■ What Is Thermal Conductivity?
Thermal conductivity is an index showing how easily a material itself conducts heat.
The unit is generally expressed in W/mK.
The larger the value, the more easily heat travels through the inside of the material.
For example,
● Air conducts heat poorly
● Resin conducts heat relatively poorly
● Aluminum and copper conduct heat well
● Graphite has high thermal conductivity depending on direction
In this way, thermal conductivity differs from material to material.
■ However, Thermal Conductivity Is the Performance of the Material Alone
This is the important point.
Thermal conductivity is basically the performance of the material alone.
In actual products, however, heat does not flow only through a single material.
Heat moves through multiple materials and interfaces, such as
Heat-generating component
↓
TIM
↓
Heat dissipation sheet
↓
Metal plate
↓
Enclosure
↓
Air
In other words, in actual thermal management, not only heat flowing inside materials,
but also heat passing between materials must be considered.
■ Heat Tends to Stop at Interfaces
What tends to be overlooked in thermal management is the interface.
An interface is the boundary where one material is in contact with another.
For example,
● Between a GPU and TIM
● Between TIM and a heat sink
● Between a heat dissipation sheet and an enclosure
● Between a battery cell and a cooling plate
● Between graphite and an insulating film, and so on.
Heat can have difficulty flowing at these interfaces.
In other words, even with high thermal conductivity materials, if heat gets clogged at the interface, it will not cool.
■ They Seem to Be in Contact, but Actually Are Not
Even if components appear to be in close contact at first glance, they are not completely in contact at the microscopic level.
Surfaces have fine irregularities.
That is why, in reality, they may be in contact only at points or on parts of the surface.
And air gets into the parts that are not in contact.
This air layer becomes the problem.
Air conducts heat extremely poorly.
In other words, even a slight air layer makes it much harder for heat to flow.
■ Even High Thermal Conductivity Materials Will Not Cool If They Are Lifting
For example, suppose you use a heat dissipation sheet with high thermal conductivity.
However, if
● The edges are lifting
● There are air bubbles
● The pressure is insufficient
● There are steps on the surface
● Foreign matter has gotten in
heat will not flow well.
This is not because the material is bad.
It is because the contact state is poor.
In other words, in thermal management,
how to bring things into contact is as important as
what to use.
■ Thickness Increases Thermal Resistance
With thermal management materials, thickness is also important.
Even with the same material, thermal resistance tends to increase as thickness increases.
For example, even with a material with high thermal conductivity, heat has difficulty flowing if it is thicker than necessary.
On the other hand, if it is too thin, it cannot absorb steps and tolerances, and air layers may remain.
In other words, thinner is better is not necessarily true.
Nor is thicker is safer necessarily true.
Thickness design suited to the application is necessary.
■ Even with High Thermal Conductivity, It Won't Work If the Direction Is Wrong
Some materials have directions in which they conduct heat more easily.
A typical example is graphite.
Graphite easily spreads heat in the in-plane direction.
However, in the thickness direction, it may not conduct heat as well as in the in-plane direction.
That is why, when using graphite,
in which direction you want to release heat
must be considered.
Even a material with high thermal conductivity will not cool as expected if the direction in which you want heat to flow does not match the direction the material is good at.
■ Heat Stops at the Place Where It Flows Least Easily
Looking at the entire heat path, if there is even one place with high thermal resistance, overall performance deteriorates.
For example, in a structure such as
Heat-generating component
↓
High-performance TIM
↓
Metal plate
↓
Air layer
↓
Enclosure
even if a high-performance TIM is used, heat gets clogged if there is an air layer along the way.
In other words, in thermal management, rather than the best material,
you need to look at the worst place.
■ Catalog Values and Mounted Values Are Different
Material catalogs list thermal conductivity and thermal resistance.
However, those are values measured under certain conditions.
In actual products,
● Pressure
● Surface roughness
● Thickness
● Application position
● Temperature
● Changes over time
● Assembly variation
are different.
That is why performance as per catalog values is not necessarily achieved.
In other words, catalog values and performance after mounting are different.
■ Why It Cools in Prototyping but Not in Volume Production
In prototyping, skilled workers do the work carefully.
They adjust positions.
They remove air bubbles.
They apply pressure.
If there is a problem, they redo it.
In volume production, however, the same thing must be repeated within automated machines and work standards.
As a result,
● Application misalignment
● Uneven compression bonding
● Air bubbles
● Thickness variation
● Peeling
● Deformation during transport
can occur.
In other words, cooling in prototyping and
cooling stably in volume production are different.
■ Problems That Occur in AI Servers
In AI servers, GPUs and power supply components generate a large amount of heat.
Even if high-performance TIM or heat dissipation materials are used, performance will not be achieved if the contact state is poor.
In AI servers in particular,
● High heat generation
● Continuous operation
● The shift to liquid cooling
● High-density mounting
● Maintainability
are required.
That is why not only the thermal conductivity of the material alone, but also thermal resistance after mounting and long-term reliability are important.
■ Problems That Occur in EVs and ESS
In EVs and ESS, thermal management materials are used between battery cells and cooling components.
However, when variation between cells, compression states, and contact states change, thermal performance also changes.
As a result, it may lead to
● Cell temperature differences
● Variation in degradation
● Shorter lifespan
● Risk of thermal runaway
In other words, in EVs and ESS, not only thermal conductivity,
but also uniformity of the contact state is important.
■ Problems That Occur in Wearables and Smartphones
In smartphones and wearable devices, thermal management components are extremely thin and small.
That is why performance changes with even slight misalignment or lifting.
In addition, because they are close to the human body, surface temperature and perceived temperature are also important.
In other words, in small devices, a difference of a few tens of μm can affect perceived quality.
■ The Researcher's Perspective: Look Not Only at Thermal Conductivity but Also at Thermal Resistance
In research and development, it is necessary to look not only at the thermal conductivity of the material alone, but also at thermal resistance in the mounted state.
What matters is
● Material thermal conductivity
● Thermal resistance in the thickness direction
● Contact thermal resistance
● Interface condition
● Pressure conditions
● Changes over time
In other words, in thermal management, material properties and
mounting state must be evaluated separately.
■ The Shop-Floor Perspective: The Cause of Not Cooling May Lie Not in the Material but in the Process
When something does not cool on the shop floor, you are tempted to suspect the material.
In reality, however, the cause is often
● How it is applied
● Compression bonding conditions
● Foreign matter
● Air bubbles
● Thickness
● Misalignment
● Edge lifting
● Release liner design
● Automation conditions
In other words, looking at the process before changing the material is important.
■ What to Check When It Doesn't Cool Despite High Thermal Conductivity
When it does not cool even though a material with high thermal conductivity is used, the following must be checked.
● Whether there is an air layer at the contact surface
● Whether pressure is sufficient
● Whether thickness is appropriate
● Whether the application position is misaligned
● Whether there are air bubbles
● Whether there is foreign matter
● Whether the directionality of the material matches
● Whether there is a clogged place anywhere in the entire heat path
● Whether the same state can be reproduced in volume production
Changing only the material without looking at these may not lead to a fundamental solution.
■ OTIS's Perspective
OTIS is not a company that develops thermal materials themselves.
However, we may be able to contribute in the area of processing that allows thermal management materials to deliver their performance in volume production processes.
What matters from OTIS’s perspective is
● Processing into the intended shape
● Handling at the intended thickness
● Increasing lamination accuracy
● Suppressing air bubbles and misalignment
● Laminating dissimilar materials
● Supporting reel supply
● Supplying stably in automated processes
● Suppressing volume production variation
Even with materials with high thermal conductivity, performance will not be achieved unless they are correctly processed, correctly applied, and correctly incorporated.
■ What OTIS Can Do
At OTIS, through
● Heat dissipation sheet processing
● TIM processing
● Graphite processing
● Insulating material processing
● Adhesive material processing
● Metal foil processing
● High-precision die-cutting
● Microfabrication
● High-precision lamination
● Lamination of dissimilar materials
● Reel supply
● Designing supply formats for automated processes
and more, we may be able to contribute to bringing thermal management materials closer to a state in which they deliver performance in volume production.
■ Outside OTIS's Expertise
On the other hand, OTIS is not a company that specializes in
● Improving thermal conductivity itself
● Material formulation design
● Filler development
● Dedicated thermal simulation analysis
● Thermal design of entire finished products
However, in the area of making high thermal conductivity materials into a form that functions correctly in volume production,
we may be able to play an important role.
■ Industries Where This Technology Will Be Important
★★★★★ AI servers
★★★★★ GPUs and semiconductors
★★★★★ EVs and batteries
★★★★★ ESS and energy storage systems
★★★★☆ Smartphones
★★★★☆ Wearable devices
★★★★☆ Robots
■ Summary
High thermal conductivity alone does not guarantee cooling
Thermal conductivity is important.
However, actual heat dissipation performance is not determined by it alone.
Heat flows not through a single material, but through multiple materials and interfaces.
That is why
● Contact state
● Air layers
● Thickness
● Pressure
● Application position
● Air bubbles
● Foreign matter
● Directionality
● Volume production variation
greatly affect performance.
In other words, in thermal management, choosing high thermal conductivity materials alone is not enough.
Mounting high thermal conductivity materials in a state where heat flows is what matters.
Thermal management is not completed by material specifications.
It is completed only when materials are mounted, in contact, and reproduced in volume production.
Column Supervisor: Koji Kakumoto (President, OTIS Co., Ltd.)
After studying abroad and working in planning and development at a trading company, he joined OTIS Co., Ltd. in 2011. While primarily working in the Corporate Planning Department, he has also served concurrently in manufacturing and technical divisions, and since 2018 has served as President, working to drive business growth and strengthen the organization.
This article is a general technical explanation intended for educational purposes and does not refer to any specific company, product, or technology.
