■ Why Do Smartphones Get Hot?
Inside a smartphone, many components operate at the same time.
● CPU
● GPU
● AI processing chips
● Battery
● Camera
● Display
● Communication module
● Charging circuit
When these operate, they consume power.
And much of the power consumed turns into heat.
In other words, it is not abnormal for a smartphone to get hot.
It is because a lot of energy is moving inside a small box.
■ Small, So Hard to Cool
The reason smartphone heat problems are difficult is simple.
It is because they are small.
Smartphones must be thin, light, and easy to hold.
Yet packed inside are
● High-performance chips
● Large-capacity batteries
● High-brightness displays
● High-performance cameras
● High-speed communication
In other words, heat sources are densely packed into a small space.
This is the essence of the smartphone heat problem.
■ Few Places for Heat to Escape
In AI servers, large fans and liquid cooling equipment can be used.
In EVs, cold plates and cooling water channels can be designed.
In smartphones, however, it is not so easy.
This is because there are constraints such as
● Wanting to make it thin
● Wanting to make it light
● Wanting to make it waterproof
● Wanting to keep it quiet
● Not wanting to compromise the design
In other words, in smartphones, a contradiction arises: you want to cool it, but there is no place to put a cooling device.
■ Hard to Fit a Fan
In smartphones, it is basically difficult to use large fans.
The reasons are that
● They make noise
● They make the device thicker
● They make it more prone to breaking
● They reduce waterproofing
● They increase power consumption
In other words, in smartphones, rather than cooling by moving air,
the idea of spreading heat internally and releasing it to the exterior becomes important.
■ Spreading Heat Is Important
In smartphones, localized heating becomes a problem.
For example, if only the area around the chip gets hot, it leads to
● Reduced processing performance
● Component degradation
● User discomfort
● Rising surface temperature
That is why, in smartphones, spreading heat concentrated at one point across a surface
is important.
What is used here are
● Graphite sheets
● Metal foils
● Vapor chambers
● Heat dissipation sheets
● Heat spreading components, and so on.
■ The Smartphone Is a Product People Touch
What matters in smartphone heat problems is
that people touch it directly.
With servers, even if the inside is hot, users rarely touch it directly.
Smartphones, however, are different.
We hold them in our hands.
We bring them close to our faces.
We put them in our pockets.
We use them for long periods.
In other words, in smartphones, not only component temperature
but also how people feel matters.
■ High Surface Temperatures Worsen the Experience
When a smartphone gets hot, users feel uncomfortable.
Even if it is not broken,
people wonder, “Is this phone okay?”
In other words, in smartphones, the heat problem is not just a technical problem.
It is also a user experience problem.
■ AI Will Increase Heat Problems Further
From now on, more and more AI processing will go into smartphones.
● Image generation
● Speech recognition
● Translation
● Photo correction
● Video processing
● Personal assistants
● Edge AI processing, and so on.
As more AI processing is done inside the device rather than only in the cloud, the load on the chips increases.
In other words, smartphones will evolve from communication devices to AI devices in the palm of your hand.
As a result, heat problems may become even more difficult.
That is why there is even the idea that embedding them directly in the human body might keep the heat from rising.
■ Heat Is Also Generated During Charging
Smartphones do not get hot only while being used.
Heat is also generated during charging.
Especially in fast charging, a large current flows, so heat is generated in the battery and charging circuit.
In other words, in smartphones,
both heat during use and heat during charging must be considered.
■ Batteries Are Vulnerable to Heat
Smartphone batteries are affected by temperature.
In high-temperature environments, degradation tends to progress.
As a result, it may lead to
● Shorter battery life per charge
● Swelling
● Shorter lifespan
● Reduced safety
In other words, smartphone heat problems relate not only to comfort,
but also to lifespan and safety.
■ Waterproofing and Heat Dissipation Are a Poor Match
Smartphones are also required to be waterproof.
However, to improve waterproofing, the inside must be sealed.
Sealing reduces airflow.
In other words, wanting to waterproof and wanting to release heat tend to clash.
This is what makes smartphone design difficult.
■ Thinness and Heat Dissipation Also Clash
Smartphones are also required to be thinner.
However, making them thinner means
● Less space for heat dissipation materials
● Shorter distance to spread heat
● Stricter contact conditions between components
In other words, wanting to make it thin and wanting to cool it also clash.
■ The Smartphone Is a Combined Problem of Heat × Waterproofing × Thinness × Comfort
Smartphone heat problems are not just about heat.
In reality,
● Heat
● Waterproofing
● Thinness
● Light weight
● Adhesion
● Insulation
● Design
● Drop resistance
● User comfort
are all intertwined at the same time.
In other words, the smartphone is a combined problem of thermal management
in itself. It is palm-sized, but technically quite difficult.
■ The Researcher's Perspective: Heat Spreading Is Important in Small Devices
In research and development,
● High thermal conductivity sheets
● Graphite
● Vapor chambers
● Thin TIMs
● Heat spreading structures
● Low-power chips, and so on are being studied.
In smartphones, it is difficult to fit large cooling devices, so
how to spread heat becomes important.
■ The Shop-Floor Perspective: Performance Changes with Lamination
Inside smartphones, many thin heat dissipation and insulating components are used.
However, if there is
● Lamination misalignment
● Air bubbles
● Lifting
● Uneven thickness
● Material deformation
● Foreign matter, the flow of heat changes.
In other words, in smartphones, just putting in heat dissipation material is not enough.
It is important to apply it in the right position and in the right condition.
■ If Contact Is Poor, Heat Will Not Escape
The inside of a smartphone is extremely narrow.
As a result, the contact state between components greatly affects thermal performance.
For example, if a heat dissipation sheet lifts even slightly, an air layer forms.
Air layers do not conduct heat well.
In other words, slight lifting
becomes large thermal resistance.
This is extremely important in smartphone thermal management.
It is questionable whether every model is cleanly laminated without air layers.
The possibility that this is leading to initial defects and heat generation in some models is probably not zero.
■ OTIS's Perspective
From OTIS’s perspective, what matters in smartphone heat problems is
● Thin-film processing
● High-precision die-cutting
● Graphite sheet processing
● Heat dissipation sheet processing
● Insulating material processing
● High-precision lamination
● Irregular-shape processing
● Reel supply
● Support for automation
● Volume production stability
For small products like smartphones,
it is important to make them thin, light, and high-precision, and make that work in volume production.
■ What OTIS Can Do
At OTIS, through
● Graphite sheet processing
● Heat dissipation sheet processing
● Insulating sheet processing
● High-precision die-cutting
● Thin-film processing
● High-precision lamination
● Stacking of irregular shapes
● Support for reel supply
● Designing supply formats for automated processes
and more, we may be able to contribute to making thermal management components for smartphones work in volume production.
In smartphones in particular, components are small and thin, so
processing accuracy
lamination accuracy
supply format are important.
■ Outside OTIS's Expertise
On the other hand, OTIS is not a company that specializes in
● Smartphone body design
● Semiconductor design
● Battery development
● OS development
● Dedicated thermal simulation analysis
However, in the area of making thermal management components inside smartphones work in volume production processes,
we may be able to play an important role.
■ Industries Where This Technology Will Be Important
★★★★★ Smartphones
★★★★★ Tablets
★★★★★ Wearable devices
★★★★☆ AR glasses
★★★★☆ Edge AI devices
★★★★☆ Compact communication devices
■ Summary
The smartphone is a collection of familiar heat problems
Smartphones are small.
Yet packed inside are
● High-performance chips
● Battery
● Communication
● Camera
● Display
● AI processing
In other words, the smartphone is the product in which
we can most closely feel the heat problems caused by miniaturization and high density.
Moreover, because people touch it directly, heat relates not only to performance but also to comfort.
That is why, in smartphone thermal management,
just being able to cool it is not enough.
Thin, light, waterproof, comfortable, and stable even in volume production.
Only when all of that works does it become true thermal management.
Having written this chapter, I once again find it hard to believe that the human brain runs on 20 W.
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.
