Thermal grease for power modules fills the microscopic spaces between a module baseplate and a heat sink or liquid-cooled cold plate. By replacing trapped air with a more conductive material, it lowers interface resistance and helps heat leave IGBT, SiC MOSFET and intelligent power modules. The right product is not simply the grease with the highest W/mK. It is the grease that forms a thin, complete and stable interface in the real assembly.
That distinction matters. A good compound can still perform badly when the heat sink is uneven, the deposited layer is too thick, the screw sequence bends the baseplate or the grease moves during thermal cycling. In other words, the module, grease, cooling surface, mounting hardware and application process behave as one system.
This guide explains how to select that system. It covers the material properties worth comparing, realistic bond line control, stencil printing, common failure modes, alternatives, standards and production qualification. Always treat the module manufacturer’s mounting instructions as the final authority for its geometry and fastening requirements.
What Does Thermal Grease Do in a Power Module Assembly?
Thermal grease does not cool a module by itself. It improves contact at the module-to-cooler interface. Even machined metal surfaces contain peaks, valleys and slight shape errors. When two dry surfaces are clamped together, those irregularities trap insulating air. Grease wets the surfaces and fills the small voids, creating a more continuous path for heat.
The heat path from the semiconductor junction to the coolant or air
Heat begins at the semiconductor junction. It then travels through the die attach, substrate, internal conductors and baseplate, where present. From there, it crosses the thermal interface material and enters a heat sink or cold plate. Finally, it moves into air or coolant.
The grease occupies only one thin part of that path, but it can become a bottleneck. Think of it like a gasket between two sections of pipe. The gasket is short, yet a poor fit can restrict the whole flow.
Engineers commonly track several resistance terms:
- Rth(j-c): junction-to-case thermal resistance inside the module.
- Rth(c-h): case-to-heat-sink resistance, strongly influenced by the interface.
- Rth(h-a) or cooler-to-fluid resistance: heat rejection from the cooling hardware.
- Rth(j-a): the complete junction-to-ambient path.
Grease selection mainly affects Rth(c-h), but a temperature problem should still be investigated across the whole stack. A better TIM cannot rescue an undersized cold plate or poor coolant flow.
Baseplate and baseplate-less modules need different thinking
A conventional IGBT module may have a metal baseplate that spreads heat and provides a mounting surface. Some newer SiC modules use thinner baseplates, exposed ceramic structures or baseplate-less designs to reduce thermal mass and internal resistance. These designs can be more sensitive to local pressure, flatness and insulation requirements.
Do not assume thermal grease provides electrical isolation. Many greases are electrically non-conductive, but the thin grease layer is not automatically a certified dielectric barrier. If isolation is part of the safety design, verify the module construction and use a qualified insulation solution.
For a broader view of grease, pads, gels and other thermal interface materials, start with the actual gap and electrical architecture before choosing a format.
Start With the Interface, Not the Thermal Conductivity Number
Thermal conductivity describes how readily heat passes through the bulk compound. It is useful, but it does not describe surface contact, final thickness or aging. Assembly-level interface resistance combines bulk resistance with the contact resistance at both surfaces. A moderate-conductivity grease that spreads into a thin, continuous film may outperform a higher-W/mK product that remains thick or leaves voids.
For a simplified uniform layer:
R = t / (k x A)
Where:
- R is bulk thermal resistance.
- t is bond line thickness.
- k is thermal conductivity.
- A is the effective contact area.
The equation is helpful, but real joints add contact resistance. Surface roughness, wetting, pressure and filler shape all affect that extra resistance. This is why a datasheet conductivity value should never be used alone to predict junction temperature.
The distinction between thermal conductivity and thermal impedance is especially useful during early screening.
| Datasheet term | What it tells you | What it does not tell you | How to use it |
| Thermal conductivity, W/mK | Bulk heat-transfer potential | Final contact quality or bond line | Compare materials tested by a clear, comparable method |
| Thermal resistance, K/W | Temperature rise per heat flow for a stated sample or assembly | Performance at another area, thickness or pressure | Use only with the stated geometry and conditions |
| Thermal impedance, K cm2/W | Area-normalized resistance under defined conditions | Exact module temperature without system data | Compare interfaces at realistic pressure and thickness |
| Viscosity | Resistance to flow during a stated test | Stencil release, slump or long-term migration by itself | Match it to printing, dispensing and handling needs |
| Operating temperature | Claimed usable temperature window | Long-term stability in your power-cycle profile | Ask for aging evidence and validate the assembly |
Seven Factors for Selecting Thermal Grease for Power Modules
The best power module grease is the material that reaches the required temperature with stable manufacturing and acceptable life. Selection should combine interface thermal impedance, achievable bond line, rheology, temperature stability, migration resistance, compatibility and supply controls. One impressive number cannot replace this balance.
1. Assembly-level thermal impedance
Ask for thermal impedance or resistance data measured at a stated thickness, pressure and temperature. The test fixture and surface finish matter, so compare like with like. If suppliers use different methods, the ranking may reflect the methods as much as the compounds.
A practical screening test uses representative metal surfaces and several bond lines. The final approval test uses the actual power module and cooler. Record coolant or heat-sink temperature, module case temperature and a repeatable electrical temperature indicator where available.
2. Achievable bond line thickness
Grease performs best when it is thin enough to limit bulk resistance yet thick enough to fill the real surface variation. The achievable bond line depends on:
- Module and cooling-surface flatness.
- Surface roughness and waviness.
- Mounting pressure and screw locations.
- Grease viscosity, yield stress and filler particle size.
- Stencil pattern, deposited volume and squeeze flow.
The effect of bond line thickness on thermal performance is often larger than buyers expect. Do not choose a nominal coating thickness before studying the mounted joint.
3. Viscosity, thixotropy and stencil printability
Viscosity is only a snapshot of flow resistance. Thixotropy describes how a material changes under shear and then rebuilds after the force is removed. A printable grease should fill the stencil openings, release cleanly, keep its pattern before mounting and spread under clamping without running everywhere.
For automated dispensing, check pressure stability, bead continuity, nozzle size, filler settling and startup after idle time. For stencil printing, check squeegee speed, angle, stencil thickness, aperture geometry and release. A material can have excellent thermal data and still be a headache on the line. That happens more than anyone likes to admit.
4. Operating and peak temperature range
Power modules experience steady heat, short overloads and repeated power cycles. A broad datasheet temperature range does not prove stable viscosity or interface resistance over years of cycling.
Define the real profile:
- Normal case and cooler temperature.
- Maximum continuous temperature.
- Short-duration peak temperature.
- Startup and shutdown frequency.
- Temperature swing and dwell time.
- Expected service life.
Request post-aging data where possible. Then repeat the thermal measurement after cycling, not just before it.
5. Pump-out, dry-out, oil bleed and volatility
These terms describe different problems:
- Pump-out is material movement caused by repeated expansion, contraction and pressure changes.
- Dry-out is the loss or redistribution of carrier fluid until the interface becomes filler-rich and less compliant.
- Oil bleed is visible separation or migration of the liquid phase.
- Volatility is the loss of lower-molecular-weight components, accelerated by heat or low pressure.
Look for stable coverage around hot areas and edges after cycling. A grease that starts with low resistance but migrates away from the active region is not a low-resistance solution in service.
6. Electrical and chemical compatibility
Confirm whether the grease is electrically insulating, slightly conductive or designed only for thermal contact. Also review its interaction with coatings, plastics, labels, seals, connectors and cleaning agents. Silicone oil migration can be unacceptable near optical surfaces, relay contacts, painting or bonding operations. In those cases, evaluate a silicone-free thermal grease or another controlled-chemistry TIM.
7. Manufacturing and supply controls
The supplier should support more than a sample syringe. Production planning needs:
- Packaging suited to printing or dispensing equipment.
- Storage temperature and shelf life.
- Conditioning, mixing or de-airing instructions.
- Batch traceability and certificate data.
- Lot-to-lot rheology and thermal controls.
- Change-notification policy.
- Technical support for stencil and validation trials.
| Engineering question | Property to review | Process risk | Recommended evidence |
| Will the module meet its temperature target? | Thermal impedance at realistic BLT | Datasheet-only selection | Module-to-cooler thermal test |
| Can the layer remain thin and complete? | Rheology, particle size, wetting | Voids or thick areas | Printed mass, thickness and transfer pattern |
| Will it survive cycling? | Pump-out and aging stability | Edge loss or dry zones | Thermal or power cycling plus post-test inspection |
| Can production apply it repeatedly? | Printability or dispensing stability | Lot and operator variation | Capability study using measured deposit mass |
| Is it compatible with the assembly? | Electrical and chemical behavior | Leakage, contamination or coating defects | Compatibility and insulation testing |
How Thick Should Thermal Grease Be on an IGBT or SiC Power Module?
There is no universal grease thickness for every IGBT or SiC power module. Many manufacturer examples sit in the tens to low hundreds of micrometres, but the approved value belongs to a specific module, cooler surface and application pattern. Follow the module mounting document, then verify the compressed interface and thermal result.
For example, Fuji Electric has published guidance around 100 micrometres for selected industrial IGBT modules, including a 100 +/- 30 micrometre example. Mitsubishi literature includes 50-100 micrometres for some module families, while other families use broader ranges. Microchip has documented a minimum value for a specified module and method. These numbers are examples, not a shared industry rule.
Also distinguish deposited thickness from mounted bond line. A stencil creates a patterned deposit with open spaces. During mounting, grease spreads and the peaks collapse. Measuring only the stencil thickness does not reveal the final interface.
A simple grease quantity estimate
For a first estimate:
Mass = coated area x average deposited thickness x density
Suppose a printed pattern has an effective coated area of 90 cm2, an average deposited thickness of 0.010 cm and a grease density of 2.6 g/cm3:
Mass = 90 x 0.010 x 2.6 = 2.34 g
This is a starting process quantity, not an approved specification. Pattern openings, squeeze-out and the mounted bond line change the real result. Weighing the module before and after printing is a useful production check because it measures deposited mass more consistently than a casual visual judgement.
The Fuji Electric mounting instructions show why grease thickness, heat-sink flatness, roughness and fastening need to be read together.
Heat-Sink Flatness, Roughness and Mounting Pressure Are Part of TIM Selection
The cooling surface can make a suitable grease look bad. Excessive flatness error creates local thick areas or dry contact. Roughness increases the volume needed to fill surface valleys. Uneven fastening changes pressure and may distort the module baseplate. Define these mechanical conditions before locking the grease and stencil.
Flatness and roughness
Flatness describes the overall shape error across the mounting area. Roughness describes the smaller texture left by machining. Both affect contact, but in different ways. A polished surface can still be bowed, and a flat surface can still be too rough.
Inspect the complete module footprint, not a small convenient patch. Module manufacturers often specify acceptable flatness and roughness for their products. Use their limits rather than copying a number from another module family.
Torque and tightening sequence
Mounting screws should usually be tightened in progressive steps and in the sequence specified by the module supplier. Pulling one corner fully down before the others can tilt or bend the module, trap grease and produce an uneven bond line.
Control the tool, torque, screw condition, washer stack and seating surface. For prototypes, mark the sequence and record actual torque. It sounds basic, but this is where quiet variation sneaks in.
Contact pattern inspection
Build sacrificial validation assemblies. Apply the specified pattern, mount the module, then remove it carefully and inspect transfer to both surfaces. Look for uncovered areas, trapped air, edge starvation and excessive squeeze-out.
Combine the visual check with temperature mapping, thermocouples or a module-specific temperature-sensitive electrical parameter. A pretty spread pattern is useful, but the thermal result still decides.
Stencil Printing vs Roller, Spatula and Automated Dispensing
Stencil printing is widely used for power modules because it controls deposited location and volume across a large baseplate. A module-specific pattern allows grease to spread into a thin layer while leaving routes for air to escape. Rollers and spatulas can work in prototypes, but they create more operator variation. Automated dispensing is useful when the pattern and material are designed together.
Why stencil printing appears in OEM guidance
Power modules are larger and mechanically different from desktop processors. A dot or X pattern that looks fine in a hobby video is not a production method for a wide IGBT baseplate.
A stencil can provide:
- Repeatable deposited mass.
- Controlled keep-out around holes and edges.
- Defined islands or tracks for spreading.
- Faster inspection and operator training.
- A measurable relationship between aperture, stencil thickness and deposit.
Infineon’s application note on screen-printing templates illustrates the module-specific nature of stencil geometry and process control.
When another method may be acceptable
A roller or controlled spatula can be practical for engineering trials, service work or low-volume builds when the module maker permits it. Dispensing may suit an automated line if the beads merge into full coverage without trapping air. Pre-applied TIM can remove an in-house printing step entirely.
Whichever method is used, define the deposited mass, position, inspection criteria and mounted result. “Apply a thin layer” is an instruction, but not yet a process.
Practical production sequence
- Inspect the module base and cooling surface for damage or contamination.
- Clean both surfaces using approved materials and allow them to dry.
- Condition the grease according to its storage and handling instructions.
- Align the module-specific stencil or dispensing fixture.
- Print or dispense the controlled pattern.
- Inspect position, completeness and deposited mass.
- Mount the module using the specified screw sequence and torque steps.
- Verify transfer pattern and thermal performance on validation units.
| Application method | Volume control | Production speed | Common risk | Best fit |
| Module-specific stencil | High after setup | High | Poor release or wrong pattern | Repeatable production |
| Automated dispensing | High with calibrated equipment | Medium to high | Bead gaps, air entrapment, filler settling | Flexible automated lines |
| Roller | Medium | Medium | Operator-dependent thickness | Approved low-volume assembly |
| Spatula or manual spread | Low to medium | Low | Uneven coverage and contamination | Prototype or service work |
| Pre-applied TIM | Very high at supplier | High in final assembly | Storage or handling damage | Clean high-volume assembly |
For general handling background, see Haktak’s guide to applying thermal grease.
Common Thermal Grease Failure Modes in Power Electronics
Most thermal grease failures are interface failures, not dramatic material breakdowns. The usual signs are rising or inconsistent temperatures, dry-looking regions, edge accumulation, squeeze-out or contamination. The root cause may be the quantity, stencil, surface, mounting process, aging behavior or cooling hardware. Diagnose the evidence before blaming the jar.
Too much grease
An overly thick layer adds thermal resistance. It may also squeeze beyond the baseplate, contaminate nearby surfaces or interfere with service. More grease does not mean more cooling. The compound is there to replace air, not to become a thick heat-spreading slab.
Too little grease or incomplete wetting
Insufficient quantity can leave air pockets, especially over a bowed baseplate or rough cooler. Local dry areas create hot spots even when the average module temperature looks acceptable. Poor stencil release or an interrupted dispensing bead can produce the same symptom.
Pump-out and edge accumulation
During every heat-up and cool-down cycle, the module and cooler expand by different amounts. Their small relative movement can push a low-stability grease away from active regions. After many cycles, material may gather near edges while the centre becomes thin or dry.
Dry-out, oil separation and viscosity drift
Heat can redistribute or remove carrier fluid. The remaining filler-rich layer becomes less able to wet changing surfaces. Oil bleed may also migrate outside the interface. Check mass, appearance and thermal resistance before and after aging.
Poor surface or mounting control
Variation in flatness, roughness, cleaning, screw torque and stencil alignment often looks like material variation. A good root-cause study compares these records with thermal data by serial number.
| Observed symptom | Likely cause | Evidence to check | Corrective action |
| High temperature from first build | Thick bond line, voids or poor cooler | Deposit mass, transfer pattern, flatness | Correct surface or pattern, then retest |
| Large unit-to-unit variation | Manual application or torque variation | Weight, stencil position, torque records | Tighten process controls and train operators |
| Temperature rises after cycling | Pump-out, dry-out or joint movement | Post-cycle coverage and thermal resistance | Screen a more stable grease or revise mechanics |
| Grease around module edges | Excess volume or squeeze flow | Printed mass and edge pattern | Reduce or redistribute the deposit |
| Coating or contact contamination | Oil bleed or incompatible chemistry | Residue analysis and compatibility test | Use controlled-bleed or silicone-free material |
Thermal Grease vs Phase Change Material, Thermal Pad and Graphite
Thermal grease is usually strongest at thin, clamped interfaces that need excellent wetting. It is not automatically best for every module. Phase-change materials can simplify handling, pads can bridge larger gaps or provide engineered insulation, and graphite can spread heat laterally. Compare the actual geometry, electrical needs, production flow and life profile.
| TIM type | Best interface | Typical bond line | Assembly advantage | Main caution |
| Thermal grease | Flat, tightly clamped module-to-cooler joint | Very thin | Excellent wetting and reworkability | Application variation, pump-out and bleed |
| Phase-change material | Thin clamped joint with clean preform or coating | Thin | Controlled placement and low mess | Must reach activation conditions and wet properly |
| Thermal pad | Larger or tolerance-varying gap | Medium to thick | Clean placement, compliance and insulation options | Higher bulk resistance and compression stress |
| Graphite sheet | Lateral heat spreading or selected thin joints | Very thin | Strong in-plane spreading | Electrical conductivity and limited conformity |
A phase-change thermal interface material may be attractive when a clean pre-applied process and stronger resistance to grease migration matter. Validate activation temperature, pressure and surface contact.
Pads are useful when there is a measurable gap or component height variation. They are not direct replacements for a thin grease joint without redesign. Graphite is highly anisotropic: excellent lateral conductivity does not automatically mean low through-plane interface resistance.
Application Examples Across Power Electronics
Power module grease appears in traction inverters, industrial drives, renewable-energy converters, UPS systems and other high-current equipment. The selection logic is similar, but the dominant stress changes. Automotive systems emphasize severe cycling and contamination control. Industrial systems may prioritize long service, repair and process repeatability. Renewable equipment adds broad ambient conditions and remote operation.
EV traction inverter and onboard charger
An EV inverter may use SiC or IGBT modules mounted to a liquid cold plate. High power density makes a small interface change visible in junction temperature. Qualification should combine coolant conditions, power cycling, vibration and chemical compatibility. The grease must also fit a controlled automotive assembly process.
Industrial motor drive and servo system
Industrial drives run for long periods and may see dust, vibration and irregular maintenance. Consistent stencil application and stable thermal resistance are often more useful than chasing the highest catalog conductivity. Rework and field replacement procedures should use the same controlled quantity as production.
Solar inverter and energy-storage PCS
Solar and battery conversion systems experience daily temperature cycles and wide ambient swings. Pump-out and dry-out screening deserve attention. Haktak’s guide to thermal paste for inverters discusses this operating context in more detail.
UPS, welding and high-power supplies
These systems may have high transient loads and forced-air or liquid cooling. Verify the peak case temperature, mounting pressure and cooler capacity. A short overload may be acceptable electrically but still create a severe interface temperature swing.
Testing Standards and a Realistic Qualification Plan
No single certificate proves that thermal grease will work beneath a power module. Standard methods provide comparable material data, while the final assembly test proves the interface. A sensible plan moves from datasheet review to controlled material tests, process capability, representative module testing and aging. Re-measure performance after environmental stress.
ASTM D5470 and TIM property measurement
ASTM D5470 is widely referenced for steady-state thermal transmission properties of thermally conductive materials. It can be used to derive thermal impedance and apparent conductivity under controlled conditions. ASTM also makes clear that an idealized method does not reproduce every real interface. Surface finish, pressure, thickness and specimen preparation still matter.
JESD51 thermal measurement concepts
The JEDEC JESD51 family provides semiconductor thermal-characterization concepts and methods. These are useful when planning junction-temperature measurements and thermal networks. They are not a standalone thermal grease certification. Apply the relevant method to the device and test objective.
IEC 60068 and application-specific environmental tests
IEC 60068 methods are commonly used as building blocks for dry heat, damp heat, temperature change, shock and vibration testing. Choose tests that reflect the product environment. Automotive, railway, renewable-energy or customer-specific programs may add their own cycles and acceptance limits.
Haktak’s overview of common TIM testing standards can help teams separate material-property tests from assembly reliability tests.
Recommended qualification ladder
- Review documentation. Check TDS, SDS, compliance, storage, shelf life and traceability.
- Run controlled TIM tests. Compare candidate materials at multiple relevant bond lines and pressures.
- Study the application process. Measure deposited mass, position, repeatability and defects.
- Test the representative assembly. Use the real module, cooler, fasteners and coolant or airflow.
- Apply environmental stress. Run thermal cycling, power cycling, humidity and vibration as required.
- Inspect and re-measure. Compare thermal resistance, coverage, bleed, migration and mechanical condition.
What to Send a Thermal Grease Supplier
A useful recommendation starts with assembly information, not a request for “the highest W/mK paste.” Share the module, cooler, temperature target, geometry, mounting process, production method and reliability profile. With that context, a supplier can narrow the chemistry and rheology, propose samples and help define a realistic validation plan.
Prepare the following brief:
- Power module manufacturer, part number and mounting guide.
- Module construction, baseplate size and active heat area.
- Heat-sink or cold-plate material, coating, flatness and roughness.
- Device loss, coolant or ambient condition and temperature limit.
- Maximum allowable interface resistance.
- Screw pattern, torque sequence and estimated clamping pressure.
- Preferred stencil, printing, dispensing or pre-applied process.
- Continuous, peak and cycling temperature profile.
- Vibration, humidity, chemicals and service-life expectations.
- Electrical insulation and silicone restrictions.
- Prototype quantity, annual volume, packaging and traceability needs.
For applications where a very thin interface is the priority, Haktak can evaluate a low thermal resistance grease against the real bond line and production method.
Need to compare candidates in your own module stack? Request samples or a material recommendation with the module drawing, cooler details and operating profile.
Frequently Asked Questions
What thermal grease is best for an IGBT power module?
The best grease is the one that meets the module temperature target at a thin, repeatable bond line and remains stable after the required cycling. Compare interface thermal impedance, printability, pump-out resistance, temperature stability and compatibility. Follow the IGBT manufacturer’s mounting instructions before selecting thickness or application pattern.
How thick should thermal grease be under a power module?
There is no universal thickness. Manufacturer examples often fall in the tens to low hundreds of micrometres, but each value belongs to a particular module and cooler condition. Use the module mounting guide, control deposited mass and verify the mounted transfer pattern and thermal result.
Is higher thermal conductivity always better for power module grease?
No. Higher W/mK can help, but a thicker layer or poor wetting can cancel the benefit. Thermal impedance at the real bond line and pressure is usually more useful. Long-term stability and production repeatability also matter.
Should thermal grease cover the entire module baseplate?
The finished interface should provide effective coverage across the intended heat-transfer area without harmful voids. That does not always mean printing a solid rectangle. Module-specific stencil patterns may use separate deposits that spread during mounting. Follow the approved pattern for the module.
Is stencil printing better than a roller or spatula?
Stencil printing is generally more repeatable for production because it controls volume and location. A roller or spatula may be accepted for prototypes, repairs or low-volume builds if the module maker permits it and the result is inspected. The method must produce the specified mounted interface.
Can thermal grease pump out during thermal cycling?
Yes. Repeated expansion and contraction can move grease away from active areas, especially when the compound, pressure and surface movement are poorly matched. Use thermal or power cycling, inspect coverage afterward and repeat the thermal measurement.
Is power module thermal grease electrically insulating?
Some products are electrically non-conductive, but this should never be assumed from appearance or from the word “grease.” Check the datasheet and safety architecture. A thin compound layer may not satisfy required dielectric, creepage or clearance protection.
When should I use phase-change material instead of thermal grease?
Consider phase-change material when you want clean placement, controlled quantity and reduced handling variation in a thin clamped joint. Confirm activation temperature, wetting, pressure and reliability. Grease may remain preferable where room-temperature wetting and easy rework are important.
How do I calculate the amount of thermal grease for a module?
Estimate mass from effective coated area, average deposited thickness and material density. Then refine it with the actual stencil pattern and mounted transfer test. In production, weighing parts before and after application is a practical way to monitor deposited quantity.
How should thermal grease be tested before mass production?
Start with material and compliance review, then compare candidates at relevant thickness and pressure. Run a process capability study, measure thermal performance in the real module-to-cooler assembly, apply the required environmental cycling, and finally inspect coverage and re-measure thermal resistance.
