Thermal Aging and Pump-Out: Common Failures in Thermal Grease Applications

Introduction: Why Reliability Matters in Thermal Grease Applications

Thermal grease plays a vital role in bridging microscopic air gaps between electronic components and heat sinks. By filling surface irregularities, it ensures efficient heat transfer and helps maintain a stable operating temperature. However, as electronic systems become more compact and power-dense, long-term reliability of the thermal interface has become just as important as initial thermal performance.

Over time, thermal grease can degrade due to thermal aging and mechanical stress, leading to failures such as dry-out, pump-out, cracking, or phase separation. These changes increase thermal resistance, reduce contact area, and can cause hot spots that shorten device lifespan. Maintaining consistent thermal conductivity over thousands of hours of operation is therefore critical for high-reliability applications like automotive electronics, power modules, and telecom systems.

Understanding Thermal Aging

Thermal aging refers to the gradual degradation of a thermal grease’s properties when exposed to elevated temperatures for extended periods. It is a chemical and physical process that affects both the base oil and the filler network.

Common mechanisms include:

• Oil separation, where the liquid phase migrates away from the filler, leaving behind a dried or crusted layer.

• Oxidation of silicone or synthetic oils, leading to viscosity changes and reduced flowability.

• Polymer crosslinking, which causes the material to harden and lose its ability to wet surfaces.

• Filler network breakdown, which disrupts thermal conduction paths.

As these processes progress, the thermal resistance (Rth) of the interface increases, and the grease loses its ability to reflow or conform under pressure. The rate of aging depends heavily on temperature, humidity, and exposure time—for example, a grease stored at 150°C for 1,000 hours may exhibit a 30–50% rise in Rth compared to its initial value.

Proper formulation and accelerated life testing are therefore essential to ensure stability under thermal load and reliable long-term performance.

The Pump-Out Phenomenon

Pump-out is another common failure mode in thermal grease applications, referring to the gradual displacement of grease from the interface during thermal cycling or mechanical vibration. This process leaves behind voids or uneven coverage, severely impairing heat transfer.

Pump-out typically occurs due to:

• Thermal expansion mismatch between the component and the heat sink, which pushes and pulls the grease during heating and cooling.

• Viscosity loss and oil migration at high temperatures, allowing the material to flow out of the contact zone.

• Mechanical stress or vibration, especially in automotive or industrial equipment, which forces the grease to move laterally.

Visual indicators include dry spots, void formation, or localized overheating in thermal maps. Once the grease is displaced, it rarely flows back, and contact resistance rises sharply. Pump-out is especially critical in assemblies with large surface areas or vertical orientations, where gravity or shear stress can accelerate material movement.

Factors Influencing Pump-Out and Aging

The long-term stability of thermal grease depends on multiple interrelated factors—both material-related and mechanical.

• Material formulation:
  The chemistry of the base oil, filler particle size, and thickener type determines how resistant the grease is to separation and flow. Low-volatility oils and high-consistency thickeners improve pump-out resistance but may increase viscosity.

• Surface roughness and contact pressure:
  Uneven surfaces create stress points where the grease can migrate under compression. Proper contact pressure ensures that the grease remains evenly spread and prevents localized displacement.

• Thermal cycling amplitude:
  The larger the temperature swing between high and low states, the greater the expansion-contraction mismatch—leading to gradual pumping over hundreds of cycles.

• Application thickness:
  A layer that is too thick tends to move or bleed under thermal stress, while a layer that is too thin cannot accommodate surface irregularities, increasing air gaps. Optimizing bond-line thickness is key to balancing stability and performance.

In short, thermal aging and pump-out are not isolated problems—they are the combined result of material design, interface geometry, and mechanical environment. Understanding these interdependencies allows engineers to select or design greases that maintain consistent performance throughout the product’s lifecycle.

Testing and Evaluation Methods

Evaluating the reliability of thermal greases requires accelerated testing that replicates real-world thermal and mechanical stresses.
One of the most common approaches is the thermal aging test, where samples are stored at 125–150°C for up to 1,000 hours. This test measures how the material’s thermal resistance and appearance change over time, simulating long-term operation in high-power environments.

To assess pump-out resistance, engineers perform temperature cycling tests, typically between –40°C and +125°C for several hundred cycles. Repeated expansion and contraction help reveal whether the grease migrates or loses coverage. Mechanical vibration tests are also used for automotive and industrial applications to simulate dynamic stress.

Standard testing references include:

• ASTM D5470: for precise measurement of thermal resistance (Rth) under defined pressure and temperature.

• ASTM D6184: to evaluate oil separation from grease under heat exposure.

Performance retention is measured by monitoring ΔRth (change in thermal resistance), visual inspection of material displacement, mass or weight loss, and microscopic observation of voids or surface dryness. Together, these evaluations help identify formulations that maintain consistent conductivity and mechanical stability over extended service life.

Engineering Solutions to Prevent Failures

Preventing thermal aging and pump-out starts with a balanced formulation design and proper application practices.

• Optimized filler particle distribution and rheology:
  Using a controlled mix of filler sizes improves particle packing and prevents oil bleed, maintaining a stable network even under repeated thermal cycling.

• Anti-bleed and anti-pump-out additives:
  Specialized polymers or surface treatments enhance oil retention and limit migration.

• Alternative TIMs for demanding environments:
  In high-cycling or vibration-prone systems (e.g., automotive ECUs or power inverters), thermal gels or phase-change materials (PCMs) may provide better long-term stability than conventional greases.

• Correct bond-line thickness and assembly pressure:
  Applying an appropriate thickness ensures uniform contact and minimizes internal shear stress. Over-compression can cause grease displacement, while under-compression may leave air voids.

• Regular validation under end-use conditions:
  Pre-production testing under realistic mounting pressures, cycling rates, and temperature gradients is the best way to verify stability before product release.

Through careful formulation and engineering validation, thermal grease performance can remain consistent even in harsh environments and extended duty cycles.

Case Examples and Comparative Insights

A practical example illustrates the importance of formulation design. In an automotive control unit operating under continuous thermal cycling, a standard thermal grease showed approximately 20% pump-out and a noticeable rise in Rth after 500 cycles. By reformulating with optimized filler distribution and anti-bleed additives, the pump-out was reduced to less than 5%, and thermal resistance remained stable throughout the test.

This comparison highlights the trade-off between cost, stability, and reworkability. For consumer products, standard grease may suffice, but for mission-critical or high-cycling environments, upgraded grease or gel-based TIMs offer significantly improved reliability and thermal consistency.

Conclusion

Both thermal aging and pump-out are driven by the combined effects of chemical degradation and mechanical stress. Over time, these mechanisms compromise thermal conductivity and create reliability risks in electronic systems.

To prevent such failures, engineers should adopt a preventive design approach—selecting materials with proven stability, validating under realistic conditions, and optimizing application parameters. Consistent performance cannot rely on initial conductivity alone; it must be sustained throughout the product’s entire lifetime.

For teams seeking improved long-term thermal performance or customized material solutions, our engineering team can help evaluate your application needs, recommend optimized TIMs, and provide sample testing for validation in your real-world environment.