Thermal Pads for IGBT Modules: What Engineers Should Check

Engineers should check thermal impedance, dielectric strength, final bond line thickness, compression force, gap tolerance, surface flatness, operating temperature, and reliability before choosing thermal pads for IGBT modules. A thermal pad for an IGBT module must move heat into the heat sink or cold plate. It may also need to provide electrical insulation. It must do both without adding too much mechanical stress.

Do not select an IGBT module thermal pad by thermal conductivity alone. A high-W/mK pad can still fail if it is too thick, too hard, under-compressed, over-compressed, or weak in dielectric performance.

The best thermal pad is the one that gives stable low thermal impedance, safe electrical isolation, and repeatable contact after mounting, thermal cycling, and long-term operation.

Why IGBT Modules Need Careful Thermal Pad Selection

IGBT modules are used where power levels are high. You find them in EV inverters, solar inverters, industrial motor drives, UPS systems, welding equipment, railway systems, and large power converters. These systems switch heavy current. They also create heat.

Heat must leave the semiconductor junction and pass through the module stack. It then moves into a heat sink, cold plate, baseplate, or housing. The thermal interface between the module and the cooling surface is a small part of the system. But it can become a large bottleneck.

An IGBT module thermal pad is often used when the interface needs one or more of these functions:

• Gap filling
• Elektrische Isolierung
• Controlled thickness
• Cleaner mounting than grease
• Repeatable assembly
• Lower stress than hard insulators
• Die-cut shape control

Power modules are not forgiving. A poor thermal interface can raise junction temperature. It can also create hot spots. Over time, that stress can reduce module life.

Das Semikron-Danfoss Application Manual Power Semiconductors is a useful industry resource because it treats power modules as full thermal, electrical, and mechanical systems, not as isolated components. That is also the right way to think about thermal pads.

Thermal pads for IGBT modules vs thermal grease

Thermal grease has been common in power modules for a long time. It can form a thin bond line. It also wets rough surfaces well. But grease can be messy. It can pump out. It can vary by application method. It also gives little control over bond line thickness unless the process is tightly managed.

Thermal pads provide a pre-formed interface. They are easier to place. They can provide insulation. They can be die-cut. They also make assembly cleaner. The tradeoff is thickness. Pads usually create a thicker interface than grease.

Thermal pads are a good fit when clean assembly, insulation, and repeatability matter. Grease may still be better when the interface is very flat, tightly clamped, and the goal is the thinnest possible bond line. HakTak’s guide on why high W/mK does not always mean better cooling performance explains why the real interface matters more than one datasheet number.

IGBT module thermal pad selection starts with power loss

Start with the heat load. Without that number, pad selection becomes guesswork.

Estimate IGBT module heat loss before choosing a thermal pad

IGBT module heat comes from several sources. The main ones are conduction loss and switching loss. Diodes inside the module may also add loss. Gate drive and layout choices can affect the final number.

Engineers should define:

• Total module loss in watts
• Loss per switch or phase leg
• Peak load condition
• Continuous load condition
• Coolant or ambient temperature
• Maximum junction temperature
• Heat sink or cold plate performance

The pad does not cool the module alone. It is part of a chain. But if the pad adds too much thermal resistance, the whole system runs hotter.

Map the IGBT heat path through the thermal interface

A simplified heat path may look like this:

1. IGBT junction
2. Chip attach and substrate
3. Module baseplate or module bottom
4. Wärmeleitpad
5. Heat sink or cold plate
6. Air or liquid cooling

Every layer adds resistance. The pad may look thin, but it sits across the full heat flow. That makes its thickness, contact, and compression important.

Use thermal impedance for IGBT module pads

Thermal conductivity is useful for screening. Thermal impedance is more useful for the final choice.

Thermal impedance includes the real pad thickness and the contact condition. That matters because a pad with a good W/mK value can still perform badly if it is too thick or does not compress well.

ASTM D5470 is often used when engineers need thermal transmission data for thermally conductive electrical insulation materials. It is relevant for TIMs used between power modules and heat sinks because it focuses on thermal impedance and apparent conductivity under controlled conditions. You can review the official method page at ASTM D5470.

HakTak also explains this in Wärmeleitfähigkeit vs. Wärmewiderstand bei der TIM-Auswahl.

Insulating thermal pads for IGBT modules

Electrical insulation is often required in power modules. Sometimes the module itself has an insulated baseplate. Sometimes it does not. Sometimes the system still needs an extra insulating layer for safety or design reasons.

Do not assume the pad is insulating. Check the data.

Check dielectric strength for IGBT thermal pads

Dielectric strength tells you how much electric field the material can withstand before breakdown. It is often listed as kV/mm or a similar unit.

For solid insulating materials, ASTM D149 is a common reference for dielectric breakdown voltage and dielectric strength. In an IGBT module design, this matters because a thermal pad may sit between a conductive module surface and a grounded or conductive cooling surface.

Important questions:

• What voltage must be isolated?
• Is the heat sink grounded?
• Is the cold plate conductive?
• Is the module base electrically isolated?
• Is there a safety margin for transients?
• What happens after compression and aging?

Breakdown voltage depends on final pad thickness

The final compressed thickness matters. A pad may be rated at a nominal thickness. After mounting, it may be thinner.

If the pad is over-compressed, dielectric margin can drop. If the pad is too thick, thermal resistance rises. This is one of the central tradeoffs.

HakTaks Artikel Elektrisch isolierende Wärmeleitpads: Wann braucht man sie? covers this point in more detail.

Do not trade safety for lower thermal resistance

A thinner pad often improves thermal performance. That does not mean it is safe. If the design requires insulation, the pad must meet that requirement after compression, temperature exposure, and aging.

Thermal design cannot be separated from electrical design here.

IGBT module thermal pad thickness

Thickness is one of the first things engineers check. It is also one of the easiest things to get wrong.

Measure the IGBT module to heat sink gap

Use the real assembly if possible. CAD dimensions are not enough.

Include:

• Module baseplate tolerance
• Heat sink flatness
• Cold plate flatness
• Surface roughness
• Mounting screw torque
• Housing tolerance
• Thermal expansion
• Any mechanical stops

Measure the minimum, nominal, and maximum gap.

Choose the thinnest IGBT thermal pad that fills the full gap

The pad should contact both surfaces at the maximum gap. It should not be crushed at the minimum gap.

The rule is practical:

Use the thinnest pad that fills the worst-case gap and stays within the safe compression range.

HakTaks Artikel Auswahl der Dicke von Wärmeleitpads für Elektronik gives a useful workflow for gap range and pad thickness.

Final bond line thickness controls thermal resistance

Bond line thickness is the final thickness after mounting. Heat must pass through that layer.

For a simple estimate:

R = t / (k × A)

Wo

• R ist thermischer Widerstand
• t is final thickness
• k ist Wärmeleitfähigkeit
• A Kontaktfläche

The equation is simple. The assembly is not. Contact resistance also matters. Surface roughness and pressure can change the result.

HakTak’s guide Wie sich die Dicke der Klebefuge auf die thermische Leistung auswirkt explains why final thickness is more important than nominal sheet thickness.

Thermal pad compression for IGBT modules

Thermal pads need compression. Without compression, the pad may not fill surface roughness. But too much compression can damage the system.

Under-compression leaves air gaps

Too little compression can cause:

• Poor contact
• High thermal impedance
• Hotspots
• Unstable test results
• Unit-to-unit variation
• Poor use of the heat sink

This often happens when the pad is too thin, too hard, or used with low mounting force.

Over-compression can damage power modules

Too much compression can cause:

• Module stress
• Baseplate bending
• Kissenextrusion
• Reduced dielectric spacing
• Wohnungsmarktverzerrung
• Uneven pressure
• Long-term compression set

An IGBT module is not a clamp for squeezing a pad as hard as possible. The goal is controlled contact.

Calculate thermal pad compression ratio for IGBT modules

Calculate compression at minimum, nominal, and maximum gap. Do not calculate it only once.

HakTaks Artikel Wärmeleitpad-Kompressionsverhältnis: Wie viel ist genug? explains the formula and the tolerance problem.

Soft vs hard thermal pads for IGBT modules

Hardness affects how much force is needed to compress the pad. It also affects contact quality.

Soft IGBT thermal pads for low-pressure interfaces

Soft pads can help when pressure is limited. They also handle small flatness errors better.

Soft pads are useful when:

• The heat sink is not perfectly flat
• The module should see lower stress
• The gap tolerance is wider
• The mounting force is limited
• Low contact resistance is needed at lower pressure

Harder IGBT thermal pads for controlled mounting

Harder pads can be easier to handle. They can hold their shape better. They may work well when the gap and pressure are controlled.

Harder pads are useful when:

• The heat sink is flat
• Mounting pressure is predictable
• Die-cut placement matters
• Rework cleanliness matters
• Dimensional control is important

Hardness should not be guessed by hand feel. ASTM D2240 is a standard reference for durometer hardness of rubber-like materials, and it is useful when comparing elastomeric pads. You can check the official page at ASTM D2240. HakTak’s guide Weiche vs. harte Wärmeleitpads: Welche sind besser? gives a more practical selection view.

Key thermal pad properties for IGBT modules

IGBT thermal pads in EV inverters and motor drives

EV inverters and motor drives are demanding. They see load changes. They see heat cycles. They may see vibration. They also need high reliability.

Thermal pads for EV IGBT modules

EV systems need stable heat transfer. A pad may need to survive:

• Temperaturwechselbeanspruchung
• Vibration
• High voltage
• Long operating life
• Coolant temperature swings
• Mechanical assembly variation

In this use case, initial thermal resistance is not enough. Aging data matters.

Thermal pads for industrial IGBT modules

Industrial drives may run for long hours. They may sit in cabinets with limited airflow. They may see dust, heat, and vibration.

The pad should be checked for:

• Long-term compression stability
• Thermal aging
• Dielectric performance
• Mounting repeatability
• Heat sink flatness sensitivity

IGBT thermal pads vs phase change materials, putty, and potting

Thermal pads are not the only TIM for power modules.

Phase change materials for IGBT modules

Phase change materials soften when the module heats up. They can improve wetting after activation. They can also be cleaner than grease.

HakTaks Artikel PCM Thermal Pads Explained explains how phase change materials behave at operating temperature.

Thermal putty for uneven power electronics gaps

Putty can help when the interface is not flat or when component heights vary. A fixed sheet pad may not handle that as well.

For uneven interfaces, HakTak’s article Wärmeleitpaste vs. Wärmeleitpad: Wie man bei ungleichmäßigen Spalten die richtige Wahl trifft is directly relevant.

Potting compounds for encapsulated power electronics

Potting compounds are used when components need encapsulation, protection, and heat transfer through a filled volume. They are not a simple pad replacement. They change the assembly and rework process.

Testing thermal pads for IGBT modules

Testing should match the real system. A flat lab coupon is useful, but it does not replace module-level testing.

Test thermal impedance at realistic pressure

The pad should be tested at a pressure that resembles the real mounting condition. If the datasheet value was measured at high pressure, it may not match your assembly.

Test dielectric strength after compression

If the pad provides insulation, test after compression or at an equivalent final thickness. Also test after heat aging if the application is severe.

Test thermal cycling and power cycling

IGBT modules heat and cool during operation. The pad must maintain contact.

Test:

• Temperaturwechselbeanspruchung
• Power cycling
• High-temperature aging
• Vibration
• Humidity if relevant
• Druckverformungsrest
• Überarbeiten

For polymer thermal property testing, ISO 22007-2 covers the transient plane heat source method for thermal conductivity and diffusivity. It can support material screening, but final IGBT module validation still needs the real assembly.

Common mistakes when selecting thermal pads for IGBT modules

The first mistake is choosing by W/mK alone. Thermal impedance, thickness, and pressure often matter more.

The second mistake is ignoring dielectric strength. IGBT systems can involve high voltage. Insulation must be proven.

The third mistake is using nominal gap only. Minimum and maximum gap drive real compression.

The fourth mistake is choosing a pad that is too thick. This can raise thermal resistance.

The fifth mistake is choosing a pad that is too hard. It may not compress enough.

The sixth mistake is over-compressing the pad. That can reduce dielectric margin and add stress.

The seventh mistake is skipping thermal cycling. IGBT modules rarely live at one steady temperature.

The eighth mistake is treating the pad as a fix for poor heat sink flatness. The mechanical design still matters.

H2: Selection checklist for IGBT module thermal pads

HakTak perspective

At HakTak, we treat IGBT thermal pads as part of the full power module design. The pad is not just a sheet of material. It must match the heat load, voltage requirement, gap, mounting force, and lifetime target.

For an accurate recommendation, engineers should provide:

• IGBT module type
• Heat loss or power dissipation
• Contact area
• Heat sink or cold plate material
• Minimum, nominal, and maximum gap
• Required dielectric strength or breakdown voltage
• Available mounting pressure or torque
• Heat sink flatness
• Operating temperature range
• Thermal cycling requirement
• Vibration or shock requirement
• Rework requirement
• Production volume and placement method

With this data, HakTak can recommend pad thickness, hardness, conductivity grade, dielectric rating, and compression range. The best answer is rarely just “use the highest W/mK.” It is usually a balance between thermal impedance, insulation, stress, and reliability.

Fazit

Thermal pads for IGBT modules need careful selection. The pad must move heat into the heat sink or cold plate. It may need to provide electrical insulation. It must compress correctly. It must survive heat, time, and cycling.

Start with heat loss, voltage isolation, contact area, and gap range. Then compare thermal impedance, thickness, hardness, compression, dielectric strength, and reliability data.

For IGBT modules, a thermal pad is not a small accessory. It is part of the power electronics reliability path. Choose it like an engineering component, not a catalog filler.

Häufig gestellte Fragen

Do IGBT modules need thermal pads?

Some IGBT modules use thermal grease, while others can use thermal pads or phase change materials. Pads are useful when clean assembly, controlled thickness, insulation, or repeatability is needed.

What is the best thermal pad for an IGBT module?

The best pad depends on heat loss, contact area, voltage isolation, gap size, compression force, thickness, hardness, and reliability requirements.

Should I choose an insulating thermal pad for an IGBT module?

Choose an insulating pad when the interface must block current between the module and a conductive heat sink, housing, or cold plate.

Is higher W/mK always better for IGBT module thermal pads?

No. Higher W/mK helps only when thickness, pressure, contact quality, and dielectric requirements are also correct.

How thick should an IGBT thermal pad be?

It should be thick enough to fill the maximum gap and thin enough to limit thermal resistance. Check final compressed thickness.

How much compression does an IGBT thermal pad need?

It needs enough compression for full contact, but not so much that it stresses the module or reduces dielectric margin.

Can thermal grease replace a thermal pad for IGBT modules?

Yes, in some flat and well-clamped interfaces. A pad may be better when insulation, clean assembly, or thickness control is needed.

Can thermal putty replace an IGBT thermal pad?

It can help in uneven or irregular gaps. For flat module-to-heat-sink interfaces, pads or grease are often easier to control.

What tests should engineers run before production?

Test thermal impedance, module temperature, dielectric strength after compression, thermal cycling, vibration, aging, and final assembly repeatability.

What data should I send to a thermal pad supplier?

Send heat loss, contact area, gap range, voltage requirement, heat sink material, pressure or torque, temperature range, and reliability conditions.