■ What Is Contact Thermal Resistance?
Two solids are stacked.
To the eye, the entire surface appears to be in contact.
But when the surfaces are magnified, they are not perfectly flat.
Metal.
Resin.
Semiconductor packages.
Cooling plates.
Every surface has fine irregularities.
That is why, in reality, there are
parts in contact
and parts not in contact.
Air remains in the parts not in contact.
This air layer makes it harder for heat to flow.
In other words, contact thermal resistance is, simply put,
the thermal resistance created because two components are not in complete contact.
■ Even If It Looks Closely Attached, It Is Not Thermally Attached
To the human eye,
it is applied neatly.
No gaps are visible.
But in the world of heat, even spaces of tens of μm can matter.
In particular, if there are
● Surface roughness
● Warping
● Tiny steps
● Foreign matter
● Air bubbles
the actual contact area becomes smaller.
In other words,
visual adhesion and thermal adhesion are not the same.
This is what makes thermal management difficult.
■ Why Is TIM Needed?
The role of TIM is not simply “putting a material with high thermal conductivity in between.”
Rather, what matters is
filling surface irregularities and gaps.
For example, consider the structure
Semiconductor
↓
TIM
↓
Heat sink
Without TIM, fine air layers remain between the semiconductor and the heat sink.
By conforming to surface irregularities and filling those gaps, TIM increases the actual contact area.
In other words, TIM is
a material that conducts heat and, at the same time, a material that creates contact.
■ Even with High Thermal Conductivity, Performance Will Not Be Achieved If It Is Too Hard
For example, there is a TIM with very high thermal conductivity.
But the material is hard.
Meanwhile, the heat-generating component and heat sink have slight warping.
In this case, the TIM cannot conform to the surfaces.
Then a state may arise where
only the center is in contact,
and the periphery is lifting.
The thermal conductivity of the material itself is high.
But the area through which heat actually enters is small.
As a result, the temperature does not drop as much as expected.
In other words, in thermal management, it is necessary to consider
a material’s thermal conductivity and its conformability to the interface as a set.
■ A 0.1 mm Lift Has Great Significance
Viewed as a mechanical part, 0.1 mm looks like a small dimension.
However, in a structure where the TIM is only about 0.2 mm thick, if a further 0.1 mm lift occurs, the effect is very large.
For example,
the design called for the TIM to be in full-surface contact.
But there is 0.1 mm of warping in one part.
In that part, the TIM is not sufficiently compressed.
An air layer remains.
As a result, local thermal resistance increases.
In other words,
even 0.1 mm, slight from the viewpoint of the whole component, is a very large dimension from the viewpoint of the thermal interface.
This is the same phenomenon as “why a 0.1 mm lift ruins thermal performance,” covered in Chapter 6,
seen from the perspective of failure cases.
■ Air Bubbles Become Small Thermal Insulation Layers
When TIM or adhesives are laminated, air bubbles sometimes get in.
Even if a bubble looks small, there is air in that spot instead of TIM.
Then heat flows around that spot.
If there is only one bubble, the effect may be small.
But if a bubble is directly above a hot spot with high heat generation, the effect on temperature becomes large.
In other words, what matters is
not only the number of air bubbles, but where they are.
■ It Cannot Be Evaluated by Bubble Ratio Alone
For example,
air bubbles cover 1% of the total area.
Looking only at the number, it seems small.
But it means something different whether that 1% is concentrated directly above the heat source or dispersed over low-heat areas.
That is why, in evaluating thermal management components, one must consider not only
“what percent bubble ratio”
but also
● Bubble position
● Bubble size
● Positional relationship with the heat source
■ Foreign Matter Also Increases Contact Resistance
A small foreign particle gets into the contact surface.
Then the component may lift around that particle.
For example, there is a foreign particle of several tens of μm on a very thin film.
The particle itself is small.
But including its surroundings, a wider area may lift.
In other words, the problem of foreign matter is
not only the area of the foreign particle itself.
That particle can change even the contact state around it.
In semiconductors and precision electronic equipment, foreign matter control also relates to thermal performance.
■ Surface Roughness Also Affects Thermal Performance
Even if a metal surface looks as flat as a mirror, fine irregularities exist.
The rougher the surface, the smaller the area where solids directly contact each other.
If the TIM is soft, it can easily fill those irregularities.
But hard materials sometimes cannot get sufficiently into the recesses.
In other words, in TIM selection,
not only the material alone, but also
the surface condition of the mating material
is important.
■ Raising Pressure Can Sometimes Reduce Contact Resistance
Pressing two components strongly together can lower thermal resistance because
● Contact points increase
● The TIM gets into irregularities
● Air bubbles decrease
● TIM thickness decreases
That is why, in TIM evaluation, the pressure condition during measurement is important.
In actual products, however, pressure cannot be raised freely.
Boards bend.
Chips break.
Enclosures deform.
Bolt loads increase.
That is why what matters is
whether performance is achieved at the actual assembly pressure.
■ Catalog Test Pressure and Actual Device Pressure Differ
For example, suppose a material manufacturer measures thermal performance under high pressure in its tests.
Meanwhile, in the actual product, only half that pressure can be applied.
In this case, the TIM may not be sufficiently compressed in the actual device, and the same performance as the catalog may not be achieved.
In other words, when comparing materials, it is necessary to look not only at
thermal conductivity, but at
under what pressure conditions the performance was measured.
■ Thickness Variation Also Changes the Contact State
Suppose TIM thickness varies as
0.20 mm,
0.22 mm,
0.18 mm.
Individually, it looks like a slight difference.
But when multiple components are laminated, these thickness differences accumulate.
For example, in a structure of
TIM
＋
Insulating film
＋
Adhesive
＋
Heat spreader
the tolerance of each material affects the final amount of compression.
In other words, it is necessary to look not only at
the tolerance of the TIM alone, but at the tolerance stack-up of the entire laminated structure.
■ Warping Makes Pressure Uneven
If a semiconductor package or cooling plate is warped, the pressure does not become uniform even when fastened with bolts.
Only the center has high pressure.
The four corners lift.
Only one side is in strong contact.
Such states can occur.
Then the amount of TIM compression also becomes non-uniform.
Thermal resistance also varies by location.
In other words, there are cases where
uniformity of contact pressure leads to uniformity of temperature distribution.
■ The Fastening Method Also Changes Thermal Performance
The same TIM.
The same heat sink.
The same semiconductor.
But if the fixing method changes, such as
4 bolts,
6 bolts,
clamps,
or springs,
the contact pressure distribution also changes.
In other words, the performance of thermal management components is affected not only by materials, but also by
mechanical fixing design.
This is where thermal design and mechanical design cannot be separated.
■ Adhesives Are Convenient for Fixing, but Increase Interfaces
Adhesives are sometimes added to fix TIM or graphite.
This is convenient for assembly.
However, if the structure becomes
Heat-generating body
↓
Adhesive layer
↓
TIM
↓
Adhesive layer
↓
Cooling component
the number of interfaces heat passes through increases.
In other words, adding adhesives may increase
● Thermal resistance
● Number of interfaces
● Thickness
That is why it is necessary
to consider the fixing method and thermal performance at the same time.
■ Full-Surface Adhesion Is Not Always the Right Answer
To fix components firmly, one is tempted to put adhesive over the entire surface.
However, adding an adhesive layer directly above the heat source may increase thermal resistance.
Depending on the application, partial adhesion such as
● Adhering only the outer periphery
● Temporarily fixing only part
● Avoiding the area directly above the heat source
may be more effective.
In other words,
where to place the adhesive is also thermal design.
■ Summary of Part 1
What we have looked at so far is not the performance of the materials themselves.
It is what is happening “between” materials.
Surfaces have fine irregularities.
There is warping.
Foreign matter gets in.
Air bubbles remain.
Thickness varies.
Pressure is not applied uniformly.
As a result,
a state arises where it looks closely attached, but is not thermally attached.
Even if thermal conductivity is raised from 3 W/m·K to 8 W/m·K,
the temperature will not drop if the material is hard and cannot conform.
TIM performance must be viewed not only in terms of “how many W/m·K,”
but also under what pressure conditions the value was measured.
That is why
adhesive put in for fixing can also increase interfaces and raise thermal resistance.
Contact thermal resistance is a problem of material selection and, at the same time,
a problem of mechanical design and process design.
In Part 2, we look at how this contact problem
appears in actual products.
Graphite.
Vapor chambers.
Liquid cooling.
EV batteries.
The more advanced the cooling technology, the more the interface becomes the final bottleneck.
Furthermore,
contact states that change over time,
why things that are fine in prototyping get worse in volume production,
actual failure cases,
and what a processing company can do.
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.
