AI data centre cooling is creating new formulation challenges for manufacturers of heat-transfer fluids, thermal interface materials, silicone compounds, encapsulants and electronic protection systems.
As accelerator power densities increase, the materials responsible for transferring heat away from high-performance computing components become increasingly important to system reliability.
A liquid-cooling loop may provide sufficient flow yet experience corrosion, particulate formation or coolant degradation. A thermal interface material may demonstrate excellent initial thermal conductivity but develop pump-out or oil bleeding after repeated power cycling. A thermally conductive gap filler may meet its thermal specification but impose excessive mechanical stress on delicate components.
These problems are connected by a common engineering challenge: thermal performance must be maintained across the entire material system, not merely demonstrated through an individual laboratory measurement.
For formulation scientists, this means that coolant chemistry, filler morphology, polymer-network design, viscosity, dielectric behaviour, material compatibility and long-term stability must be considered together.
This technical guide examines the most important material-level problems in AI data centre cooling and provides practical troubleshooting strategies for developing more reliable coolants, thermal interface materials, dielectric fluids and protective polymer systems.
1. Which Thermal Management Materials Are Used in AI Data Centres?
Modern AI computing systems use several thermal-management architectures, and the required material properties vary considerably between them.
A useful starting point is to distinguish direct-to-chip liquid cooling, immersion cooling and the thermal materials used within electronic packages.
Direct-to-chip liquid cooling
In direct-to-chip cooling, a liquid coolant circulates through cold plates thermally connected to processors, accelerators or other high-power components.
Heat passes from the electronic component through its package and thermal interfaces into the cold plate before being transferred to the circulating liquid.
Typical material considerations include:
Coolant heat capacity, density and viscosity.
Corrosion inhibition and metal compatibility.
Microbial and particulate contamination control.
Elastomer, seal and polymer compatibility.
Cold-plate material selection.
Long-term fluid stability.
The coolant usually circulates within a contained liquid loop rather than directly surrounding the electronic circuitry.
Consequently, a fluid suitable for direct-to-chip cooling is not automatically appropriate for immersion cooling.
Single-phase immersion cooling
In single-phase immersion cooling, suitable electronic components are immersed in a dielectric fluid that remains liquid during normal operation.
Heat is transferred to the surrounding fluid, which is circulated or otherwise cooled through an external heat-removal system.
The fluid must combine appropriate thermal and flow properties with electrical insulation, chemical stability and compatibility with the immersed materials.
Two-phase immersion cooling
Two-phase systems use a dielectric fluid that undergoes boiling and condensation within the intended operating conditions.
The fluid's boiling characteristics, latent heat, dielectric performance and chemical stability become especially important.
The material must also remain compatible with the electronic assembly and the system's seals, plastics and other components.
Thermal interface and protective materials
Regardless of whether air, liquid or immersion cooling is used, semiconductor packages may require:
Thermal interface greases and pastes.
Phase-change thermal interface materials.
Thermal pads and gap fillers.
Thermally conductive adhesives.
Electronic encapsulants.
Underfills and protective compounds.
These materials perform different functions.
A TIM primarily reduces thermal resistance between mating surfaces. A gap filler accommodates larger geometric gaps. An underfill can improve the mechanical reliability of electronic interconnections. An encapsulant provides protection and may assist thermal management when specifically formulated for that purpose.
Treating them as interchangeable can lead to inappropriate material selection.
2. Water vs Glycol: What Is the Best Coolant for AI Data Centre Liquid Cooling?
Selecting a coolant for direct-to-chip cooling requires more than choosing the liquid with the highest thermal conductivity.
Water provides attractive heat-transfer properties, including relatively high specific heat capacity and low viscosity.
However, long-term operation also depends on corrosion control, microbial stability, water quality and compatibility with the entire wetted-material system.
Water-based formulations containing ethylene glycol or propylene glycol may offer useful freeze protection and other system-specific advantages.
The trade-off is that increasing glycol concentration generally reduces the mixture's heat capacity and increases viscosity compared with water at a given temperature.
The practical consequences include changes in pumping requirements and heat-transfer capability.
Why coolant heat capacity matters
The sensible heat transported by a liquid can be estimated from:
Q = ṁ × Cp × ΔT
Where:
Q is heat-transfer rate.
ṁ is coolant mass-flow rate.
Cp is specific heat capacity.
ΔT is the temperature difference between coolant inlet and outlet.
For the same heat load and permitted coolant temperature rise, a fluid with lower specific heat capacity requires a greater mass-flow rate.
However, the final hydraulic requirement also depends on fluid density, viscosity, channel geometry and system design.
This is why coolant formulation must be evaluated alongside cold-plate and circulation-system requirements.
Why glycol concentration should not be maximized
A common mistake is increasing glycol content without establishing whether the application actually requires additional freeze protection.
Higher glycol concentration may increase viscosity and pumping energy requirements.
It may also alter inhibitor performance, elastomer compatibility and long-term degradation behaviour.
The correct glycol concentration should be selected according to the system's operational and storage conditions, approved coolant specification and overall reliability requirements.
Comparing coolant platforms
| Coolant platform | Potential advantages | Main considerations |
|---|---|---|
| Treated water | Strong heat-transfer properties and relatively low viscosity | Corrosion, water quality, microbial control, particulate contamination |
| Water/propylene glycol | Freeze protection and flexible formulation possibilities | Reduced heat capacity, increased viscosity, inhibitor compatibility |
| Water/ethylene glycol | Effective freeze protection and established industrial experience | Toxicity considerations, viscosity, corrosion protection |
| Dielectric liquid for appropriate specialized loops | Electrical insulation for specific cooling architectures | Lower heat-transfer properties in many systems, materials compatibility and fluid specification |
Practical recommendation: Choose the coolant using the actual heat-transfer and reliability requirements of the cooling loop. Do not substitute an automotive coolant or generic industrial heat-transfer fluid without verifying that its complete additive package and performance are compatible with the intended data-centre system.
3. Why Does Copper Cold-Plate Corrosion Occur in Liquid-Cooled AI Servers?
Corrosion is one of the most important material-reliability concerns in liquid-cooled computing systems.
Copper and copper alloys are widely used in cold plates because of their favorable thermal conductivity and manufacturability.
However, a cooling system may contain other wetted materials, including stainless steel, aluminum, plated metals, polymers and elastomers.
This combination creates a more complicated chemical environment than a simple copper-and-water laboratory test.
Common contributors to cold-plate corrosion
Dissolved oxygen and fluid chemistry
Oxygen availability, pH, dissolved ions and other coolant constituents influence electrochemical corrosion processes.
The severity depends on the metal, surface condition, operating temperature and chemical environment.
Mixed-metal interactions
Where dissimilar metals are electrically connected through an electrolyte, galvanic corrosion may become relevant.
However, simply finding two metals in a system does not prove galvanic corrosion is occurring.
The actual electrical connection, fluid conductivity, electrochemical potential and wetted geometry must be considered.
Inadequate inhibitor protection
A corrosion inhibitor may perform well on one metal while providing insufficient protection for another.
Inhibitor concentration, compatibility and long-term stability must be established across the complete wetted-material system.
Contamination
Chlorides, aggressive ions, manufacturing residues and cleaning chemicals can change corrosion behaviour.
Particle contamination can also contribute to localized problems or indicate material degradation elsewhere in the loop.
Why deionized water is not automatically corrosion-free
Deionized water can have very low initial ionic conductivity.
However, this condition may change after contact with metals, polymers, seals, fittings and other system materials.
Ions can enter the circulating fluid through dissolution, leaching, corrosion or external contamination.
Therefore, low initial conductivity does not guarantee long-term chemical compatibility.
Practical corrosion troubleshooting
If a coolant develops discoloration, suspended particles or unexpected changes in conductivity, investigate systematically.
Begin by measuring:
pH.
Electrical conductivity.
Relevant dissolved metal concentrations.
Particulate contamination.
Inhibitor concentration or condition, where measurable.
Fluid appearance and oxidation indicators.
Use an appropriate analytical method, such as ICP-OES or ICP-MS, to determine whether dissolved copper, aluminum, iron or other metals are increasing.
Microscopy and elemental analysis of collected particles may help establish whether deposits originate from corrosion, manufacturing debris or another source.
Compare the results with retained fresh coolant and the system's baseline specification.
How to improve corrosion protection
A useful development approach evaluates the complete inhibitor package rather than one inhibitor in isolation.
Potential formulations may include compatible corrosion-inhibiting chemistries, pH-control components and other necessary stabilizers.
However, additives must be evaluated for solubility, deposit formation, thermal stability and compatibility with the coolant-loop materials.
An inhibitor that protects copper but produces insoluble deposits in narrow channels may create another reliability problem.
The objective is sustained corrosion control without compromising coolant cleanliness, hydraulic performance or material compatibility.
4. Why Does Data Centre Coolant Become Cloudy or Form Deposits?
Cloudiness and particulate formation can interfere with the performance of direct-to-chip cooling systems.
In narrow cold-plate passages, particulate contamination or deposited material may contribute to flow restriction and increased pressure drop.
However, not every cloudy coolant is experiencing corrosion.
Possible mechanisms include:
Inhibitor precipitation.
Incompatible additive interactions.
Corrosion-product formation.
Microbial growth.
Residual manufacturing contamination.
Polymer or elastomer degradation.
Contamination introduced during filling or maintenance.
How to identify the source
The investigation should begin with physical and chemical characterization rather than immediate additive correction.
Collect representative samples using clean, compatible containers and a controlled sampling procedure.
Measure particle concentration and distribution where appropriate.
Separate a portion of the suspended material for analysis.
FTIR, elemental analysis, microscopy, thermal methods or other suitable techniques may help identify the particles.
For example, a deposit rich in copper may support a corrosion-related hypothesis.
An organic deposit containing components of the inhibitor package may indicate additive precipitation or degradation.
Where biological contamination is possible, use validated microbiological methods.
Why adding more inhibitor may make the problem worse
If precipitation is caused by incompatibility or excessive additive concentration, introducing more corrosion inhibitor can increase deposits rather than restore performance.
Similarly, uncontrolled pH adjustment can destabilize certain additive systems.
The correct solution depends on the identified material.
A robust coolant formulation must remain stable under expected temperature, concentration and contamination conditions, not only when freshly blended.
5. How Do You Test Coolant Compatibility With Elastomers, Seals and Plastics?
A coolant may show excellent thermal properties and adequate corrosion resistance while still creating problems with hoses, O-rings, gaskets and other polymeric components.
The relevant interactions depend on fluid chemistry and the elastomer or plastic formulation.
A material identified as EPDM, for example, is not a single universal chemical composition.
Different grades can contain different fillers, plasticizers, curing systems and stabilizers.
Common compatibility mechanisms
Swelling
Fluid absorption can increase the dimensions or mass of a polymer.
Excessive swelling may affect sealing performance or mechanical integrity.
Extraction
Coolant constituents may extract plasticizers or other ingredients from a polymer.
This can lead to changes in hardness, flexibility or dimensional behaviour.
Chemical degradation
Some polymers may experience chemical changes under prolonged exposure to incompatible fluids, elevated temperature or aggressive additives.
Loss of mechanical properties
Changes in tensile strength, elongation, compression set or hardness may affect long-term reliability.
Practical compatibility study
Prepare representative polymer specimens using the actual production-grade materials.
Expose them to the candidate coolant under controlled conditions.
Compare exposed samples with untreated references.
Measure:
Mass change.
Dimensional change.
Hardness.
Tensile properties where appropriate.
Compression set for sealing applications.
Visual evidence of cracking, swelling or degradation.
Testing should be performed at relevant temperatures and exposure durations.
Where feasible, evaluate the actual assembled sealing configuration in addition to simple material coupons.
A material may demonstrate acceptable mass change but still produce unacceptable leakage because of changes in compression behaviour.
Do not assume that a published generic compatibility chart is sufficient to qualify a specific coolant–seal combination.
6. What Makes a Good Dielectric Fluid for AI Data Centre Immersion Cooling?
Immersion cooling introduces a different set of material requirements because the fluid may be in direct contact with electronic components.
The coolant must provide the required electrical insulation while transferring heat and maintaining compatibility with the immersed assembly.
Important characteristics include:
Dielectric breakdown strength.
Volume resistivity.
Thermal properties.
Viscosity.
Chemical and oxidation stability.
Flash point and fire-safety characteristics.
Material compatibility.
Environmental and regulatory considerations.
Single-phase dielectric fluid selection
Single-phase immersion systems may use suitable engineered hydrocarbon fluids, synthetic esters or other qualified dielectric-liquid platforms.
These technologies have different property balances.
Low viscosity may improve circulation and heat transfer, but viscosity alone does not establish thermal performance.
Specific heat capacity, density, thermal conductivity and operating-temperature dependence must also be considered.
For hydrocarbon-based fluids, oxidation stability and fire-safety requirements deserve attention.
For ester-based fluids, chemical stability, moisture sensitivity and material compatibility depend on the particular fluid chemistry.
Two-phase dielectric fluid selection
Two-phase immersion technology relies on vaporization and condensation.
The fluid must have an appropriate boiling behaviour under the system's actual operating conditions.
Additional considerations include vapor pressure, latent heat, materials compatibility, fluid loss control, condensation performance and safety.
Historical use of fluorinated fluid chemistries has also made environmental persistence, regulatory compliance and supply-chain availability important selection criteria.
Alternative fluids should not be selected simply because they match the boiling point of a previous product.
The complete safety, thermal, electrical, materials and environmental requirements must be requalified.
Why dielectric fluid resistivity changes during operation
A fluid may meet its electrical specification when new but change after prolonged contact with the immersed system.
Possible causes include contamination, moisture ingress, degradation products and extraction of substances from components.
The resulting changes may affect fluid resistivity or other electrical characteristics.
A material compatibility programme must therefore examine both sides of the interaction:
Does the fluid damage the component?
Does the component change the fluid?
This distinction is particularly important when evaluating electronic plastics, cable insulation, sealants, labels, connectors and elastomeric parts.
The Open Compute Project has published guidance specifically addressing these compatibility mechanisms.
7. Why Do Immersion-Cooling Fluids Cause Elastomer Swelling or Additive Leaching?
An immersion fluid may contact hundreds of material interfaces throughout its operating life.
These include polymer housings, elastomeric seals, cable jackets, adhesives, labels and protective coatings.
Even when these components remain physically intact, slow extraction or absorption can create important changes.
Fluid absorption and swelling
Some polymers absorb portions of the immersion fluid.
This may change dimensions, mechanical properties or sealing behaviour.
The extent of swelling depends on chemical compatibility, temperature and exposure duration.
Additive extraction
Plasticizers, processing aids and other low-molecular-weight ingredients may migrate from polymer components into the immersion fluid.
This can cause the polymer to become harder or more brittle.
At the same time, extracted substances may change the fluid's chemical or electrical properties.
Deposit formation
Extracted materials may remain dissolved under one operating condition but precipitate or deposit under another.
This can create contamination even when the original polymer component appears satisfactory.
Practical laboratory investigation
Expose representative component materials to the actual proposed immersion fluid.
Measure mechanical and dimensional changes in the components.
Separately analyze the fluid for:
Changes in dielectric properties.
Extracted organic constituents.
Particles or deposits.
Acidic or oxidative degradation products.
Changes in viscosity or appearance.
Accelerated exposure can be useful for screening, but excessively high temperatures may introduce mechanisms that would not occur under normal operation.
Use realistic service conditions and justified acceleration factors wherever possible.
The preferred material combination must satisfy both component durability and fluid-property retention.
8. Why Do Thermal Interface Materials Pump Out During GPU Thermal Cycling?
Thermal interface material pump-out is a significant reliability concern for power electronics and high-performance computing assemblies.
A TIM is used to reduce thermal resistance between mating surfaces, such as an electronic package and its heat spreader or cooling plate.
Thermal greases and pastes can provide good initial contact because they conform to microscopic surface irregularities.
However, repeated temperature changes can alter the distribution of the material.
What causes TIM pump-out?
Different materials within an electronic package expand and contract at different rates.
During thermal cycling, the resulting mechanical movement can impose repeated shear and compression on the thermal interface layer.
For susceptible pastes and greases, this can progressively displace material away from parts of the contact region.
This phenomenon is commonly referred to as pump-out.
As the interface loses uniform coverage, voids or poorly contacted regions may develop.
The effective thermal resistance can increase.
Importantly, the severity depends on more than TIM formulation.
Relevant factors include:
Package and heat-spreader deformation.
Thermal expansion mismatch.
Temperature-cycling amplitude.
Clamp pressure.
Bond-line thickness.
TIM viscoelastic behaviour.
Filler concentration and distribution.
Interface geometry.
Why higher viscosity does not automatically prevent pump-out
Increasing viscosity or yield stress may improve resistance to material displacement.
However, the same modification can reduce the ability of the TIM to wet surfaces and form a thin bond line.
A very stiff formulation may remain in place yet produce greater initial thermal resistance.
Therefore, a pump-out-resistant TIM must balance rheological stability with conformability and contact performance.
Practical pump-out investigation
Begin by measuring thermal resistance at a controlled bond-line thickness and pressure.
Expose representative assemblies to a defined thermal- or power-cycling programme.
Monitor changes in thermal resistance over time.
Where possible, examine the interface after testing to identify material migration, voiding or changes in coverage.
Compare the measured change against a reference TIM under identical conditions.
A thermal paste with excellent initial conductivity but rapidly increasing interface resistance may be less suitable than a lower-conductivity material that maintains stable contact throughout service.
9. TIM Pump-Out vs Dry-Out vs Oil Bleeding: How Do You Identify the Difference?
Several distinct instability mechanisms can produce increasing thermal resistance.
They should not be treated as interchangeable.
Pump-out
Pump-out involves displacement of TIM material caused by mechanical and thermal cycling.
Evidence may include redistribution of paste, edge accumulation and changes in interface coverage.
Oil bleeding
Oil bleeding involves separation or migration of the liquid binder phase from a filled material.
This may produce visible oil-rich regions or deposits.
Bleeding can alter the local filler-to-binder ratio and change the rheology or thermal properties of the remaining material.
Dry-out
The term dry-out is commonly used when a thermal grease loses sufficient liquid-phase functionality to become less conformable or otherwise degraded.
The mechanism may involve volatilization, binder migration, separation or chemical changes, depending on the material.
Not every apparent dry-out is caused by evaporation.
A careful investigation should distinguish these mechanisms rather than relying only on appearance.
Recommended diagnostic approach
| Observed behaviour | Possible mechanism | Useful investigation |
|---|---|---|
| TIM accumulates near interface edges | Pump-out | Thermal cycling, interface imaging and material-distribution analysis |
| Oil appears around the TIM deposit | Binder bleeding | Controlled storage, mass balance and oil-separation testing |
| Paste becomes stiff or crumbly | Binder loss, separation or chemical degradation | Rheology, microscopy and chemical analysis |
| Thermal resistance rises without visible migration | Interfacial voiding, contact changes or material degradation | Thermal impedance measurement and interface inspection |
| Material hardens after prolonged heating | Binder degradation or other chemistry-dependent changes | Thermal aging, rheology and spectroscopic analysis |
These are starting hypotheses, not definitive identification criteria.
A complete failure analysis should include the material's initial composition and actual service conditions.
10. How Can TIM Thermal Conductivity Be Improved Without Making Viscosity Too High?
Thermal interface materials are frequently formulated using thermally conductive fillers dispersed in a polymeric or liquid binder.
Candidate filler systems include:
Aluminum oxide.
Aluminum nitride.
Boron nitride.
Selected silica-containing or other engineered mineral systems where suitable.
Electrically conductive carbon materials for applications that permit them.
Metallic fillers for appropriate specialized systems.
Each has different thermal, electrical, processing and cost characteristics.
Why filler loading improves conductivity
A thermally conductive filler can increase the effective thermal conductivity of the composite when appropriately dispersed.
At higher concentrations, contacts between particles can contribute to heat-transfer pathways.
However, the response is affected by particle shape, size distribution, orientation, packing and interfacial thermal resistance.
The relationship between filler concentration and effective conductivity is not necessarily linear.
Why viscosity rises at high filler loading
As filler concentration increases, less free binder remains between particles.
Particle interactions become more significant, and the composition may develop higher viscosity or yield stress.
High-aspect-ratio particles and strongly interacting surfaces can further increase rheological resistance.
This creates a central trade-off:
Increasing filler loading may improve bulk thermal conductivity while making the material more difficult to dispense, spread or compress into a thin interface.
Bimodal and multimodal filler design
Combining different particle sizes can improve packing efficiency in selected systems.
Smaller particles may occupy spaces between larger particles, potentially allowing increased filler concentration at an acceptable viscosity.
However, the result depends on particle morphology, surface treatment and actual particle-size distribution.
An unsuitable combination can increase viscosity or promote segregation.
Filler surface treatment
Surface treatment can influence particle wetting, agglomeration and interactions with the binder.
For silicone-based TIMs, compatible surface chemistry can help control filler dispersion and processing behaviour.
However, excessive surface treatment or poorly selected coupling chemistry may create unwanted changes in thermal performance, storage stability or cure behaviour.
Why bond-line thickness can matter more than headline conductivity
For a simplified uniform interface, material thermal resistance is related to:
R = BLT / (k × A)
Where:
R is thermal resistance.
BLT is bond-line thickness.
k is effective thermal conductivity.
A is heat-transfer area.
Actual assemblies also have contact resistances, nonuniform pressure and other complexities.
Nevertheless, the relationship illustrates an important principle.
A material with higher conductivity can still produce greater total thermal resistance if it requires a substantially thicker bond line or makes poor surface contact.
Therefore, select TIM candidates based on measured thermal impedance under representative pressure and geometry, not conductivity alone.
11. How Do You Prevent Silicone Thermal Gap Fillers From Settling or Bleeding?
Silicone gap fillers are commonly used where relatively large or variable gaps must be bridged while maintaining mechanical compliance.
These systems often contain substantial concentrations of thermally conductive particles.
The high density of some fillers can create sedimentation concerns during storage.
The liquid binder may also separate or bleed if the formulation lacks sufficient physical stability.
Why filler settling occurs
Settling is influenced by differences in density between the filler and the continuous phase, particle characteristics, viscosity and the structure of the uncured formulation.
A higher-viscosity binder may slow sedimentation, but viscosity alone may not provide adequate long-term stability.
A controlled yield-stress structure can be useful for maintaining particle distribution during storage.
However, excessive yield stress may make dispensing or mixing difficult.
Why two-part gap fillers present an additional challenge
Two-part systems may contain different reactive ingredients and filler packages in Parts A and B.
If filler settling occurs unevenly, material dispensed later from a cartridge or container may no longer match the original formulation composition.
This can affect thermal properties, viscosity and curing behaviour.
In some cases, it can also alter the effective mixing ratio of reactive ingredients delivered by the equipment.
Practical storage-stability study
Store the material under controlled conditions in representative packaging.
Examine samples taken from different locations within the container where practical.
Compare:
Filler concentration or ash content.
Density.
Viscosity and yield behaviour.
Thermal performance.
Cure characteristics after mixing.
Visible oil separation.
For cartridge products, evaluate first-, middle- and last-dispensed material.
This can reveal nonuniformity that might not appear in a small laboratory storage sample.
Improving gap-filler stability
Potential approaches include controlled particle-size distribution, compatible surface treatment, optimized binder viscosity and suitable rheology modifiers.
Changes should be evaluated against dispensing pressure, mixing uniformity, compressibility and final thermal resistance.
The goal is to maintain a uniform material that can still be applied and compressed within the required manufacturing process.
12. Why Do Thermally Conductive Encapsulants and Underfills Crack During Thermal Cycling?
Encapsulants and underfills support electronic reliability by protecting components and managing mechanical stresses.
However, the protective material itself can introduce reliability problems when its thermal expansion, modulus, cure behaviour or adhesion are poorly matched to the assembly.
Coefficient of thermal expansion mismatch
Electronic packages often combine silicon, metals, ceramics, solder joints, organic substrates and polymers.
These materials expand and contract differently with temperature.
When bonded together, differences in dimensional response generate mechanical stresses.
An encapsulant with inappropriate thermal expansion or stiffness may increase stress concentrations near sensitive interfaces.
Why adding thermally conductive filler is not always beneficial
Increasing mineral filler loading can improve effective thermal conductivity and may reduce the composite's coefficient of thermal expansion.
However, higher filler concentration can also increase modulus, viscosity and processing difficulty.
The resulting material may transfer more stress to delicate components if its compliance becomes insufficient.
Therefore, minimizing thermal expansion is not the only objective.
The formulation must maintain an appropriate balance among:
Thermal conductivity.
CTE.
Modulus.
Tg.
Adhesion.
Moisture absorption.
Cure shrinkage.
Processing behaviour.
Cure shrinkage and residual stress
Thermosetting encapsulants develop their polymer networks during curing.
Changes in density and dimensions during reaction and subsequent cooling can generate residual stresses.
The magnitude depends on resin chemistry, cure conversion, filler content, adhesion and temperature history.
High cure temperatures are not automatically preferable simply because they accelerate processing.
The final thermal and mechanical condition of the electronic assembly must be considered.
Moisture-related problems
Moisture absorption can alter polymer properties and produce swelling stresses.
Moisture may also interact with interfacial defects or contribute to reliability concerns during thermal exposure.
Underfill voids, inadequate wetting and contamination can compound these problems.
Practical reliability testing
A useful encapsulant or underfill investigation may include:
DSC for cure behaviour and Tg.
DMA for temperature-dependent modulus.
TMA or another suitable method for thermal expansion.
Moisture absorption testing.
Cure shrinkage characterization.
Microscopy for voids and cracking.
Adhesion measurements.
Thermal cycling of representative assemblies.
The test programme should examine both the material and the electronic assembly.
A cured polymer specimen may satisfy its material specification while still generating excessive stress in the finished package.
13. Why Does a High-Thermal-Conductivity Material Still Produce Poor Cooling Performance?
One of the most common selection errors is treating a single thermal-conductivity number as a direct indication of cooling performance.
Consider two thermally conductive gap-filler candidates.
Material A has a higher reported thermal conductivity.
Material B has a somewhat lower conductivity but conforms more effectively to the surfaces and forms a thinner, more uniform interface.
Under the actual assembly conditions, Material B may provide lower measured thermal impedance.
The reason is that the thermal performance of the installed interface depends on several factors.
Interface geometry
A thick bond line introduces a larger thermal conduction path.
Surface contact
Microscopic voids and incomplete wetting can increase contact resistance.
Pressure
Clamping or compression influences how the material conforms and distributes within the interface.
Filler distribution
Nonuniform filler packing or segregation can produce local variations in thermal properties.
Long-term stability
Pump-out, bleeding, dry-out and mechanical changes can alter the interface after initial qualification.
Measurement conditions
Reported conductivity and impedance values may be obtained using different test methods, pressures, specimen thicknesses or temperatures.
These values should not be compared without checking the measurement conditions.
ASTM D5470 provides a recognized approach for measuring thermal transmission properties of relevant materials under defined laboratory conditions.
However, laboratory thermal impedance is not automatically equivalent to performance in every installed electronic assembly.
The more useful specification is the thermal resistance of the complete interface under representative installation and service conditions, together with its change after aging.
14. AI Data Centre Thermal Materials: Practical Troubleshooting Matrix
The following matrix connects observed problems with possible material mechanisms and diagnostic approaches.
| Observed problem | Possible material mechanism | First diagnostic investigation | Corrective direction |
|---|---|---|---|
| Copper cold-plate corrosion | Inadequate inhibitor protection, aggressive ions or incompatible coolant chemistry | Dissolved metals, pH, conductivity and surface analysis | Optimize coolant chemistry and wetted-material compatibility |
| Coolant becomes cloudy | Precipitation, corrosion products or contamination | Particle and chemical characterization | Identify deposit source before modifying additives |
| Increasing cooling-loop pressure drop | Particles, deposits, fouling or other hydraulic restriction | Filter analysis, fluid cleanliness and flow testing | Correct contamination source and system cleanliness |
| Elastomer swelling | Fluid absorption or incompatible chemistry | Immersion exposure and mechanical-property changes | Select compatible seals and coolant components |
| Dielectric fluid loses electrical performance | Contamination, extraction, moisture or degradation | Resistivity, dielectric and chemical analysis | Improve compatibility and fluid-quality control |
| GPU TIM pumps out | Thermal-cycling deformation and insufficient material stability | Cycled thermal impedance and interface imaging | Optimize rheology, bond line and package compatibility |
| TIM exhibits oil bleeding | Binder–filler separation or migration | Oil separation and rheological testing | Improve binder–filler stability |
| Thermal grease dries or hardens | Binder loss, degradation or structural change | Thermal aging and chemical characterization | Reassess binder chemistry and stability |
| TIM conductivity rises but installed performance worsens | Excessive bond-line thickness or contact resistance | Thermal impedance under controlled pressure | Optimize filler design, compliance and BLT |
| Gap filler settles in storage | Insufficient suspension structure | Top/middle/bottom composition and rheology | Improve particle packing and yield behaviour |
| Underfill develops voids | Inadequate wetting, entrapped air, residues or dispensing conditions | Microscopy and process analysis | Optimize wetting, dispensing and cure |
| Encapsulant cracks during cycling | CTE mismatch, excessive modulus or residual stress | DMA, TMA, fracture inspection | Rebalance network, filler and cure properties |
A reliable troubleshooting programme should confirm the failure mechanism before implementing a major formulation change.
Many apparent thermal-management failures originate from interactions between materials rather than a single defective ingredient.
15. Practical Case Study: Solving TIM Degradation in a Liquid-Cooled AI Accelerator
Consider a hypothetical qualification project involving a high-power accelerator mounted beneath a liquid-cooled cold plate.
The cooling system initially meets its target temperature under steady operation.
However, after repeated power cycling, the measured component-to-coolant temperature difference gradually increases.
The coolant flow rate, inlet temperature and cold-plate pressure drop remain within their expected ranges.
This suggests that the thermal problem may not originate primarily from the circulating liquid.
The development team begins investigating the thermal interface between the accelerator package and the cold plate.
Step 1: Confirm the thermal path
Record the relevant operating conditions:
Component power.
Coolant inlet and outlet temperatures.
Coolant flow rate.
Cold-plate pressure drop.
Component temperature.
Controlled ambient conditions.
Compare current data with the original performance baseline.
If coolant conditions remain stable while component temperature rises at the same power, increasing resistance elsewhere in the thermal path becomes a plausible explanation.
Step 2: Examine the interface
Remove representative assemblies using a controlled procedure.
Inspect the TIM for changes in coverage, material migration, void formation or hardening.
Where pump-out is suspected, compare the interface with an uncycled reference.
Document the results rather than relying only on visual impressions.
Step 3: Compare alternative TIM formulations
Prepare a controlled material study.
| Trial | Material variation | Main technical question |
|---|---|---|
| A | Existing reference grease | What is the original thermal and cycling performance? |
| B | Grease with revised rheological structure | Can pump-out resistance improve without excessive thermal impedance? |
| C | Alternative filler-packing system | Can a thinner bond line be achieved at acceptable viscosity? |
| D | Suitable phase-change TIM candidate | Does another material architecture offer better cycling stability? |
| E | Suitable compliant TIM candidate | Does controlled compliance improve contact retention? |
The candidates should be selected only where they are compatible with the assembly's thermal, electrical, mechanical and processing requirements.
Step 4: Evaluate thermal performance after cycling
Measure initial thermal impedance under controlled conditions.
Repeat measurements after defined cycling intervals.
Compare the rate of change rather than relying only on the initial value.
A formulation that maintains stable thermal resistance may be preferable even if its original conductivity is not the highest among the candidates.
Step 5: Examine mechanical and processing constraints
Evaluate whether the improved TIM requires different clamping pressure, bond-line control or dispensing conditions.
An apparently attractive formulation may impose excessive mechanical load on the package or become difficult to apply consistently.
Step 6: Requalify the complete assembly
Once the most promising material is identified, test it in the representative package and cold-plate configuration.
Verify that the improvement remains valid under realistic power cycling, temperature conditions and mechanical tolerances.
This case illustrates why thermal-management material development must connect formulation properties with installed system performance.
16. How Should Thermal Management Materials Be Qualified for Long-Term AI Data Centre Reliability?
A useful qualification programme begins with the specific material's function.
Coolants, dielectric fluids, TIMs and encapsulants should not be judged by identical performance metrics.
However, they should be evaluated using a consistent reliability philosophy.
Recommended material qualification matrix
| Material | Critical initial properties | Important aging and reliability tests |
|---|---|---|
| Direct-to-chip coolant | Viscosity, heat capacity, pH, conductivity, corrosion protection | Corrosion exposure, fluid stability, metals analysis, material compatibility and cleanliness |
| Immersion dielectric fluid | Dielectric properties, viscosity, thermal properties, safety characteristics | Electrical-property retention, oxidation stability, component compatibility and extractables |
| Thermal grease or paste | Thermal impedance, rheology, bond-line behaviour | Pump-out, bleeding, dry-out, thermal cycling and storage stability |
| Thermal gap filler | Thermal impedance, compressibility, viscosity, filler uniformity | Sedimentation, mechanical cycling, oil bleeding and temperature aging |
| Thermally conductive adhesive | Thermal properties, adhesion, cure and mechanical behaviour | Thermal cycling, environmental aging, interface durability and mechanical retention |
| Encapsulant or underfill | Cure characteristics, CTE, modulus, Tg and adhesion | Moisture exposure, thermal cycling, cracking, voiding and dimensional reliability |
Qualification should compare initial performance with performance after defined environmental or operational exposure.
The relevant test conditions, acceptance limits and sampling plan must be established for the particular equipment and material.
No single accelerated test can guarantee service life without appropriate correlation.
Why interaction testing is essential
A coolant may pass its laboratory formulation stability test while remaining incompatible with a particular seal material.
A TIM may pass an initial thermal impedance test but degrade under package-specific cycling.
An encapsulant may demonstrate excellent bulk thermal conductivity while introducing unacceptable mechanical stress.
Therefore, the qualification programme should include representative material combinations and assemblies wherever possible.
The most important question is not simply whether each material passes its own specification.
It is whether the entire thermal-management material stack remains compatible and functional throughout the intended operating conditions.
Frequently Asked Questions About AI Data Centre Cooling Materials
What is the best coolant for AI data centre liquid cooling?
There is no universally best coolant. Treated water and suitable water–glycol systems are common candidates for direct-to-chip cooling, depending on environmental conditions and system specifications. The correct formulation must balance heat-transfer properties, viscosity, corrosion protection, materials compatibility and long-term fluid quality.
Why does copper corrode in data centre liquid-cooling systems?
Copper corrosion can be influenced by pH, dissolved ions, oxygen, contamination, inhibitor performance and interactions within the complete wetted-material system. The appropriate response is to characterize the fluid and corrosion products, identify the relevant mechanism and qualify the corrective chemistry.
Can deionized water be used directly as a server coolant?
Deionized water may be part of an approved cooling strategy, but its initial low ionic conductivity does not establish long-term compatibility. Fluid chemistry changes during circulation, so corrosion, contamination and compatibility controls must be defined for the specific system.
Why does a glycol coolant become cloudy?
Cloudiness may result from additive precipitation, corrosion products, contamination or degradation. Particle analysis and chemical characterization are needed to determine the cause. Increasing inhibitor concentration without identifying the mechanism can create further instability.
Which dielectric fluid is most suitable for immersion cooling?
Selection depends on the immersion architecture, operating temperature, electrical requirements, thermal properties, materials compatibility, safety and environmental constraints. Engineered hydrocarbons, synthetic esters and other suitable dielectric technologies offer different performance balances. The complete fluid–component combination must be qualified.
Why does a GPU thermal interface material pump out?
Thermal expansion and mechanical deformation during repeated temperature cycling can displace susceptible greases and pastes from the interface. TIM rheology, bond-line thickness, clamping pressure and package geometry influence the severity.
How can thermal paste pump-out be reduced?
Possible strategies include optimizing binder rheology, filler packing, bond-line thickness, interface pressure and material architecture. Each change must be assessed against initial thermal impedance and cycling stability. Increasing viscosity alone does not guarantee improved performance.
Why does silicone TIM oil bleed during storage?
Oil bleeding may occur when the binder phase separates or migrates within the highly filled formulation. The behaviour depends on binder chemistry, filler surface properties, loading and storage conditions. Evaluate oil separation and rheological stability before changing the formulation.
How can I increase TIM conductivity without making viscosity too high?
Evaluate filler particle-size distribution, packing efficiency, morphology, surface treatment and binder compatibility. Bimodal filler combinations can improve packing in suitable systems, but the complete formulation must be tested for thermal impedance and dispensing behaviour.
Why does a thermal gap filler settle during storage?
High-density particles can migrate in a formulation that lacks sufficient suspension stability. Particle interactions, binder rheology and yield behaviour influence settling. Storage studies should assess material uniformity throughout the container and after dispensing.
Why do electronic encapsulants crack during thermal cycling?
Thermal expansion mismatch, excessive stiffness, cure shrinkage, interfacial defects and moisture-related effects can contribute to cracking. Evaluate the encapsulant's CTE, Tg, modulus, cure behaviour and adhesion alongside the mechanical behaviour of the complete assembly.
Does a higher thermal conductivity always mean better cooling?
No. Installed thermal performance also depends on bond-line thickness, contact resistance, pressure, surface conformity and material stability. A higher-conductivity material can produce worse performance if it forms an excessively thick or poorly contacting interface.
What tests are most useful for qualifying thermal-management materials?
The correct tests depend on the material. Coolants require fluid-quality, corrosion and compatibility studies. TIMs require thermal impedance and cycling evaluation. Dielectric fluids require electrical and chemical stability measurements. Encapsulants require thermomechanical, adhesion and assembly-level reliability testing.
Final Perspective: AI Cooling Reliability Begins With Material Compatibility
AI data centre thermal management is no longer determined only by the capacity of pumps, heat exchangers and cooling infrastructure.
The materials connecting heat-generating components to cooling systems play an essential role in both immediate thermal performance and long-term reliability.
Direct-to-chip coolants must maintain appropriate thermal and hydraulic behaviour while protecting the wetted materials against corrosion and degradation.
Immersion dielectric fluids must retain electrical and chemical performance while remaining compatible with the immersed assembly.
Thermal interface materials must provide low installed thermal resistance without unacceptable pump-out, bleeding or degradation.
Gap fillers, encapsulants and underfills must balance thermal performance with mechanical compliance, cure control and environmental stability.
For formulation scientists, the most valuable approach is to develop these materials against the requirements of the complete thermal-management architecture.
A material that delivers an impressive initial data-sheet value is useful. A material that retains the required performance after realistic operating and aging conditions is the one that should advance toward qualification.
The central opportunity is to engineer performance and reliability together rather than treating durability as a final verification step.
Advanced Technical Training: Formulating Thermal Management Materials for AI Data Centres
For coolant formulators, electronic-material R&D scientists, silicone chemists, thermal interface material developers and advanced-materials specialists seeking deeper practical knowledge, OnlyTRAININGS offers the expert-led training:
Formulating Thermal Management Materials for AI Data Centres: Coolants, TIMs, Dielectric Fluids & Encapsulants
The training focuses on the formulation science and qualification challenges associated with high-power computing materials.
Key technical subjects include:
Selection and formulation of direct-to-chip liquid coolants.
Water and glycol heat-transfer-fluid chemistry.
Corrosion inhibitors and copper compatibility.
Elastomer, polymer and wetted-material compatibility.
Single-phase and two-phase immersion-cooling dielectric fluids.
Dielectric performance and alternative fluid-platform evaluation.
Thermal interface material filler selection and rheology.
TIM pump-out, dry-out, oil bleeding and thermal-cycling reliability.
Thermally conductive gap fillers and silicone formulation strategies.
Encapsulants, underfills, moisture resistance and cure stress.
Thermal-material qualification and practical failure diagnosis.
The training connects polymer, adhesive, coolant and electronic-material formulation decisions to the requirements of advanced AI computing environments.
It is designed for professionals who need to evaluate candidate materials, improve formulation robustness and make technically defensible material-selection decisions.
Explore the AI Data Centre Thermal Management Materials Training on OnlyTRAININGS
Technical References and Further Reading
1. Open Compute Project: Cold Plate Cooling Sub-Project.
Industry guidance, requirements and specifications addressing direct liquid cooling, cold plates, coolant fluids and interoperability.
https://www.opencompute.org/projects/cooling-environments/cold-plate/
2. Open Compute Project: OAI System Liquid Cooling Guidelines (2023).
Guidance covering cold-plate design, coolant selection, wetted-material compatibility, thermal performance and system validation.
https://www.opencompute.org/documents/oai-system-liquid-cooling-guidelines-in-ocp-template-mar-3-2023-update-pdf
3. Open Compute Project: Material Compatibility in Immersion Cooling, Revision 1.0 (2022).
Technical reference on dielectric-fluid interactions with electronics, polymers, elastomers and other materials.
https://www.opencompute.org/documents/material-compatibility-in-immersion-cooling-document-version-1-0-nov-28-2022-1-pdf
4. Open Compute Project: Immersion Cooling Sub-Project.
Guidelines and contributions covering single-phase and two-phase immersion cooling, dielectric fluids and compatibility.
https://www.opencompute.org/projects/cooling-environments/immersion/
5. Schneider Electric: Comparison of Dielectric Fluids for Immersive Liquid Cooling of IT Equipment, White Paper 291.
Discussion of dielectric fluid platforms, safety, environmental considerations and material compatibility.
https://www.se.com/us/en/download/document/SPD_WP291_EN/
6. ASTM D5470-17(2024): Standard Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials.
Reference method for evaluating thermal impedance and apparent thermal conductivity.
https://store.astm.org/standards/d5470
7. Söhl, S. and Eisele, R. (2019). Impact of the Pump-Out-Effect on the Thermal Long-Term Behaviour of Power Electronic Modules. Microelectronics Reliability, 100–101, 113479.
https://doi.org/10.1016/j.microrel.2019.113479
8. National Renewable Energy Laboratory: Degradation Characterization of Thermal Interface Greases.
Research on TIM degradation mechanisms, including pump-out and dry-out during thermal and power cycling.
https://research-hub.nlr.gov/en/publications/degradation-characterization-of-thermal-interface-greases-2/
9. Underfill: A Review of Reliability Improvement Methods in Electronics Production (2025).
A review discussing void formation, thermal expansion mismatch, warpage and electronic-package reliability.
https://pmc.ncbi.nlm.nih.gov/articles/PMC12389694/
AI data centre thermal management materials, Data centre liquid cooling coolant, thermal interface material pump-out, cold plate corrosion, dielectric immersion cooling fluid, thermally conductive gap fillers
Technical note: The troubleshooting procedures and illustrative case study are intended for professional formulation development. Material selection, test conditions, electrical safety, chemical compatibility and qualification criteria must be validated for the specific cooling architecture and electronic assembly. The discussed material families are not universally interchangeable.
