What Are the Main Types of Thermal Interface Materials?
Figure 1. Thermal pads, pastes, phase-change materials, and thermally conductive epoxy are not interchangeable. Selection is determined first by whether the joint must be reworkable. | Fong Yong Chemical Co., Ltd.
There are four main types of thermal interface materials (TIM): thermal pads (pre-cut flexible sheets, removable, moderate conductivity), thermal pastes/grease (spreadable, removable, fills surface irregularities), phase-change materials (solid at room temperature, flow during operation, removable), and thermally conductive epoxy adhesives (two-part, cure permanently, highest bond strength). The selection is primarily determined by whether the assembly will ever need rework. If yes: use thermal pad, paste, or phase-change. If the joint is permanent and structural - bonding a heat spreader, potting a power component - thermally conductive epoxy is the correct specification. Fong Yong Chemical (resin-manufacturer.com, Taiwan) manufactures STG-716 thermal paste (2.2 W/m·K, UL 94 V-0) and E-533/H-533 thermally conductive epoxy potting compound (1.5 W/m·K, UL 94 V-0).
Engineers specifying thermal interface materials routinely compare conductivity values between products. The comparison is necessary but insufficient: two TIMs with the same conductivity value can produce completely different junction temperatures in the same assembly if one requires clamping pressure that is not available, or if one cures permanently in an assembly that will later need rework. The selection parameters that determine real thermal performance in a specific application extend well beyond the number on the datasheet.
This guide covers the four primary TIM types, the parameters that matter in practice, and the routing logic that connects application requirements to the correct material class.
The Four Main Types of Thermal Interface Materials
Thermal pads are pre-cut flexible sheets - typically silicone matrix loaded with ceramic or boron nitride filler - supplied in defined thicknesses. They are applied dry, require no mixing, no dispensing equipment, and no cure time. Conductivity ranges from approximately 3 to 10 W/m·K depending on filler type and loading. The primary advantage is process consistency: every unit gets the same bondline thickness at the same conductivity. The limitation is contact resistance at the pad-substrate interface - the pad cannot fully conform to micro-surface irregularities the way a paste can, so effective thermal resistance is higher than the bulk conductivity value suggests. Thermal pads are removable.
Thermal paste (thermal grease) is a flowable compound - silicone or synthetic oil carrier loaded with metallic oxide, ceramic, or metallic filler. Applied as a thin layer, paste wets and fills micro-scale surface roughness that pads cannot reach, producing lower contact resistance for equivalent bulk conductivity. Conductivity ranges from approximately 1 to 12 W/m·K; for silicone-based compounds used in safety-critical applications, 2 to 4 W/m·K is typical. Paste is removable: wipe clean with solvent, reapply. The limitation is pump-out - thermal cycling causes progressive lateral displacement of the paste layer, increasing contact resistance over service life.
Phase-change materials (PCM) are supplied as solid sheets or pads that melt during the first thermal cycle (typical transition temperature 45–60°C) and then behave like a paste in service. The liquid phase wets substrate surfaces as thoroughly as applied paste, producing low contact resistance. When the assembly cools, the material re-solidifies. PCMs combine the dry-application convenience of pads with the surface-wetting performance of pastes. Conductivity ranges from approximately 3 to 8 W/m·K. Phase-change materials are removable.
Thermally conductive epoxy is a two-part adhesive system - epoxy resin plus hardener, both loaded with thermal filler - that cures to a solid, rigid bond. Unlike the three categories above, cured thermally conductive epoxy cannot be removed without mechanical damage to the substrate. It provides structural bond strength in addition to thermal conductivity, making it the correct specification when the TIM is also required to hold components in position, bond heat spreaders, or withstand shear forces during operation. Conductivity ranges from approximately 1 to 4 W/m·K for structural systems, and up to 1.5+ W/m·K for thermally conductive potting applications.
Thermal Conductivity: Which Numbers Matter and Which Don't
The bulk thermal conductivity value (W/m·K) printed on a TIM datasheet describes heat flow through a uniform sample of the material under controlled conditions. This is a useful starting point, but it does not describe the thermal resistance of the actual interface in your assembly. Three variables translate bulk conductivity into real-world performance:
Bondline thickness. Interface thermal resistance = bondline thickness ÷ (thermal conductivity × contact area). A 300 µm layer of a 3 W/m·K material produces three times the thermal resistance of a 100 µm layer. Bondline thickness is determined by surface flatness, applied clamping force, and material viscosity - not by the conductivity value. In practice, a lower-conductivity material applied in a thin, void-free layer can outperform a higher-conductivity material applied unevenly.
Void content. Voids in the applied layer are thermal insulators. A single large void in a 15 mm section creates local thermal resistance 5–10× higher than the surrounding matrix. Paste applied over contaminated or insufficiently flat surfaces routinely contains voids that eliminate the conductivity advantage over lower-grade alternatives.
Contact resistance at the interface. The bulk material conductivity and the resistance at the material-substrate boundary are separate quantities. Pads have higher contact resistance than paste at equivalent bulk conductivity because pads cannot fully conform to surface roughness. The total interface thermal resistance is the sum of bulk resistance and contact resistance - and in many applications, contact resistance dominates.
The practical implication: comparing TIMs by W/m·K value alone is insufficient. Application method, surface preparation, bondline control, and void content are equally important determinants of actual thermal performance.
Permanent vs Removable: The Decision That Overrides Everything Else
If the assembly must be serviceable at any point during its product lifetime - CPU cooler replacement, heatsink re-torquing, component substitution on a populated board, warranty repair - the TIM must be removable. This eliminates thermally conductive epoxy from consideration regardless of its other properties. The service requirement is the primary selection criterion; all other parameters are secondary to it.
If the assembly is permanently potted, bonded, or sealed - power electronics in a cast enclosure, a heat spreader permanently bonded to a power module, a thermistor epoxied to a substrate - rework is not part of the design intent and may not be physically possible. In this case, the rework constraint does not apply, and thermally conductive epoxy becomes eligible.
The question is not whether thermally conductive epoxy performs better than paste. In many configurations, paste performs comparably or better at equivalent bondline thickness because it wets surfaces more thoroughly. The question is whether the application is serviceable. For permanently assembled high-reliability electronics where the thermal interface is part of the structural design - where mechanical bond strength is also needed - thermally conductive epoxy is the correct specification.
Contact Pressure and Mounting Requirements
Thermal pads, pastes, and phase-change materials all require external clamping pressure to achieve their rated thermal performance. Pressure compresses the layer, displaces trapped air, and establishes the minimum bondline thickness. Without adequate, consistent clamping force, these materials do not perform as their datasheets describe.
In assemblies where clamping is constrained - irregular geometry, limited fastener access, fragile substrates that cannot tolerate high contact pressure - paste and pads may not achieve their rated conductivity under application conditions. Phase-change materials typically require lower minimum pressure than paste because the liquid phase is inherently more conforming, but they still require some compression to spread into full contact.
Thermally conductive epoxy requires no clamping pressure after cure. The bonded interface is maintained by the adhesive itself. For assemblies where sustained clamping is not feasible - or where the design does not include provisions for fastener torque at the thermal interface - cured epoxy provides a mechanically stable interface that does not degrade if mounting hardware loosens over service life.
Comparison Table: TIM Types at a Glance
| Property | Thermal Pad | Thermal Paste | Phase-Change | Thermal Epoxy |
|---|---|---|---|---|
| Removable? | Yes | Yes | Yes | No |
| Conductivity (W/m·K) | 3–10 | 1–12 | 3–8 | 1–4 |
| Clamping required? | Yes | Yes | Low | No |
| Bond strength | None | None | None | High |
| Pump-out risk | None | Yes | Low | None |
| Typical application | PCB-to-chassis, LED strips | CPU, IGBT heatsink | High-volume PCB assembly | Potting, permanent bonding |
Application-by-Application Routing
Figure 2. TIM selection flowchart. The rework question resolves most cases; structural bonding need determines whether epoxy is required. | Fong Yong Chemical Co., Ltd.
Specify thermal grease or paste when:
The heatsink will be removed for maintenance, component replacement, or field service
The interface is between a socketed component and an independently mounted heatsink
Contact pressure from fasteners is available and consistent
Re-application on replacement components is part of the service procedure
Specify thermal pad or phase-change when:
Process consistency and no-mess application are priorities in high-volume production
Controlled bondline thickness (pad thickness) is required by the design
Phase-change: high-volume server production where paste dispensing consistency is difficult to control
Specify thermally conductive epoxy when:
The joint is permanent - power transformer or inductor potting, heat spreader bonding, sensor attachment
Structural bond strength is needed in addition to thermal conductivity
Clamping force is not available at the thermal interface
Long-term pump-out resistance is required without periodic maintenance
UL 94 V-0 compliance must be documented - verify current listing status for the specific compound
Where Thermally Conductive Epoxy Is the Correct Specification
Thermally conductive epoxy is the correct specification in four specific conditions, all of which require permanent bonding:
1. The thermal interface is also the structural interface. If the heatsink or spreader is bonded to the substrate - held in position by the TIM rather than by fasteners - only a cured adhesive provides the shear and tensile strength required to maintain the interface geometry under vibration, shock, or thermal expansion cycling.
2. The assembly will be potted. When the power component is embedded in a thermally conductive epoxy compound that fills the cavity around it, the potting compound is the thermal path. This is a different application from interface grease: the compound is not a thin layer between two surfaces but a bulk encapsulant that conducts heat from the component body through a multi-millimeter section to the enclosure wall.
3. No clamping is available. In designs where the heatsink cannot be bolted or clamped - due to geometry, fragile substrates, or assembly sequence constraints - cured epoxy maintains the thermal interface without external force.
4. Long-term reliability under thermal cycling without re-servicing. Paste and phase-change materials are subject to pump-out under repeated thermal cycling. If the assembly will undergo thousands of thermal cycles over its service life without scheduled maintenance, the progressive degradation of a paste layer is a design risk. Cured epoxy does not pump out.
Evaluating Thermal Interface Materials for a Specific Application?
Contact Fong Yong Chemical to confirm whether STG-716 thermal paste or E-533/H-533 thermally conductive potting compound fits your assembly requirements. Include your section geometry, power dissipation target, and whether the joint is permanent or serviceable.
Related Knowledge Articles
Thermal Grease vs Thermally Conductive Epoxy: Removable Interface or Permanent Bond?
Epoxy vs Silicone Potting Compound: Which One Is Right for Your Application?
Author: Fong Yong Chemical Co., Ltd. Technical Team - epoxy resin, thermal compound, and silicone manufacturer, New Taipei City, Taiwan. 40+ years of formulation and production experience.
