Comment choisir l'épaisseur du pad thermique pour l'électronique

To select thermal pad thickness for electronics, first measure the real gap between the heat source and the cooling surface, then choose a pad that fills the maximum gap while staying within the recommended compression range at the minimum gap. The selected pad should create full contact without excessive mechanical stress, and its final compressed thickness should keep thermal resistance within the design target.

En termes simples : choose the thinnest thermal bloc-notes that reliably fills the full gap across tolerance variation without over-compressing the assembly.

Thermal pad thickness should not be selected by nominal gap alone. Engineers also need to consider gap tolerance, compression ratio, pad hardness, contact pressure, thermal conductivity, thermal impedance, electrical insulation, surface flatness, vibration, thermal cycling, and long-term compression set.

Why Thermal Pad Thickness Matters

Thermal pads are used to transfer heat from electronic components to heat sinks, metal housings, chassis, cold plates, or enclosures. They are commonly used when the gap is too large or uneven for thermal grease but still needs a clean, repeatable thermal path.

Thermal pad thickness matters because heat must travel through the pad before reaching the cooling surface. A thicker pad usually creates higher thermal resistance. A thinner pad usually improves heat transfer, but only if it fully contacts both surfaces.

This creates a practical tradeoff:

• If the pad is too thin, it may not touch both surfaces.
• If the pad is too thick, it may increase thermal resistance and mechanical stress.
• If the pad is too hard, it may not compress enough.
• If the pad is too soft, it may deform too much or be difficult to handle.

The right thickness balances thermal performance and mechanical safety.

For the broader relationship between thickness and heat transfer, see HakTak’s article: Comment l'épaisseur du joint de colle affecte la performance thermique.

What Is Thermal Pad Thickness?

Thermal pad thickness is the supplied thickness of the pad before assembly. Common thicknesses may include 0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm, 3.0 mm, and higher, depending on the material and application.

However, engineers should distinguish between three thickness values.

The most important value for thermal performance is usually the final compressed thickness, also called bond line thickness. This is the actual thickness heat must pass through during operation.

Start with Real Gap Measurement

The first step is to measure the actual gap. Do not rely only on CAD nominal dimensions unless the design is still in early concept stage.

Real gaps can differ from CAD because of:

• Component height tolerance
• PCB thickness tolerance
• Solder joint height
• Planéité du radiateur
• Tolérance d'ajustement
• Variation du couple de serrage de la vis
• Board warpage
• Thermal expansion
• Assembly stack-up

Measure Minimum, Nominal, and Maximum Gap

A good thickness selection process needs three values:

• Minimum gap: the smallest possible distance after tolerance stack-up
• Nominal gap: the expected design gap
• Maximum gap: the largest possible distance after tolerance stack-up

For example, a design may have a nominal gap of 1.0 mm, but the real tolerance range may be 0.8 mm to 1.25 mm. A pad selected only for 1.0 mm may fail if the assembly reaches either extreme.

Use Physical Validation

Useful gap measurement methods include:

• Feeler gauges
• Clay or compressible gap measurement films
• Laser displacement measurement
• CMM inspection
• Cross-section analysis
• Trial assembly with witness material
• Mechanical tolerance stack-up analysis

For high-volume production, physical validation is important because small tolerance errors can create large thermal variation across units.

Understand Compression Ratio

Thermal pads are designed to compress. Compression helps the pad fill surface roughness, remove air gaps, and improve contact. But the pad must stay within a safe compression range.

Compression ratio can be calculated as:

Compression ratio = (original thickness – compressed thickness) / original thickness × 100%

For example, if a 1.5 mm pad is compressed to 1.2 mm:

(1.5 – 1.2) / 1.5 × 100% = 20% compression

HakTak has a dedicated guide on this topic: Comment la compression affecte la performance des coussinets thermiques.

Why Compression Window Matters

A thermal pad must work across the full gap range.

If the minimum gap is very small, the pad may be over-compressed. If the maximum gap is large, the pad may not make enough contact.

This is called the compression window.

The selected pad thickness must survive all three conditions.

A Practical Thermal Pad Thickness Selection Method

A practical selection process looks like this:

• Measure the minimum, nominal, and maximum gap.
• Identify available clamping pressure or screw force.
• Choose candidate pad thicknesses that cover the maximum gap.
• Check compression at minimum and nominal gap.
• Review pad hardness and compression-deflection data.
• Estimate final bond line thickness.
• Compare thermal impedance at realistic pressure.
• Check mechanical stress and electrical insulation.
• Validate in the real assembly.
• Test after thermal cycling and aging.

This process takes more work than simply choosing a pad equal to the nominal gap, but it reduces field failures and production variation.

Example: Selecting Pad Thickness from Gap Range

Assume an electronic assembly has:

• Minimum gap: 0.85 mm
• Nominal gap: 1.00 mm
• Maximum gap: 1.20 mm
• Target compression: 15-30%

Possible pad choices:

In this case, a 1.2 mm pad may be acceptable only if the maximum gap still receives enough contact pressure. A softer 1.5 mm pad might work if the assembly can tolerate higher compression at the minimum gap. The final decision requires force, thermal, and mechanical validation.

Thermal Resistance and Pad Thickness

Thermal resistance increases as thickness increases. For a simplified heat path:

R = t / (k × A)

Où :

• R est la résistance thermique
• t is final pad thickness
• k est la conductivité thermique
• A est la zone de contact

This means pad thickness can override conductivity. A thick high-W/mK pad can perform worse than a thinner moderate-W/mK pad if the final bond line is much larger.

For a deeper explanation, see HakTak’s article: Pourquoi une conductivité thermique élevée (W/mK) ne signifie pas toujours de meilleures performances de refroidissement.

Conductivity Is Not Enough

A 10 W/mK pad is not automatically better than a 5 W/mK pad. If the 10 W/mK pad is too thick, too hard, or poorly compressed, its real thermal impedance may be higher.

Engineers should compare:

• Conductivité thermique
• Impédance thermique
• Épaisseur finale comprimée
• Pression de contact
• Compression range
• Mouillage de surface
• Stabilité à long terme

L'article de HakTak Conductivité thermique vs Impédance thermique dans la sélection des TIM explains why thermal impedance is often more useful than W/mK alone.

Pad Hardness and Thickness Selection

Thickness cannot be selected without hardness. A thick soft pad and a thick hard pad behave very differently.

Soft Pads

Soft thermal pads are useful when:

• Pressure is limited
• Components are fragile
• Surfaces are uneven
• PCB bending must be minimized
• Multiple component heights need coverage

Soft pads can fill gaps more easily, but they may be more delicate, more difficult to handle, or less dimensionally stable.

Hard Pads

Harder pads can be easier to handle and more dimensionally stable. They may work well when the gap is controlled and the assembly provides enough pressure.

However, hard pads can create stress if they are too thick or insufficiently compressed. They may also fail to conform to rough or uneven surfaces.

Thickness and Force Must Be Matched

The right question is:

Can this bloc-notes thickness reach the required final bond line thickness within the available force limit?

If the answer is no, the pad may look good on a datasheet but fail in the product.

Surface Flatness and Contact Area

Thermal pad thickness also depends on how flat and parallel the surfaces are.

If the heat sink and component are flat and parallel, a thinner pad may work. If the surfaces are tilted, warped, or rough, a thicker or softer pad may be needed to maintain contact.

Common causes of uneven contact include:

• Heat sink machining tolerance
• Enclosure deformation
• PCB warpage
• Component height variation
• Uneven screw torque
• Thermal expansion mismatch

If one side of the interface has a 0.5 mm gap and the other side has a 1.0 mm gap, a uniform pad may be compressed unevenly. This can create hot spots and mechanical stress.

For multi-component assemblies, thermal putty or gap filler may sometimes be better than a standard pad. HakTak’s article Pâte thermique ou pad thermique : comment choisir pour les espaces irréguliers explains this tradeoff.

Electrical Insulation and Thickness

Many thermal pads are electrically insulating. In power electronics, the pad may need to provide both heat transfer and dielectric isolation.

Thicker pads may increase dielectric strength, but they also increase thermal resistance. Thinner pads may improve thermal performance, but they must still meet insulation requirements.

Engineers should check:

• Rigidité diélectrique
• Résistivité transversale
• Breakdown voltage
• Creepage and clearance requirements
• Compression effects on insulation
• Aging after thermal cycling

C'est particulièrement important dans :

• EV inverters
• Alimentations électriques
• Modules IGBT
• MOSFET assemblies
• LED drivers
• Entraînements de moteurs industriels

When insulation is safety-critical, thermal selection must be coordinated with electrical safety requirements.

Application-Based Thickness Guidance

The table below gives general guidance. Final selection should always be validated in the real assembly.

When a Thermal Pad May Not Be the Best Choice

Thermal pads are useful, but they are not always the best TIM.

Choose thermal grease or paste when:

• The interface is very thin and flat
• Strong clamping pressure is available
• Clean handling is less important
• The lowest bond line thickness is needed

L'article de HakTak Conseils pour appliquer la pâte thermique et fonctionnement explains grease application considerations.

Choose PCM thermal pads when:

• You need pad-like handling
• The interface can reach activation temperature
• Better wetting is needed after heating
• Production repeatability matters

See PCM Thermal Pads Explained for more detail.

Choose thermal putty or gap filler when:

• Gaps are uneven
• Component heights vary
• Pressure is limited
• The assembly has complex geometry

Choose thermally conductive potting compounds when:

• Components need encapsulation
• Environmental protection is required
• Heat must move through a filled volume
• Mechanical support is needed

HakTak’s Guide to Thermally Conductive Potting Compounds in Electronics explains this category.

Testing Thermal Pad Thickness

Thermal pad selection should be confirmed by testing. Datasheet values are useful, but they cannot replace assembly validation.

Useful tests include:

• Gap measurement
• Compression-deflection testing
• Thermal impedance testing
• Device temperature testing
• Electrical insulation testing
• Cyclage thermique
• Vibration testing
• Compression set testing
• Rework evaluation

ASTM D5470 is commonly referenced for thermal transmission properties of thermally conductive electrical insulation materials and is widely relevant to TIM impedance testing. Engineers can review the official ASTM listing here: ASTM D5470.

For broader thermal property measurement of polymeric materials, ISO 22007-2 covers the transient plane heat source method for determining thermal conductivity and thermal diffusivity of plastics. The official ISO page is here: ISO 22007-2:2022.

L'article de HakTak Common TIM Testing Standards Engineers Should Know gives a practical overview of how these methods fit TIM selection.

Common Mistakes When Selecting Thermal Pad Thickness

The first mistake is matching pad thickness only to the nominal gap. Real assemblies have tolerance ranges.

The second mistake is choosing a thicker pad “just to be safe.” Excess thickness increases thermal resistance and may create mechanical stress.

The third mistake is ignoring hardness. A pad that is too hard may not compress enough, while a very soft pad may deform too much.

The fourth mistake is comparing thermal conductivity without checking final thickness. A high-W/mK pad can still perform poorly if the bond line is thick.

The fifth mistake is testing at unrealistic pressure. A pad tested at high lab pressure may not perform the same in a low-pressure device.

The sixth mistake is ignoring long-term compression set. A pad may lose contact pressure after aging or thermal cycling.

The seventh mistake is forgetting electrical insulation. A thinner pad may improve heat transfer but fail dielectric requirements.

HakTak Perspective

At HakTak, thermal pad thickness is treated as both a thermal and mechanical design decision. A pad must fill the gap, compress safely, maintain contact, meet insulation needs, and keep thermal resistance low over time.

For a reliable recommendation, engineers should provide:

• Minimum, nominal, and maximum gap
• Zone de contact
• Heat source power
• Surface materials
• Available pressure or screw torque
• Component height tolerance
• PCB stiffness
• Required insulation
• Plage de températures de fonctionnement
• Vibration and thermal cycling requirements
• Rework expectations
• Production assembly method

With this information, a supplier can recommend thickness, hardness, conductivity grade, compression range, and material type more accurately.

The best thermal pad thickness is not the thickest option that fills the gap. It is the thinnest option that fills the worst-case gap, compresses safely in the best-case gap, and maintains reliable thermal contact through the product life.

Conclusion

Selecting thermal pad thickness for electronics requires more than matching a pad to the nominal gap. Engineers must account for gap tolerance, compression ratio, pad hardness, final bond line thickness, thermal resistance, contact pressure, electrical insulation, and long-term reliability.

In general, thinner pads reduce thermal resistance, but only when they fully contact both surfaces. Thicker pads can handle larger gaps, but they increase thermal resistance and may create mechanical stress. The right pad thickness is the one that works across the full tolerance range while keeping the assembly thermally and mechanically stable.

For production electronics, the final choice should be validated through realistic compression, thermal impedance, device temperature, and reliability testing.

Foire aux questions

How do I choose thermal pad thickness?

Measure the minimum, nominal, and maximum gap, then choose a pad that fills the maximum gap while staying within the recommended compression range at the minimum gap.

Should a thermal pad be thicker than the gap?

Usually yes, the uncompressed pad is often slightly thicker than the gap so it can compress and make full contact. The amount depends on the pad’s recommended compression range.

What happens if a thermal pad is too thin?

If the pad is too thin, it may not contact both surfaces, leaving air gaps and increasing thermal resistance.

What happens if a thermal pad is too thick?

If the pad is too thick, it can increase thermal resistance, bend the PCB, stress components, deform the housing, or become difficult to assemble.

Does thicker thermal pad mean better cooling?

No. Thicker pads usually increase thermal resistance. A thicker pad is useful only when needed to fill the gap.

How much should a thermal pad compress?

There is no universal value. The correct compression depends on pad material, hardness, thickness, gap tolerance, and available pressure. Supplier data and assembly testing should be used.

Is thermal conductivity more important than thickness?

Both matter. Thermal conductivity helps, but final compressed thickness and contact quality strongly affect real thermal performance.

Can I stack thermal pads to increase thickness?

Stacking pads is usually not recommended because it adds extra interfaces and contact resistance. A single correctly selected pad is usually better.

Are soft thermal pads better for uneven gaps?

Soft pads can conform better under low pressure, but they may have handling or stability tradeoffs. For highly uneven gaps, thermal putty or gap filler may be more suitable.

Should thermal pad thickness be tested after aging?

Yes. Compression set, thermal cycling, and vibration can change contact pressure over time, so reliability testing is important.