How to Select h-BN TIM Format: Paste vs Pad vs Gel for Variable Gaps

Direct Answer

Main failure reason: Pre-formed pads cause high mechanical stress on components when compressed to accommodate large tolerance stacks, whereas gels flow to minimize bond line thickness without significant pressure. ^[S7][S14]

Context

• Hexagonal boron nitride (h-BN) is a preferred ceramic filler for thermal interface materials (TIMs) in high-voltage applications due to its high dielectric breakdown strength (30-40 kV/mm) and intrinsic thermal conductivity. ^[S10]
• The primary challenge with h-BN is its platelet anisotropy; ^[S20][S24]
• in-plane thermal conductivity is significantly higher than through-plane conductivity, meaning the orientation of particles during processing dictates final performance. ^[S20][S24]
• Engineers must choose between three formats: pastes (greases) for thin bond lines (<0.1 mm), pre-formed pads for defined gaps (0.5–5.0 mm), and dispensable gels (gap fillers) for variable gaps with low stress requirements. ^[S4][S7]
• In automotive ECUs and power modules, tolerance stack-ups from casings and heat sinks often create unpredictable gap variations that static pads cannot accommodate without excessive compression force. ^[S14][S22]

Decision Logic

Format: Engineering Decision Table

Mechanism

Mechanism family: Particle Orientation & Rheology

• h-BN platelets tend to align in the direction of flow during dispensing (gels) or calendering (pads), which can negatively impact through-plane conductivity if platelets lay flat perpendicular to heat flow. ^[S19][S24]
• Dispensable gels utilize shear-thinning behavior to flow under low pressure during assembly, then recover viscosity to maintain shape, minimizing stress on mating components compared to the elastic compression required for pads. ^[S16][S23]
• Thermal pastes rely on wetting action to displace air at microscopic levels but lack the cross-linked polymer network needed to resist pump-out during thermal expansion cycles (CTE mismatch). ^[S4][S21]

Data Points

• h-BN filled composites exhibit shear-thinning where viscosity decreases with increasing shear rate, enabling automated dispensing; ^[S16][S23]
• typically, gels function effectively in gaps from 0.5 mm to 4.0 mm. ^[S16][S23]
• Silicone-based thermal pads typically require 10-50% compression to achieve rated thermal impedance, which can translate to significant closure force in large-area applications. ^[S14]

Practical Evaluation Checklist

• Measure effective thermal resistance (R_th) using ASTM D5470 steady-state method at minimum and maximum expected gap thicknesses. ^[S14]
• Check dispense rate and viscosity stability over time to ensure the h-BN filler does not settle or cause needle clogging. ^[S16]
• Validate pump-out resistance using a vertical slide test or accelerated mechanical cycling that simulates CTE-induced warpage. ^[S21]
• Compare assembly force requirements: record the load necessary to compress the TIM to the target bond line thickness (BLT). ^[S8][S14]
• Screen for dielectric breakdown voltage if the TIM also serves as the primary electrical isolation layer (h-BN advantage). ^[S10]

NOT suitable when…

• The application requires clean, solvent-free removal for frequent maintenance (gels and pastes leave residue). ^[S7][S11]
• The gap is extremely large (>5 mm) and vertical, where uncured gels may slump or flow due to gravity and vibration. ^[S22]
• Immediate handling is required after assembly, but the selected gel requires a heat or moisture cure cycle. ^[S9]

Common Misconceptions

• Does higher bulk thermal conductivity always mean lower thermal resistance? -> No, contact resistance and bond line thickness often dominate performance. because A 3 W/mK gel with a 0.1 mm BLT outperforms a 5 W/mK pad that can only compress to 0.5 mm due to hardness or tolerance limits. ^[S7][S9]

Decision Next Step

Switch approach when:

• Tolerance analysis shows gap variation exceeds the recommended compression range (10-40%) of a pad. ^[S14]
• Clamping force is limited by PCB flexure or fragile component packages. ^[S6][S22]

Do not switch yet when:

• The interface must provide structural support or vibration damping that only a solid pad can offer. ^[S22]

Next step: Review ASTM D5470 Testing Procedures

Related Technical Paths

• Compare: Why specify 99.9% vs 99.5% purity hexagonal boron nitride for TIMs?
• Failure mode: Why can high h-BN filler loading in TIMs worsen device temperatures?
• Adjacent path: Can h-BN replace Al₂O₃ in my TIM format? (Evaluation Plan)

Evidence Boundary Line

Evidence is valid for silicone and epoxy-based matrices filled with hexagonal boron nitride; phase-change materials (PCMs) and metal-based TIMs are excluded.

Sources

1. [S1] Boron Nitride and Graphene Thermal Pads: In-depth Technical Difficulties (Sheen Thermal)
2. [S4] Boron Nitride for Thermal Management (Momentive Technologies)
3. [S6] Thermal Pads vs Thermal Putty (T-Global Technology)
4. [S7] Thermal Gap Filler vs Thermal Gap Pads (Sheen Technology)
5. [S8] P-THERM Thermal Interface Materials Selection Guide (Polymer Science)
6. [S9] Thermal Gel vs. Thermal Pad: Key Differences Explained (Trumonytechs)
7. [S10] What Are the Characteristics of Hexagonal Boron Nitride? (Stanford Advanced Materials)
8. [S11] Thermal Gel vs. Thermal Pad (Gallop Innotek)
9. [S14] Thermal Gap Pad Compression: Optimizing Performance (Modus Advanced)
10. [S16] Design of Highly Thermally Conductive Hexagonal Boron Nitride Composites (ACS Applied Polymer Materials)
11. [S19] Sequential Dual Alignments Introduce Synergistic Effect on Thermal Conductivity (OSTI)
12. [S20] Thermal Conductivity of Polymer-Based Composites with Magnetic Alignment (PubMed)
13. [S21] Reliability Testing Of Thermal Greases (Electronics Cooling)
14. [S22] Thermal Gel vs. Gap Pad for EV Power Control Units (Taxo Tape)
15. [S23] Controlling Shear Rate for Designable Thermal Conductivity (PMC)
16. [S24] Hexagonal Boron Nitride In Heat Spreaders: In-Plane vs Through-Plane (Eureka)

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Published: 2026-02-08

Last updated: 2026-02-08