continuously to
higher temperatures (up to 200°C), and up to 300°C for shorter durations.
Silicone’s properties remain intact during temperature fluctuations.
A wide range of products can be made from different
formulations of cured silicone elastomers. These include adhesive sealants,
micro thin coatings, soft cured gels and rubbers of varying hardness. As bonds
made with silicones can be acted upon by small mechanical loads, their primary
use is in the form of sealants.
A humidity of 5%-95% is required to cure single component
silicone adhesives. That apart, a temperature of between (5°C and 40°C) is
needed for the curing process. The adhesive film’s thickness will determine how
long complete curing takes, and can run into several days. For thickness of
only a few millimeters, the adhesive cures fully in 24 hours.
During battery design, engineers may concern themselves
with two main tasks – installing each battery cells and the other is evacuating
the heat, also known as thermal conductivity. Ensuring thermal conductivity
involves integrating a cooling system with cooling fluids, usually under the
battery modules, with the goal of reducing heat generated by the battery cells.
Thermally conductive materials are used to facilitate conduction between the
cell and cooling plate. The cavities or roughness of the surface are ‘filed’ to
avoid air void. As these materials have high thermal conductivity, the heat
moves down directly to the colling system for removal.
So, it follows that cells should adhere effectively to the
cooling plate, which means the thermally conductive material should maintain
bonding for a long time. For reliable binding over the entire battery life, the
right selection of thermally conductive materials is necessary.
When high thermal conductivity is required, silicone
adhesives are a popular option. Their thermal conductivity can be improved by
adding thermally conductive fillers to the formulation while also maintaining
bonding performance. Silicones can be purpose-designed to offer the right level
of adhesion and safety.
The most sensitive circuits and their components made use
of thermal silicone sealants. Thermally conductive silicone is essential to
dissipating heat away from heat-generating components and the surrounding
equipment. The adhesive ensures that the component functions as expected even
if the appliance heats up. Sealants also assist in the transfer of the device
heat to heat sink.
Silicone adhesives are a sought-after choice for their
ability to remain flexible across a very wide range of temperatures and
operating conditions. They are also resistant to water and a variety of
chemicals. They find their utility in harsh environments, bonding, sealing and
protecting modern miniaturized electronics effectively.
Types of thermally
conductive silicones
Silicone potting compounds are useful for heat generating
circuits that require potting or encapsulation in a heatsink enclosure. This
provides both heat dissipation and environmental protection.
Silicone thermal gap filler pastes are an alternative to
prefabricated thermal pads. They offer better thermal dissipation compared to
rigid pads. As they can be dispended by automated units, they are used in
sectors that require a high production throughput and lower mechanical pressure
on components. Silicone thermal gap filler pads can be cut to size and applied
by hand. The thickness of the gap pad contributes to higher thermal resistance
in comparison with thermal pastes.
Thermal adhesives sold as fluids can have different
mechanical and adhesive properties once they are cured. Some may peel away
easily; others provide moderate adhesive, while some also provide permanent
adhesion. They may have a rubbery soft texture upon curing or an inflexible state.
How to choose the
right thermal interface material – Important points
- The thermal resistance between joining parts depending
on
(a) thermal conductivity of the thermal interface material
(b) bond line thickness, the amount of the material used
between the joining items
(c) contact resistance between the different types of
materials at the interfaces that make contact with one another.
- To improve heat transfer, the following options are
available:
·
increasing thermal conductivity of the thermal
interface material
·
reducing the bond line thickness between the
electronic component and heat sink
·
reducing contact resistance by using liquid-dispensed
thermal interface materials
·
increasing the mounting pressure for solid
thermal interface materials.
You can use highly thermally conductive RTVs as a default.
RTV silicone (room-temperature-vulcanizing silicone) is a type of silicone
rubber that cures at room temperature, forming a flexible rubber. It can be an
adhesive sealant, encapsulating potting compound or molding rubber. RTV
silicone is made from a component system comprising a base and a curative, and
first involves mixing rubber with a curing agent. Compounds containing tin or
platinum are commonly used as a catalyst during preparation.
Benefits of our
thermally conductive silicone sealants and adhesives
·
Fast cure and good adhesion
·
High thermal conductivity
·
Low thermal resistance
·
Wide operating temperature range
·
Compatible with high-temperature lead-free
processing
·
Minimal ionic impurities & excellent
dielectric properties
·
UV exposure, ozone and weathering have no impact
on silicone.
Applications
•
Component bonding
•
Gasket fabrication
•
PC board heat sink
•
Electronic component heat transfer
•
Adhesive sealing of components
•
Part fixturing with heat dissipation capability
•
Power unit adhesive
|
