For example, suppose temperature evaluation is done with 10 prototypes and the average temperature falls below the target value.

Looking only at the average, thermal management appears to have succeeded.

In reality, however,

・Products that cooled very well

・Products just barely within target

・Products that exceeded the target

may be mixed together.

In volume production, the average alone is not enough.

What customers need is not products that cool on average.

It is that whichever product is used, it falls within the necessary temperature range.

In other words, what matters in volume production is checking

・Average value

・Maximum value

・Minimum value

・Variation

・Outliers

・Changes over time

The stability of the entire distribution is more important than the peak performance of one piece.

When temperature differences are found in mass-produced products, people sometimes think,

“Perhaps the material’s thermal conductivity varied.”

Of course, variation in material properties must be checked.

However, actual thermal performance is not determined by the material’s thermal conductivity alone.

It also changes with, for example,

・Component thickness

・Adhesive layer thickness

・Contact area

・Air bubbles

・Foreign matter

・Burrs

・Warping

・Lamination position

・Application position

・Compression ratio

・Contact pressure

・Surface condition

In other words, the thermal performance of the finished product is the result of combining material properties and processing state.

Even if the material’s thermal conductivity is the same, temperature changes if the contact state differs.

Conversely, even if there are some differences in material properties, it may be possible to keep thermal performance as a product within a certain range by stabilizing contact area and compression state.

In volume production design, it is better not to assume that “the same material means the same performance.”

Multiple factors are involved in volume production variation of thermal management components.

Organizing typical ones gives the following.

Factors related to materials

・Lot differences in thermal conductivity

・Differences in base material thickness

・Differences in adhesive thickness

・Differences in hardness

・Differences in compression characteristics

・Differences in surface roughness

・Differences in moisture absorption state

・Changes due to storage period

Factors related to die-cutting

・Variation in outer dimensions

・Misalignment of hole positions

・Burr height

・Rough cut surfaces

・Die wear

・Adhesive stringing

・Interlayer delamination

・Remaining scrap

・Component deformation

Factors related to lamination

・Stacking position misalignment

・Air bubbles

・Wrinkles

・Foreign matter

・Differences in nip pressure

・Tension differences

・Lamination speed

・Material temperature

・Adhesive squeeze-out

Factors related to reel supply

・Variation in component pitch

・Curl from winding

・Winding tightness

・Changes in peeling force

・Stretching of the carrier film

・Differences between start and end of winding

・Changes due to storage period

・Shifting during transport

Factors related to assembly

・Application position

・Application pressure

・Compression bonding time

・Dirt on component surfaces

・Warping of mating components

・Fastening force

・Amount of compression

・Whether reapplication occurred

These factors do not necessarily act alone.

For example,

the material is slightly thick.

The adhesive layer is also slightly thick.

Burrs are slightly high.

Compression bonding pressure is slightly low.

Each is within the allowable range, but insufficient contact may occur as a result of them overlapping.

What matters in volume production is not looking at just one factor, but considering the state in which multiple variations overlap.

It is sometimes assumed that if all drawing dimensions are within specification, product function is also guaranteed.

In thermal management components, however, there may be differences in thermal performance even within dimensional specifications.

For example, suppose the tolerance for outer dimensions is ±0.2 mm.

If the component is on the +0.2 mm side, it can sufficiently cover the heat source.

On the −0.2 mm side, however, it may not be able to cover the edge of the heat source.

Even if it passes on the drawing, it may not secure the area necessary for thermal function.

In addition, even if hole positions are within tolerance, depending on their position relative to the heat source, they may narrow the effective cross-section through which heat flows.

In other words, dimensional tolerances must be decided not from the range that can be processed,

but from the range in which thermal function can be maintained.

Controlling all dimensions with the same strictness raises processing costs.

On the other hand, controlling all dimensions with general tolerances may fail to protect thermal performance.

What becomes important here is clarifying functionally critical dimensions.

For example,

・The width of the part covering the heat source

・The area in contact with the cooling plate

・The overlap between heat dissipation material and insulating material

・Positions that prevent exposure of conductive material

・The width of narrow heat path parts

・Reference holes recognized by automated machines

・Product pitch on the reel

may need to be controlled more strictly than general outer dimensions.

On the other hand, for edges that do not affect heat or assembly, relatively wide tolerances can sometimes be set.

What matters is not making the entire drawing strict.

Understanding which dimensions determine thermal performance and concentrating control there

is what matters.

In thermal management components, thickness is important.

When heat flows in the thickness direction, the thicker the component generally is, the longer the distance heat travels.

On the other hand, with flexible TIM, steps between components and surface irregularities cannot be filled unless there is a certain thickness.

In other words, there is the relationship that

・If too thick, thermal resistance increases

・If too thin, gaps cannot be filled

Furthermore, during assembly, the amount of compression changes with component thickness.

When placed in the same gap,

a thick component is strongly compressed.

A thin component is insufficiently compressed.

As a result,

・Contact pressure

・Actual contact area

・Material deformation

・Load on surrounding components

change.

Thickness variation does not merely change the distance heat travels.

It changes the pressure state during assembly itself.

For example, suppose a component consists of four layers:

・Thermally conductive sheet

・Adhesive material

・Insulating film

・Protective layer

If each layer has small thickness variation, they accumulate in the finished product thickness.

Even if each layer is within specification, if all overlap on the thick side, the finished product becomes thick.

If all overlap on the thin side, the finished product becomes thin.

In multilayer components, it is necessary to check not only the tolerance of each material, but also

the total thickness tolerance after lamination.

In addition, in thickness measurement, soft materials are measured thinner when pressed strongly.

Values can also change with probe load, measurement time, and measurement position.

That is why, for flexible materials,

・Measurement method

・Measurement load

・Measurement position

・Settling time before measurement

must also be standardized.

If the measurement method differs, the same component may be recorded as having different thicknesses.

In air bubble management, judgment is sometimes made only on “whether there are air bubbles or not.”

In reality, however, the effect on thermal performance changes with

・Size

・Number

・Total area

・Position

・Shape

・Continuity

A single small air bubble at an edge.

An air bubble of the same size directly above the heat source.

Multiple air bubbles connected, creating a large non-contact area.

These should not necessarily receive the same judgment.

What matters especially is the position of the air bubble.

Directly above the heat source.

The position where heat exits to the cooling plate.

The path along which heat spreads in the in-plane direction.

Air bubbles in these positions may have a large effect even if small.

Therefore, air bubble criteria must be decided according to the thermal design, such as

・Using the same criteria for the entire surface

・Using strict criteria only for critical areas

・Setting prohibited areas

Even if foreign matter is small, if it is hard, it may lift the component around it.

And if it is conductive foreign matter such as metal particles or graphite particles, it may affect insulation.

In other words, the effect of foreign matter differs depending on its

・Size

・Thickness

・Hardness

・Conductivity

・Position

Hard foreign matter directly above the heat source causes local poor contact.

Conductive foreign matter at insulating edges raises electrical risk.

Even a soft fiber, if long and continuous, may create a passage for air.

Therefore, in foreign matter management, it is necessary to consider not only the number, but also the type and functional position.

In die-cutting, dies and blades wear little by little.

Burrs that were small when the die was new may become larger as the number of processed pieces increases.

At this point, while finished product dimensions remain within specification, only the edge quality may deteriorate.

That is why dimensional inspection alone may not sufficiently detect die wear.

When burrs increase, they may

・Lift the component

・Make contact pressure uneven

・Damage insulating films

・Generate conductive chips

・Snag on automated machines

In volume production, it is necessary not to use the die until it can no longer cut,

but to manage it before it affects thermal function and ease of assembly.

Die and blade replacement timing is sometimes managed only by the number of processing shots.

Shot count is an important indicator.

However, even with the same shot count, the progress of wear changes depending on

・Material hardness

・Presence of adhesive material

・Filler content

・Material thickness

・Processing speed

・Cleaning condition

That is why die life is better judged by combining

・Shot count

・Burr height

・Cut surface condition

・Dimensional change

・Processing load

・Adhesive buildup

・Product peeling condition

In other words, die management is not counting the number of uses, but

monitoring changes in the processing state.

If equipment suddenly stops, it is easy to notice an abnormality.

Die wear, however, in many cases progresses little by little.

Burrs increased by a few μm compared with yesterday.

Adhesive stringing increased slightly.

Resistance during scrap removal became a little stronger.

These small changes accumulate, and at some point affect thermal performance and automated supply.

However, because the change is gradual, it is sometimes overlooked on the shop floor as “the usual state.”

What becomes important here are

・Recording the initial state

・Periodic measurement

・Trend management

・Replacement criteria

・Rules for judgment in abnormal situations

It is necessary to look not only at pass or fail, but at which direction quality is moving.

In lamination, even if the settings are the same, the actual state is not always the same.

For example,

・Adhesive material adheres to the roller surface

・Roller temperature rises

・The pressure mechanism changes

・Material diameter decreases and the tension state changes

・Material lots switch

・Processing speed is changed

Even if the same pressure is displayed on the settings screen, the pressure distribution across the material width may have changed.

As a result,

・Air bubbles increase

・Adhesive squeezes out on only one side

・Wrinkles occur

・Thickness differs between left and right

・Stacking position shifts

In volume production, it is necessary to check not only the equipment settings, but

what state is actually being created in the component.

In reel processing, the diameter of the raw material roll changes as the remaining material decreases.

If the unwinding or winding diameter changes, the force applied to the material may change even with the same equipment settings.

When tension changes,

・Stretching of the carrier film

・Component pitch

・Lamination position

・Warping

・Winding state

may change.

There is no problem in the first half of the reel, but the pitch changes in the second half.

It peels easily at the start of winding, but the peeling state changes at the end of winding.

Such phenomena are hard to find with sampling inspection at only one place.

Therefore, in reel volume production, it is important to check conditions and quality at

・The start of winding

・The middle

・The end of winding

For reel-supplied products, it is important that components can be peeled stably from the carrier.

If peeling force is too high, the component deforms.

If too low, the component lifts during transport.

What is even more problematic is that peeling force changes within the reel.

Winding tightness.

Storage time.

Temperature.

Humidity.

Lot of the release film.

Condition of the adhesive material.

These may change peeling force.

Because automated machines operate under fixed conditions, if peeling force changes greatly,

・It cannot be peeled

・The component flies off

・It remains on the carrier

・The suction position shifts

・The equipment stops

may result.

In automation, it is not enough that it peels on average.

It is necessary that even at maximum and minimum, it is within a range the equipment can handle stably.

The final application position is not determined by processing accuracy alone.

For example, it is determined by the overlap of

・The reference position relative to the component’s outer shape

・The placement position on the carrier

・Reel feed accuracy

・Image recognition accuracy

・Nozzle suction position

・Rotation during transport

・Positional accuracy of the application equipment

Even if the processing manufacturer controls the component position to ±0.1 mm, a further ±0.1 mm of variation may be added on the customer’s equipment side.

In the final product, these errors combine.

Therefore, at the design stage, it is necessary to consider how to allocate tolerances to each process.

Even if only processing is made extremely high-precision, the final position will not be stable if variation on the application equipment side is large.

Conversely, even if equipment accuracy is high, it is meaningless if the component position on the carrier is unstable.

In volume production design, the accuracy of the entire process must be looked at.

Thermal management components conform to surface irregularities and increase contact area by receiving a certain pressure between the heat-generating body and the cooling component.

However, if pressure during assembly varies, thermal resistance also changes.

If pressure is too low,

・Insufficient contact

・Remaining air bubbles

・Insufficient conformity to surface irregularities

occur.

If pressure is too high,

・Overcompression of flexible materials

・Adhesive squeeze-out

・Breakage of graphite and ceramic materials

・Excessive stress on thin chips

may occur.

In addition, when variation in component thickness and variation in fastening parts overlap, the actual contact pressure changes even with the same fastening conditions.

Therefore, for thermal management components, it is better not to consider component thickness management and

pressure management during the customer’s assembly separately.

As we have seen, variation in thermal performance does not arise only from material properties.

Die-cutting, lamination, reel supply, assembly.

Each process has factors that create variation.

Moreover, these factors do not necessarily act alone.

Small variations overlapping can produce the result of insufficient contact.

Even if drawing dimensions are within specification, thermal performance is not necessarily guaranteed.

What matters is understanding which dimensions determine thermal performance.

In the next part, we look at how such variation is faced and managed on the volume production floor.

 

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.