Thermal Grease Formulation: PDMS Base with Al₂O₃, BN, and ZnO Fillers
title: "Thermal Grease Formulation: PDMS Base with Al₂O₃, BN, and ZnO Fillers" description: "How thermal interface greases are formulated from PDMS silicone oil combined with thermally conductive fillers—filler selection, loading levels, and performance trade-offs." section: "midstream"
What Thermal Grease Does
Thermal interface materials (TIMs) fill the microscopic air gaps between a heat source (CPU, power transistor, LED package) and a heat sink. Air has thermal conductivity of ~0.025 W/m·K; a well-formulated thermal grease reaches 3–12 W/m·K. The grease's function is not to conduct heat itself—the filler does that—but to displace air and conform to both surfaces under the mechanical pressure of mounting hardware.
PDMS silicone oil (typically 50–5,000 cSt) is the dominant carrier because it remains fluid over –50 to +200 °C, does not dry out, is electrically insulating, and is chemically inert to most electronics materials. Metal-based and phase-change alternatives exist for extreme-performance applications, but silicone-based grease covers >60% of the TIM market by volume.
Filler Selection
| Filler | Thermal Conductivity | Density | Cost | Limitation |
|---|---|---|---|---|
| Al₂O₃ (alumina) | 20–36 W/m·K | 3.9 g/cm³ | Low | Moderate hardness; abrasive at high loading |
| ZnO | 25–54 W/m·K | 5.6 g/cm³ | Low | High density increases grease weight |
| BN (hexagonal) | 60–300 W/m·K (in-plane) | 2.1 g/cm³ | Medium–High | Anisotropic; requires orientation control |
| AlN | 170–200 W/m·K | 3.3 g/cm³ | High | Moisture sensitive; hydrolyzes to Al(OH)₃ |
| Diamond | 1,000–2,200 W/m·K | 3.5 g/cm³ | Very High | Reserved for aerospace/high-value electronics |
Alumina is the dominant filler in commercial thermal greases. It is available in spherical grades (best packing efficiency) and angular grades (higher surface area for bonding), typically at d50 = 1–10 µm.
Formulation and Loading
Typical thermal grease formulation: 75–92% filler by weight in PDMS base. At these loadings (above the critical pigment volume concentration), filler particles are in direct contact, forming conductive pathways. The silicone matrix fills inter-particle voids and provides the grease's conformability.
Key formulation challenges:
- Pump-out: High-loading greases can separate under repeated thermal cycling as PDMS migrates under mechanical pressure. Surface-treating the filler with silane coupling agents (e.g., KH-570) or using higher-viscosity base oils reduces pump-out.
- Bleed: Low-viscosity PDMS can migrate laterally from the bond line. Partial cross-linking of the PDMS matrix (thixotropic grades) controls bleed.
- Viscosity: The formulated grease must be printable (screen or stencil) at 50,000–500,000 cP, yet flow under clamping pressure (50–200 kPa).