Why can high h-BN filler loading in TIMs worsen device temperatures?
Direct Answer
Main failure reason: High loading of hexagonal boron nitride in polymer TIMs can sharply increase viscosity and modulus, driving thicker and more non-uniform bond lines, trapped air and pump-out-driven coverage loss so that the net interface resistance and device temperatures can worsen instead of improve. [S1][S2][S4][S6]
Context
- Hexagonal boron nitride (h-BN) is widely used as an electrically insulating, thermally conductive filler in polymer-based thermal interface materials for power electronics modules and CPU packages. [S2][S4][S8]
- For particle-laden polymer TIMs, increasing ceramic filler loading raises bulk thermal conductivity but also increases yield stress and viscosity, which tends to thicken the bond line and elevate contact resistance if assembly pressure and wetting are not co-optimized. [S1][S3][S7]
- In practice, the dominant penalties at very high filler loading often come from process and reliability effects such as voiding, incomplete spreading, and pump-out or dry-out under thermal or power cycling, which progressively increase junction-to-case thermal resistance even when bulk conductivity is high. [S4][S6][S8]
- Bond line thickness, true contact area and void content are controlled not only by TIM formulation but also by surface roughness, clamping pressure, and warpage; [S1][S4][S9]
- trapped air in roughness valleys and TIM-free regions can strongly increase interface resistance relative to a fully wetted contact. [S1][S4][S9]
Decision Logic
Format: Engineering Decision Table
| Engineering Variable | Material | Incumbent | Engineering Decision Signal |
|---|---|---|---|
| Application-stage viscosity and achievable BLT under real clamping conditions | high-loading h-BN filled polymer TIM between device lid or baseplate and heatsink | silicone-oil-based thermal grease with ZnO/Al2O3 ceramic fillers as a reworkable paste TIM between device baseplate or lid and heatsink, optimized for thin bond line thickness and moderate pump-out resistance | If the h-BN TIM behaves paste-like only under aggressive mixing but appears stiff or stringy during dispensing or lid attach, expect thicker and more variable bond lines than with the incumbent grease even though bulk conductivity is higher. [S1][S2][S4] |
| Void tendency and ability to expel trapped air | high-loading h-BN filled polymer TIM | silicone-oil-based ZnO/Al2O3 thermal grease | If coupon builds with the h-BN TIM repeatedly show corner or edge void bands, or residual bubble populations after degassing and attach, while the incumbent grease wets the same geometry cleanly, void-driven interface resistance is likely to dominate and can erase any conductivity benefit. [S4][S6][S9] |
| Spreading, coverage and sensitivity to placement tolerances | high-loading h-BN filled polymer TIM | silicone-oil-based ZnO/Al2O3 thermal grease | If small changes in dispense volume or component alignment cause the h-BN TIM to leave uncovered regions on rough or stepped surfaces where the incumbent still maintains full coverage, the new formulation is more likely to create hotspots in production. [S4][S8] |
| Pump-out and dry-out under thermal or power cycling | high-loading h-BN filled polymer TIM | silicone-oil-based ZnO/Al2O3 thermal grease | If large-area cycling fixtures or in-situ imaging show faster retreat of the h-BN TIM from high-strain regions or earlier development of voided or cracked bands than the incumbent grease, the risk of junction temperature drift and field returns increases. [S6][S7] |
| Contact uniformity and hotspot formation | high-loading h-BN filled polymer TIM | silicone-oil-based ZnO/Al2O3 thermal grease | If thermal mapping on test vehicles shows sharper local temperature peaks and stronger dependence on clamping flatness with the h-BN TIM than with the incumbent, the high-loading formulation is amplifying contact non-uniformity and should be treated as higher risk. [S1][S8][S9] |
Mechanism
Mechanism family: Viscosity-driven BLT growth, voiding and pump-out at high h-BN loading
- As h-BN loading increases, rheological studies on silicone and epoxy systems show strong growth in viscosity and yield-like behavior, which resists squeeze-out and makes it harder to form thin, uniform bond lines under typical assembly pressures. [S1][S2][S3]
- Thick, highly filled TIM layers deform less under clamping and surface roughness, so micro-valleys on mating surfaces remain partially filled with air rather than TIM, increasing thermal contact resistance despite higher bulk conductivity. [S1][S4][S9]
- High-viscosity, highly filled formulations are more difficult to degas and spread, so residual voids and incomplete coverage become more likely, which numerical and experimental studies link directly to higher junction-to-ambient thermal resistance and hotspots. [S4][S6][S9]
- Under thermal and power cycling, stiff, highly filled greases show stronger pump-out and dry-out, where material migrates away from high-strain regions and leaves voids or dry residues that progressively increase interface resistance. [S1][S6][S7]
- For anisotropic fillers such as h-BN platelets, high loading and processing can align particles in-plane, improving lateral conductivity while offering less benefit through the thickness, so any increase in bond line thickness and contact resistance directly harms the relevant thermal path. [S3][S4]
Data Points
- Silicone thermal grease filled with h-BN nanosheets shows sharply rising viscosity with filler content; [S2]
- in one study, viscosity increased from about eighty pascal-seconds for the unfilled commercial grease to roughly double that value at a modest h-BN loading, and the authors reported that such hyperviscosity hindered both processing and the ability of the grease to conform to mating surface topography. [S2]
- Epoxy composites using alumina spheres plus boron nitride platelets exhibit both higher thermal conductivity and higher viscosity as BN content increases; [S3]
- dynamic rheology showed a transition toward more solid-like behavior as BN fraction was raised, and an optimal BN level was identified where thermal conductivity gains were significant yet processability remained acceptable. [S3]
- For PDMS-based TIMs with h-BN and graphene fillers, through-plane thermal conductivity increased several-fold over neat PDMS at high total filler loadings, but when the uncured composite viscosity exceeded several hundred pascal-seconds, poor degassing and filler agglomeration led to a reduction in effective thermal conductivity at the highest loadings. [S4]
- Large-area tests of commercial thermal greases using an ASTM D5470-derived steady-state stand and thermal cycling between subzero and elevated temperatures have shown that pump-out and dry-out can measurably increase thermal resistance within a few tens of cycles, with acoustic and visual inspection revealing migration of grease away from the center and the formation of voided regions. [S5][S6][S7]
Practical Evaluation Checklist
- Measure the complex viscosity and yield behavior of the candidate high-loading h-BN TIM at the intended dispense and lid-attach temperature and shear rates, and compare qualitatively with the incumbent silicone-oil ZnO/Al2O3 grease to ensure it still flows and levels without excessive stringing or elastic recoil. [S2][S3][S4]
- Measure bond line thickness and its spatial uniformity on coupon fixtures that mimic the device-to-heatsink stack-up, using representative surface roughness and clamping, and confirm that the h-BN TIM does not produce systematically thicker or more uneven bond lines than the incumbent under the same assembly conditions. [S1][S4][S5]
- Check for trapped air and incomplete coverage by performing x-ray, scanning acoustic microscopy or cross-sectional inspection on assembled coupons after dispense and attach, focusing on corners, edges and rough regions where highly filled formulations are most prone to leave voids. [S4][S6][S9]
- Validate thermal impedance on coupons using an ASTM D5470-type steady-state apparatus or equivalent guarded heat flow setup across multiple clamping pressures, and ensure that the h-BN TIM shows a clear and repeatable advantage over the incumbent at the bond line thicknesses your process can realistically achieve. [S5][S6]
- Screen pump-out behavior by running accelerated thermal or power cycling on large-area fixtures that replicate baseplate-to-heatsink interfaces, imaging the grease layer periodically to quantify coverage loss, void growth and retreat from high-strain regions. [S6][S7]
- Compare junction-to-case or junction-to-ambient thermal performance on instrumented test vehicles for both the h-BN TIM and the incumbent grease under identical power maps and clamping schemes, and look for any reversal where the higher-conductivity TIM yields higher measured junction temperature. [S8][S10]
- Record any changes in thermal impedance, hotspot patterns and visible TIM distribution before and after an initial block of thermal cycles so that early degradation trends for the high-loading h-BN system are captured and can be contrasted with the incumbent. [S6][S7][S9]
NOT suitable when…
- High-loading h-BN TIMs are a poor fit when the assembly cannot guarantee reasonably uniform and stable clamping pressure, because their elevated yield stress and stiffness then promote thick, non-uniform bond lines and high contact resistance compared with more compliant greases. [S1][S3][S4]
- They are risky in applications with strong coefficient-of-thermal-expansion mismatch and frequent thermal or power cycling where interfaces are difficult to rework, since pump-out and dry-out driven coverage loss has been shown to increase thermal resistance and junction temperature over relatively short cycling histories. [S6][S7][S8]
- High-viscosity h-BN TIMs are not advisable where manufacturing cannot tightly control mixing, degassing and dispense patterns, because such formulations are more prone to retain voids and agglomerates that make thermal performance highly sensitive to lot-to-lot and operator variation. [S2][S4][S9]
Decision Next Step
Switch approach when:
- Consider switching to a high-loading h-BN TIM when coupon-level ASTM D5470-style measurements show consistently lower thermal impedance than the incumbent grease at the bond line conditions your process can realistically maintain, and this improvement persists across repeated assemblies. [S2][S4][S5]
- Switch when large-area thermal cycling fixtures and in-situ imaging indicate that the h-BN TIM maintains coverage and exhibits equal or slower pump-out and void growth than the incumbent over the relevant cycling envelope. [S6][S7]
- Switch when package-level junction-to-case or junction-to-ambient characterization on JEDEC-style boards confirms that device temperatures are lower or at least no worse than with the incumbent across representative power profiles and clamping schemes. [S8][S10]
Do not switch yet when:
FAQ
Q: Why can a higher bulk thermal conductivity h-BN TIM still lead to a hotter device?
A: Because junction temperature is dominated by total interface resistance rather than bulk conductivity alone, a high-loading h-BN TIM that is much more viscous can form a thicker, less uniform bond line, trap more air in roughness valleys, and lose coverage through pump-out during cycling, so the effective thermal path from junction to heatsink becomes worse than with a more compliant but lower conductivity grease.
Related Technical Paths
Evidence Boundary Line
This Technical Insight covers polymeric h-BN filled thermal interface materials under clamped chip-to-sink style interfaces and excludes solders, liquid metals and unconstrained gap fillers.
Sources
- [S1] Thermal Interface Materials: A Brief Review of Design Characteristics and Materials (Electronics Cooling)
- [S2] Three-Dimensional Heterostructured Reduced Graphene Oxide–Hexagonal Boron Nitride-Stacking Material for Silicone Thermal Grease with Enhanced Thermally Conductive Properties (Nanomaterials)
- [S3] Rheological Properties and Thermal Conductivity of Epoxy Resins Filled with a Mixture of Alumina and Boron Nitride (Polymers)
- [S4] Thermal Interface Materials with Hexagonal Boron Nitride and Graphene Fillers in PDMS Matrix: Thermal and Mechanical Properties (Energies)
- [S5] ASTM D5470-17(2024) Standard Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials (ASTM International)
- [S6] Degradation Characterization of Thermal Interface Greases (NREL)
- [S7] Visualization test method to evaluate pump-out phenomena of thermal grease during thermal cycling (Transactions of the JSME)
- [S8] Thermal Interface Materials for Power Electronics Applications (NREL)
- [S9] On the assessment of voids in the thermal interface material on the thermal performance of silicon chip packages (Microelectronics Reliability)
- [S10] Thermal Characterization of Packaged Semiconductor Devices (Renesas Electronics)
Reviews design drivers for TIMs and shows that bond line thickness increases with filler loading due to higher yield stress, competing with gains in thermal conductivity and impacting contact resistance and pump-out.
Demonstrates that adding h-BN nanosheets to silicone thermal grease sharply increases viscosity and limits usable loading, and compares this with a graphene–h-BN hybrid that achieves higher conductivity at lower viscosity.
Studies epoxy composites with alumina and boron nitride platelets, showing that higher BN content increases viscosity and viscoelastic modulus while boosting thermal conductivity, with an optimal BN level balancing processability and performance.
Investigates PDMS-based TIMs with h-BN and graphene fillers, reporting large gains in through-plane thermal conductivity at high filler loading but also strong viscosity increases, voiding and reduced performance when viscosity becomes too high.
Defines a steady-state guarded heat flow method used widely to measure thermal impedance and apparent thermal conductivity of TIMs, including greases, gels and pads, under controlled pressure and temperature.
Characterizes commercial thermal greases using an ASTM D5470-based tester and large-area fixtures, showing that pump-out and dry-out during temperature cycling increase thermal resistance and degrade interface coverage.
Uses a bimetal plate fixture and ultrasonic imaging to visualize thermal grease pump-out under thermal cycling, linking void formation and grease movement to increased thermal contact resistance.
Provides an overview of TIMs in power electronics, highlighting that the grease layer often dominates package thermal resistance and that reducing interface resistance requires controlling bond line thickness, contact and reliability.
Uses analytical and numerical models to quantify how void concentration and distribution in TIM layers affect junction-to-air thermal resistance, showing that voids can significantly degrade package thermal performance.
Summarizes JEDEC JESD51 series methods for measuring junction-to-ambient, junction-to-case and junction-to-board thermal resistances, and discusses how these system-level metrics relate to package and TIM behavior.
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