Heat dissipation sheets, graphite, metal foil, insulating films, and adhesive materials supplied by materials manufacturers are not finished parts as they are.

To incorporate them inside products, things such as

・An outer shape that fits the heat-generating component

・Holes to avoid screws and connectors

・Cutouts to ensure insulation distance

・Reference shapes that determine the application position

・Surfaces that automated machines can pick up by suction

・Carrier configurations that are easy to peel

are necessary.

In other words, through processing, thermal management materials change from materials into functional parts.

At this point, it is not enough simply to be processed into the shape on the drawing.

Even after processing, they are required to be

・Flat

・Not cracked

・Not stretched

・Without layer misalignment

・Free of air bubbles

・Free of foreign matter

・Securing the necessary contact area

In processing thermal management materials, shape accuracy and interface quality must be achieved at the same time.

Viewed as a dimension of a mechanical part, 0.1 mm can look like a small difference.

In thermal management, however, 0.1 mm of lifting is by no means small.

For example, suppose a 0.1 mm gap arises between a heat-generating component and a heat dissipation sheet.

What exists in that gap is, in many cases, air.

The thermal conductivity of air is extremely low compared with typical metals, graphite, heat dissipation sheets, and thermally conductive adhesives.

That is why even a slight air layer obstructs the flow of heat.

What matters is not only the thickness of the gap.

When lifting occurs, problems arise such as

・The actual contact area decreases

・Heat concentrates in the contact parts

・Local heat flux becomes high

・Hot spots remain

・Temperature differences arise between contact and non-contact parts

If the center of the component lifts by 0.1 mm, heat tries to avoid the lifted part and pass through the surrounding contact parts.

As a result, the flow of heat takes a detour.

In other words, 0.1 mm of lifting

is not a problem of the heat path becoming 0.1 mm longer.

It is a problem of the very area through which heat can pass decreasing.

The reason air bubbles and lifting are problems is not only that the thermal conductivity of air is low.

The increase in interfaces is also a major factor.

Originally, if two materials are in close contact, heat flows along the path

Heat-generating body

↓

TIM

↓

Heat spreader plate

However, when an air layer gets between the TIM and the heat spreader plate, the structure becomes

TIM

↓

Air

↓

Heat spreader plate

At this point, newly added are

・The interface between TIM and air

・The interface between air and the heat spreader plate

In other words, just one air layer getting in increases the interfaces heat must pass through.

At interfaces, thermal resistance arises due to fine surface irregularities and insufficient contact.

That is why air bubbles lower thermal performance for multiple reasons:

・They contain air, which conducts heat poorly

・They reduce contact area

・They increase interfaces

Even metal plates, films, and heat dissipation sheets that look flat at first glance have irregularities at the microscopic level.

Even when two materials are stacked, the entire surface is not actually in contact.

Fine high points touch each other, and small gaps remain between them.

Soft TIM and adhesive materials have the role of conforming to these irregularities and filling the spaces.

However, if

・Compression bonding pressure is insufficient

・The material is too hard

・There is foreign matter on the surface

・Thickness is insufficient

・The material is warped

they cannot sufficiently conform to the surface irregularities.

As a result, even if they appear closely bonded, the actual contact area is insufficient.

In lamination for thermal management, what matters is not

applying the material,

but increasing the contact points through which heat actually flows.

Air bubbles during lamination do not occur by chance.

There are reasons why they occur.

Typical causes include the following.

・Lamination speed is too fast

・Compression bonding pressure is insufficient

・The edges of the material make contact first

・The base material is warped

・There is foreign matter on the lamination surface

・The adhesive material cannot conform to surface irregularities

・Roller width or pressure is not appropriate for the material width

・Tension differences during lamination are large

・Material temperature is low and the adhesive material is hard

For example, if a component with a large area is brought into full-surface contact all at once, the air in the center can lose its escape route.

That is why, in actual processing, measures are needed such as

・Applying in order from one end

・Pushing air outward with a roller

・Adjusting compression bonding speed

・Stabilizing material temperature

・Using a vacuum environment as necessary

For air bubble countermeasures, inspecting finished products and removing defects is not enough.

It is important to create a process in which air bubbles are unlikely to occur from the start.

Even air bubbles of the same size have different effects depending on where they are.

An air bubble at an edge far from the heat source.

An air bubble directly above the heat source.

An air bubble at the exit for heat to the cooling plate.

Even if they are the same size, their effects on thermal performance differ.

Air bubbles directly above hot spots in particular may raise local temperatures.

In addition, an air bubble in the center of a heat spreading material can obstruct the path by which heat spreads to the surroundings.

That is why, in air bubble management, not only

・The number of air bubbles

・The diameter of air bubbles

・The total area of air bubbles

but also where the air bubbles are is important.

When the tension and feed speed during lamination are not matched, wrinkles occur in thin films, graphite, and metal foil.

Where there are wrinkles,

・Material overlaps

・It becomes locally thicker

・The surroundings lift

・Compression bonding pressure becomes uneven

・Adhesive layer thickness changes

For example, if only the crest of a wrinkle presses strongly against the cooling plate and its surroundings lift, contact area decreases significantly.

Furthermore, if a hard wrinkle acts like a protrusion, it can apply local stress to the mating component.

In other words, wrinkles are not cosmetic defects, but functional defects that generate thermal resistance and stress at the same time.

When things such as

・Chips from cutting

・Metal particles

・Graphite particles

・Adhesive residue

・Film fragments

・Fibers

・Dust

get onto the lamination surface, a step forms between materials.

Even if the foreign matter itself is extremely small, it can lift the material around it as well.

In other words, foreign matter acts as a minute spacer.

If a single 30 μm piece of foreign matter is trapped, it is conceivable that not only the area directly above it but several mm around it gently lift.

In addition, if it is conductive foreign matter, it may cause

・Insulation defects

・Short circuits

・Electrical leakage

・Noise

In thermal management components, foreign matter management is not done only for cosmetic quality.

It is necessary to maintain thermal contact and electrical safety.

Clean environments are sometimes thought of as being for optical components and semiconductor components.

However, in thermal management components too, the processing environment is important to prevent lifting and poor contact due to foreign matter.

Especially in

・Large-area lamination

・Thin adhesive materials

・Soft TIM

・High-precision insulating components

・Transparent films

・Components for medical applications

small foreign matter affects product function.

Not only cleaning equipment, but also

・How materials are unpacked

・Workers’ clothing

・Transport routes

・Collection of scrap

・Static electricity countermeasures

・Material exposure time

must be considered.

Die-cutting is the process of cutting materials into the shape on the drawing.

In thermal management components, however, the way heat flows changes with the outer shape.

For example,

・The area covering the heat source

・The direction in which heat spreads

・The area in contact with the cooling plate

・Heat paths shaped by holes and cutouts

・The width of narrow connecting parts

are determined by the die-cut shape.

If the outer shape is too small, it cannot sufficiently cover the heat-generating part.

If a hole is too close to the heat source, the effective heat spreading area decreases.

If there is a narrow bridge-like part, the cross-sectional area through which heat can pass becomes small.

In other words, die-cutting is not

a process that cuts materials into a shape that fits in the product,

but a process that creates the passage through which heat flows.

In die-cutting, there is a gap between the upper and lower blades, or between the punch and die.

This gap is generally called clearance.

If the clearance is not appropriate,

・Burrs become larger

・The cut surface becomes rough

・The material is stretched

・The edge deforms

・Interlayer delamination occurs

may result.

With metal foil, if the clearance is too large, the material is drawn in before it is sheared, and burrs tend to occur.

On the other hand, if it is too narrow, the processing load rises, which can lead to blade wear and chipping.

The appropriate clearance differs depending on

・Material type

・Material thickness

・Hardness

・Laminated configuration

・Presence of an adhesive layer

In other words, in die design, it is necessary to consider not only outer dimensions but also

the state of the edge after cutting.

Because burrs occur on edges, they tend to be thought of as a problem only at the edges.

In thermal management components, however, slight burrs change the contact state of the entire component.

For example, suppose a 50 μm burr occurs on the edge of metal foil.

If that burr touches the cooling plate or enclosure first, the component can tilt with the burr as a fulcrum.

Then

・Only the edge makes strong contact

・The center lifts

・The opposite edge also does not make contact

・Compression bonding pressure becomes uneven

may result.

In other words, a 50 μm burr does not necessarily affect only a 50 μm range.

It is conceivable that poor contact arises over a range of several mm to tens of mm, starting from the burr.

Heat spreading materials include

・Copper foil

・Aluminum foil

・Graphite

・Conductive sheets, and so on.

These conduct heat easily, but also conduct electricity.

That is why burrs and protrusions on edges may

・Pierce insulating films

・Come close to circuits

・Contact other metal parts

・Generate conductive particles

It is meaningless to cause insulation defects as a result of using conductive materials to improve thermal performance.

In other words, in conductive thermal management components,

edge quality determines both heat and electricity.

Even if processing is clean during prototyping, the same state does not necessarily continue in volume production.

As die-cutting is repeated, blades wear.

As wear progresses,

・Burrs increase

・The cut surface becomes rough

・Adhesive stringing increases

・Incomplete cuts occur

・Dimensions change

・Processing load increases

may result.

What matters is that replacing blades only after they can no longer cut at all is too late.

Before burr height and edge condition that affect thermal performance exceed allowable values, replacement and maintenance must be done while watching

・Number of processing shots

・Dimensional changes

・Burr height

・Cutting load

・Appearance

Volume production quality is not the quality of the first piece made with a new die.

It is quality that can be maintained even as the die wears.

Adhesive materials are often laminated into thermal management components.

However, adhesive materials are not materials that can be cut simply, like metals or hard films.

Adhesive materials have the characteristics of

・Sticking to blades

・Being stretched

・Stringing

・Squeezing out from the edges

・Springing back after cutting

・Changing hardness with temperature

When adhesive material stuck to the blade accumulates, it can be transferred to the next product or reduce cutting accuracy.

In addition, even if it appears to be cut during processing, the adhesive material alone may remain connected by a thin thread.

If the component is peeled in this state,

・The product may deform

・The carrier may lift

・The adhesive material may stretch

・Scrap may adhere to the product

For thermal management components including adhesive materials, what is needed is not conditions for cutting the base material,

but conditions for stably separating the entire laminate.

The hardness and stretch of adhesive materials, foams, and resin films change with temperature.

When the temperature is high,

・Adhesive material softens

・Stringing increases

・It sticks to blades more easily

・The outer shape deforms more easily

When the temperature is low,

・Material becomes harder

・Cracking and chipping increase

・Bonded interfaces peel more easily

may result.

That is why, even with the same die and the same material, the processing state can differ between summer and winter, morning and night.

In die-cutting thermal management materials,

the temperature and humidity of the processing room

the storage state of materials

material temperature before processing are also part of the process conditions.

Graphite sheets are used as materials that spread heat in the in-plane direction.

On the other hand, they have unique difficulties during processing.

Graphite has the properties of being

・Brittle

・Prone to chipping

・Prone to shedding particles

・Prone to peeling in layers

・Prone to retaining folds and wrinkles

・Harder to handle the thinner it gets

If the edge chips during die-cutting, the heat spreading area becomes smaller than the designed outer shape.

If graphite particles are generated, they become a cause of foreign matter and conductive contamination.

In addition, if fine cracks form during processing, the continuity for spreading heat may be impaired even if it is hard to see from the outside.

In graphite processing,

・Blade shape

・Processing pressure

・Support film

・Adhesive layer configuration

・Scrap removal method

・Transport tension

must be designed appropriately.

Copper has high thermal conductivity and is used as a heat spreading material and conductive component.

However, thin copper foil has the characteristics of

・Stretching easily

・Deforming easily

・Producing burrs easily

・Retaining fold marks easily

・Changing dimensions easily under tension

If the material is stretched during die-cutting, hole positions and outer dimensions can change.

In addition, if tension during reel transport is too strong, dimensions in the length direction may change before and after processing.

With thin copper foil, not only cutting the material but

processing it without stretching, without bending, and while keeping it flat is important.

Aluminum foil is lightweight, conducts heat, and is used in thermal management applications.

However, compared with copper, it is softer and tends to wave when thin.

If tension and roller pressure during processing are uneven,

・Waviness

・Wrinkles

・Stretching at the edges

・Warping

・Burrs

may occur.

The adhesiveness of aluminum foil can also change depending on the state of surface treatment and the oxide film.

That is why, in lamination,

・Surface condition

・Compatibility with adhesive materials

・Compression bonding conditions

・Pretreatment

must also be checked.

Foams and soft TIM are used for step absorption and compression conformity.

However, soft materials are compressed during die-cutting.

If the entire material is crushed before the blade enters, its shape springs back after cutting and dimensions can change.

In addition, with soft materials,

・The cut surface may tilt

・The edges may be crushed

・The thickness may not recover

・Adhesive may squeeze out

・Narrow shapes may deform

Even with the same dimensions on the drawing, dimensions and thickness can appear different depending on the load during measurement.

In other words, with flexible materials,

the processing method

the measurement method

the compression state in use must be considered together.

Thermally conductive sheets containing a high proportion of ceramic filler have high thermal conductivity and insulation, but can be hard and brittle.

If processing conditions are not matched,

・Edge chipping

・Fine cracks

・Particle shedding

・Interlayer delamination

・Cracking around holes

can occur.

Stress tends to concentrate especially at small-diameter holes, narrow bridge shapes, and sharp corners.

Even if the material’s thermal performance is high, if fine cracks occur due to processing, mechanical reliability and insulation may decline.

In reel supply, half-cut processing, which cuts the product layer and leaves the release film or carrier, is used.

If the half-cut is too shallow,

・The product may not separate completely

・The component may stretch when peeled

・The stacking position may shift

・The edges may deform

Conversely, if it is too deep,

・The carrier may be scratched

・The carrier may break during transport

・It may rupture under reel tension

・The component position may become unstable

If the component deforms when peeled, the application position and contact state also change.

In other words, half-cut accuracy affects not only suppliability, but also the final thermal contact state.

After die-cutting, the unnecessary parts other than the product are removed.

This process is sometimes called scrap removal (weeding).

Thermal management materials include

・Narrow shapes

・Small holes

・Complex cutouts

・Soft adhesive materials

・Brittle graphite

so scrap removal can be difficult.

If force is applied to the product side during scrap removal,

・The shape may stretch

・The component may move on the carrier

・Graphite may crack

・The adhesive surface may be exposed

・Foreign matter may adhere

Die-cutting does not end at the moment the blade cuts.

Processing extends all the way to removing the scrap and leaving the product in a stable state.

Die-cutting is not simply a process that cuts materials into a shape that fits in the product. It is a process that creates the passage through which heat flows.

Burrs create lifting. Air bubbles increase interfaces. Foreign matter becomes a spacer. Wrinkles make pressure uneven.

All of these are problems that occur not outside the material, but in the heat path itself.

However, the processing of thermal management materials does not end here.

Even if the material can be cut into the correct shape, there is the next process. That is lamination.

The process of laminating multiple materials together and creating the interfaces through which heat flows. The shape created by die-cutting becomes a functional part only through lamination.

In the next article (6-6-2), we will organize the problems of interface quality and volume production stability in lamination.

6.Microfabrication and Heat Problems

6-6-2. Die-Cutting and Lamination of Thermal Management Materials (Part 2) ― Not a Process of Laminating Materials, but a Process of Creating Interfaces Through Which Heat Flows ―

 

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.