For example, suppose 100 W of heat is being generated by a small semiconductor.

If you try to hand that 100 W to a heat sink over a small area as is, heat concentrates in that part.

Only the area directly above the heat source becomes hot.

The surrounding area is still at a low temperature.

In other words, the entire heat sink is not being fully used.

So the structure becomes

Small heat source

↓

Heat spread over a wide area

↓

Heat dissipation by the entire heat sink

This makes it possible to use a large effective area of the cooling component.

The important role of a vapor chamber is

not discarding heat.

It is quickly moving heat from a small heat source to a wide area.

If it is only a matter of spreading heat, copper or aluminum plates can do it too.

Copper has high thermal conductivity and is extremely good as a heat-spreading material.

Then why is a vapor chamber needed?

In a metal plate, heat moves by thermal conduction inside the material.

The farther from the heat source, the more resistance there is before heat is transferred.

In particular, under conditions where

・The heat source is very small

・Heat generation is large

・You want to move heat over a wide range

・You want a thin structure

a simple metal plate alone may not provide sufficient heat spreading.

In a vapor chamber, by using the movement of internal vapor,

heat is carried over a wide range without relying only on thermal conduction inside the metal.

This is the big difference.

A vapor chamber is generally a thin sealed container.

Inside, there are

・Working fluid

・Space in which vapor moves

・A wick structure for returning liquid

Even if it looks like a thin metal plate from the outside, inside there is a mechanism that circulates heat.

The basic operation is a cycle of

1.The working fluid evaporates due to heat from the heat-generating part

2.The vapor moves to the low-temperature side

3.The vapor condenses on the low-temperature side

4.The condensed liquid returns to the heat-generating part through the wick

5.It evaporates again

When a liquid changes from liquid to vapor, a lot of thermal energy is required.

In other words, at the heat-generating part,

heat is absorbed as the liquid evaporates.

That vapor moves to another place and turns back into liquid at a cold place.

When it condenses, it releases the heat it was holding.

In other words, heat is carried in the form of

Heat absorbed at the heat-generating part

↓

Moves as vapor

↓

Heat released at the low-temperature part

This is the basic principle of a vapor chamber.

In metals, heat is transferred via atoms and electrons.

In a vapor chamber, in addition to that,

the vapor itself moves.

That is why heat can be dispersed over a relatively wide area even with a thin structure.

It is particularly well suited to applications that spread heat concentrated at one point across an entire surface.

Inside a vapor chamber, a structure called a wick is provided.

The role of the wick is

to return condensed liquid to the heat-generating part.

The liquid moves inside the wick using capillary action.

For wicks,

・Metal mesh

・Sintered metal

・Fine grooves

・Porous structures

and the like are used.

If this return of liquid stops, working fluid is no longer supplied to the heat-generating part.

Then there is no liquid left to evaporate.

This is an important point that governs the performance of a vapor chamber.

If heat generation becomes too large, the speed at which liquid returns may not keep up with the speed at which liquid evaporates at the heat-generating part.

Then liquid at the heat-generating part becomes insufficient.

This state is generally called dry-out.

When dry-out occurs,

・Heat transport performance may drop

・The temperature of the heat-generating part may rise sharply

・Hot spots may occur

In other words, vapor chambers also have the limit that

they cannot carry heat infinitely.

In smartphones and thin electronic devices, one wants to make vapor chambers thin as well.

However, making them thinner also makes the internal space smaller.

Then

・The space in which vapor moves becomes narrower

・The wick structure also becomes thinner

・The amount of working fluid is limited

・Internal flow resistance becomes larger

may result.

In other words, thinning and heat transport capacity are not simply compatible.

The thinner it is made, the more important the design accuracy of the internal structure becomes.

Conversely, making a vapor chamber larger seems as if it would allow more heat to be spread.

However, as the area increases, problems arise such as

・The distance internal vapor travels becomes longer

・The distance for returning liquid also becomes longer

・Flatness control becomes difficult

・Strength must be secured

・Manufacturing accuracy becomes stricter

In other words, miniaturization has limits, and enlargement also has design challenges.

A technology very similar to the vapor chamber is the heat pipe.

The basic principle is shared.

Internal liquid evaporates,

moves as vapor,

condenses on the low-temperature side,

and the liquid returns.

The difference lies mainly in the form in which heat is moved.

Heat pipes often have long, thin tubular structures.

That is why they are good at carrying heat from point A to point B.

Vapor chambers, on the other hand, are flat and good at spreading heat from a small heat source over a wide area.

In other words, it is easy to understand if you think of it as

Heat pipe = “carrying” heat

Vapor chamber = “spreading” heat

In practice, however, their applications sometimes overlap.

Graphite is also used as a material for spreading heat in the in-plane direction.

Graphite has the characteristics of being

・Very thin

・Lightweight

・Highly thermally conductive in the in-plane direction

Vapor chambers and graphite can both be used for heat spreading, but their mechanisms are different.

Graphite spreads heat by thermal conduction of the material itself.

Vapor chambers carry heat by evaporation and condensation of the working fluid.

That is why, depending on the application,

・Graphite

・Metal plates

・Heat pipes

・Vapor chambers

are used selectively or in combination.

This is important.

A vapor chamber spreads heat.

However, it does not make heat disappear.

The spread heat must ultimately be handed to

・Heat sinks

・Enclosures

・Air

・Cold plates

・Liquid cooling systems

In other words, it is used as

Heat-generating body

↓

TIM

↓

Vapor chamber

↓

Heat sink

↓

Air

or as

Heat-generating body

↓

TIM

↓

Vapor chamber

↓

Cold plate

↓

Liquid

A vapor chamber is a heat-spreading component within a cooling system.

Even if the vapor chamber itself is high-performance, it is meaningless if it cannot receive heat from the heat-generating body.

For example, if

・There are air bubbles in the TIM

・The TIM is too thick

・Contact pressure is insufficient

・The vapor chamber is warped

・The application position is shifted

heat does not sufficiently enter the vapor chamber.

In this state,

before the heat transport capacity of the vapor chamber even comes into play, heat is stopped at the entrance.

Therefore,

it is not that it cools because a vapor chamber was adopted;

being able to put heat correctly from the heat source into the vapor chamber is the first condition.

A vapor chamber is a flat component.

That is why the flatness of the contact surface is important.

If there is warping,

・Only the center makes contact

・The edges lift

・Pressure becomes high in only some areas

・TIM thickness varies by location

may result.

The smaller and denser the heat-generating part becomes, the more local contact conditions affect temperature.

In other words, for a vapor chamber, not only the internal structure but also the surface accuracy of the outside is important.

There is only a limited gap between the vapor chamber and the heat-generating component.

Vapor chamber.

TIM.

Insulating material.

Frame.

The thicknesses of these accumulate and determine the final contact pressure.

The vapor chamber on the thick side.

The TIM also on the thick side.

The enclosure tolerance on the narrow side.

When these overlap, local pressure may become high.

Conversely, when they overlap on the thin side, insufficient contact may result.

In other words,

it is necessary to look not only at the dimensional tolerance of the vapor chamber alone, but at tolerance accumulation with surrounding components.

A vapor chamber has a hollow internal structure.

That is why, when made very thin, attention must be paid to deformation due to external force.

For example, if local pressure is applied due to

・Fastening force during assembly

・Thickness variation of surrounding components

・Drop impact

・Enclosure deformation

it may deform.

If deformation is large,

・The vapor space may become narrower

・The wick structure may be affected

・The contact surface may deform

In other words, precisely because a vapor chamber is thin, dimensional and pressure control of surrounding components is important.

Working fluid is sealed inside.

In other words, a vapor chamber is also a sealed container.

If the internal working fluid or vapor leaks, it stops functioning.

That is why

・Joining

・Welding

・Sealing

・Airtightness

are important.

This is an important technology of vapor chamber manufacturers themselves, and differs from OTIS’s area of expertise.

However, the point that

“the more a component transfers heat, the more sealing technology actually matters too” is shared with liquid cooling.

Vapor chambers are often made of metal.

That is why, when used near electronic circuits,

electrical insulation

must also be considered.

For example, the structure may be

Vapor chamber

↓

Insulating film

↓

Electronic components

However, adding an insulating film means that layer also becomes thermal resistance.

Thinner is advantageous for heat.

But if too thin, the risks of

・Burrs

・Foreign matter

・Pinholes

・Dielectric breakdown

become higher.

In other words, it is necessary to achieve both high heat transfer and high insulation reliability.

Insulating the entire surface makes it easier to increase safety.

However, extending the insulating film more than necessary may add extra layers to the heat path.

So designs such as

・Insulating only the necessary parts

・Securing distance from conductive parts

・Providing openings

・Partial lamination

can be considered.

However, partial processing requires control of

・Misalignment

・Steps

・Edge lifting

This is an area where precision processing becomes important.

In thin products, vapor chambers are sometimes fixed with adhesive materials.

However, while adhesive materials have the advantages of

・Fixing

・Vibration absorption

・Improved ease of assembly

they have the problem that they become thermal resistance.

Making the adhesive layer thinner makes it easier to reduce thermal resistance.

However, if too thin,

・It cannot conform to surface irregularities

・Adhesive strength becomes insufficient

・Warping cannot be absorbed

In other words, a contradiction arises between the thickness necessary to adhere and

wanting it thin to pass heat.

When full-surface adhesive increases thermal resistance, placing adhesive only in the necessary places can be considered.

For example,

・Fixing only the outer periphery

・Temporarily fixing only part

・Placing adhesive while avoiding the area directly above the heat source

This may make it possible to reduce adhesive in the heat path.

On the other hand, effects on

・Fixing strength

・Vibration

・Warping

・Component position

must also be considered.

Here, where to place the adhesive itself becomes thermal design.

Semiconductors, memory, power supply components and so on are not necessarily all placed at the same height.

When a flat component such as a vapor chamber is placed, gaps arise between it and

・Tall components

・Short components

So steps are absorbed using

・TIM

・Gap fillers

・Flexible thermally conductive sheets

However, the thicker the material, the larger thermal resistance tends to become.

That is why it is necessary to achieve both

absorbing differences in height and

shortening the heat path.

One of the advantages of a vapor chamber is that it can spread heat received from multiple heat sources over a wide range.

For example,

GPU.

Memory.

Power supply components.

It is conceivable to bring multiple heat sources into contact with a single vapor chamber.

However, each component differs in

・Heat generation

・Height

・Flatness

・Allowable temperature

・Required contact pressure

When bringing multiple components into contact with a single flat plate,

the pressure optimal for one component is not necessarily optimal for another.

That is why

・TIM thickness

・Material hardness

・Partial gap fillers

・Step design

become important.

As we have seen, a vapor chamber is not just a metal heat-spreading plate.

Inside, liquid evaporates, moves as vapor, condenses on the low-temperature side, and returns to the heat-generating part again.

Through this cycle, it carries heat from a small heat source to a wide area.

However, its performance has limits.

Making it thinner narrows the internal space and lowers heat transport capacity.

If heat generation is too large, the supply of liquid cannot keep up and dry-out occurs.

If there is warping or thickness variation, contact with the TIM deteriorates and heat does not even get in.

In other words, a vapor chamber is a component whose real capability is governed not only by the performance of its internal structure, but also by the external contact state.

Trying to receive multiple heat sources with one plate increases the difficulty even further.

In Part 2, we look at how vapor chambers with these characteristics are used in actual products, and what role OTIS plays among them.

 

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.