In modern power electronics, especially in IGBT modules and MOSFET-based systems, thermal management is not a “supporting role” anymore—it is a core design constraint. As switching devices become smaller and power densities rise, engineers are forced to rethink how heat is transferred from the semiconductor junction to the heatsink.
Two commonly used solutions often come up in design discussions: ceramic sheets (ceramic insulators/substrates) et thermal pads (thermal interface materials). They may look similar in function at first glance—both sit between hot components and heatsinks—but in reality, they behave very differently.
This article breaks down their differences from a practical engineering perspective so you can choose the right solution for reliability, cost, and thermal performance.
Why Thermal Interfaces Matter in IGBT and MOSFET Systems
Before comparing materials, it helps to understand the problem they are solving.
In power modules, heat flows through several stages:
Junction → package → interface layer → heatsink → ambient air
At each interface, microscopic air gaps form due to surface roughness. These air gaps are terrible conductors of heat. In fact, trapped air acts like an insulator and dramatically increases thermal resistance.
That is why les matériaux d'interface thermique (MIT) exist—to replace air with something more thermally conductive and mechanically compliant. As research shows, TIMs are specifically designed to reduce contact résistance thermique between mating surfaces and improve heat dissipation efficiency.
In IGBT systems, this is especially critical because most failures are thermally induced rather than electrical.
What Is a Thermal Pad?
A coussin thermique is a soft, pre-formed sheet made from silicone-based or non-silicone elastomers filled with thermally conductive particles.
It is designed to:
- Fill air gaps between uneven surfaces
- Fournir une isolation électrique
- Be easy to assemble in mass production
- Absorb mechanical tolerance variations
Thermal pads are widely used in MOSFET VRMs, IGBT driver boards, and general power electronics assemblies because of their convenience.
From a thermal standpoint, they are a compliance material—meaning they deform under pressure to improve contact between surfaces.
However, their performance depends heavily on:
- Épaisseur
- Hardness (Shore OO)
- Thermal conductivity (W/m·K)
- Compression force applied
Even under ideal conditions, thermal pads generally have moderate conductivité thermique, and their performance can vary significantly with pressure and aging.
What Is a Ceramic Sheet?
A ceramic sheet (often aluminum oxide Al₂O₃ or aluminum nitride AlN) is a rigid insulating substrate used between semiconductor devices and heatsinks.
Unlike thermal pads, ceramic sheets are:
- Hard and non-compressible
- Extremely thermally conductive (especially AlN)
- Excellent electrical insulators
- Mechanically stable under high temperature and voltage
Typical thermal conductivity ranges:
- Alumina (Al₂O₃): ~15–35 W/m·K
- Aluminum nitride (AlN): ~170–230 W/m·K
This is a major leap compared to standard polymer-based thermal pads.
Ceramic sheets are often used in:
- High-voltage IGBT modules
- EV inverter stacks
- Industrial motor drives
- High-reliability MOSFET power stages
They are not just TIMs—they are also structural and dielectric components in many designs.
Key Difference: Compliance vs Structural Function
The most important distinction is this:
Thermal pads = compliant interface material
Ceramic sheets = structural thermal insulator
Coussinets thermiques adapt to surfaces. Ceramic sheets require precise surfaces.
That single difference drives almost every other design decision.
If surfaces are not perfectly flat, ceramic sheets cannot fill gaps effectively. Thermal pads, however, deform and compensate.
So:
- Thermal pad = forgiving
- Ceramic sheet = precise but high performance
Thermal Conductivity: The Biggest Performance Gap
Heat transfer efficiency is where ceramic sheets clearly outperform thermal pads.
Thermal pads rely on polymer matrices, which inherently limit heat flow. Even high-end pads struggle to reach the performance of ceramics.
Ceramic materials like AlN allow near-metal-level heat transfer while maintaining electrical insulation.
In practical systems:
- Thermal pads: suitable for moderate power dissipation
- Ceramic sheets: designed for high power density modules
This is why ceramic substrates are common in modern IGBT modules used in EV traction inverters.
Electrical Insulation and Safety Considerations
Both materials provide electrical insulation, but in different ways:
Coussinets thermiques :
- Good dielectric strength
- Flexible but can degrade over time
- Performance depends on compression stability
Ceramic sheets:
- Extremely high dielectric breakdown strength
- Stable under high voltage and temperature
- No mechanical creep or deformation
For high-voltage switching environments (like 600V–1200V IGBT systems), ceramics provide a more predictable insulation barrier.
Mechanical Behavior Under Pressure
This is where thermal pads dominate.
Coussinets thermiques :
- Compress easily
- Adapt to uneven surfaces
- Absorb vibration and thermal expansion mismatch
Ceramic sheets:
- Require flat, machined mounting surfaces
- Brittle under mechanical stress
- Depend on precise mounting torque and flatness
In real-world manufacturing, this means thermal pads are easier and cheaper to implement.
Ceramic sheets require stricter assembly control.
Fiabilité et stabilité à long terme
Over long operating cycles, especially in power electronics, materials age differently.
Thermal pads may:
- Pompage sous cyclage thermique
- Dry or harden over time
- Lose performance due to compression set
Ceramic sheets:
- Do not degrade thermally
- Maintain performance over decades
- Are resistant to thermal cycling fatigue
This is why ceramics are often chosen for automotive-grade and industrial-grade systems where maintenance is impossible.
Cost and Manufacturing Trade-Offs
Cost plays a major role in material selection.
Coussinets thermiques :
- Low cost
- Easy automation
- Suitable for mass production consumer electronics
Ceramic sheets:
- Higher material and machining cost
- Require precision assembly
- Used in high-value systems (EV, industrial drives, aerospace)
So the decision is often not purely technical—it is economic.
Application Scenarios: When to Use What
Utilisez des coussins thermiques lorsque :
- Surfaces are not perfectly flat
- Cost is a major constraint
- Power density is moderate
- You need fast assembly and tolerance compensation
Typical cases:
- Consumer power supplies
- Motherboard MOSFET cooling
- Low-to-mid power converters
Use ceramic sheets when:
- High voltage insulation is critical
- Thermal resistance must be minimized
- Long-term reliability is required
- Mechanical precision is available
Typical cases:
- IGBT traction inverters
- EV power modules
- High-power industrial MOSFET systems
Hybrid Designs (The Real Industry Solution)
In practice, engineers often combine both technologies.
A common stack might look like:
IGBT → graisse thermique → ceramic substrate → thin thermal coussinet → heatsink
This hybrid approach balances:
- Ceramic performance
- Pad compliance
- Manufacturing tolerances
It reflects a simple reality: no single material solves all constraints.
Brève conclusion
Thermal pads and ceramic sheets are not competitors—they are solutions for different engineering problems.
Thermal pads prioritize flexibility, cost efficiency, and ease of assembly. Ceramic sheets prioritize thermal performance, electrical insulation, and long-term stability.
Choosing between them depends on whether your design is driven by manufacturing simplicity or high-power reliability.
FAQ
Are ceramic sheets better than thermal pads?
Yes for performance, but not always practical. Ceramics are better thermally and electrically, but less flexible.
Can thermal pads replace ceramic insulators?
Only in low-to-medium power designs. They cannot match ceramic thermal or voltage performance.
Why are thermal pads still widely used?
Because they are cheap, easy to apply, and can handle uneven surfaces.
Do ceramic sheets need thermal paste?
Often yes, to reduce micro air gaps and improve contact efficiency.
Which is better for IGBT modules?
Ceramic sheets are generally preferred for high-power IGBT modules, especially in EV and industrial systems.