Thermal interface materials come in many forms. To keep things simple, we group them by function into three categories: thermal gap fillers, thermal encapsulants and potting materials and thermally conductive adhesives.
Product Types
Product Comparison
Define Your Thermal Interface Material Requirements, from Concept to Finished Product
Thermal interface materials help electronic devices dissipate heat efficiently, but requirements differ from device to device and are set by the product's use, production lifecycle, package design and operating conditions.
Thermal conductivity and thermal resistance affect heat transfer differently at different bond line thicknesses:
Thin - below 75 µm: thermal resistance is decisive.
Medium - 75 to 250 µm: thermal conductivity and thermal resistance both influence heat transfer.
Thick - above 250 µm: thermal conductivity is decisive.
Thermal Properties
Electrical Properties
Almost all electronic devices require thermal interface materials with good electrical insulation. Two parameters matter most: dielectric breakdown voltage and volume resistivity.
High-speed, high-frequency devices also require attention to permittivity and dielectric loss to avoid electromagnetic interference.
Crosslinkable, curable thermal interface materials require attention to hardness, tensile strength, elongation at break and compressive stress.
For fragile chips such as BGA packages, choose materials with low hardness and low modulus to avoid crushing the chip.
Mechanical Properties
Processability
Thermal grease can be applied in several ways - screen printing, blade coating, manual roller application - each with different requirements for flowability and thixotropy that deserve close attention.
Thermal gels are classic non-Newtonian fluids, and measuring and characterizing their viscosity accurately is far more complex than it appears.
For products such as thermal gels, extrusion rate is a simpler, more effective and more intuitive characterization method than viscosity.
Evaluate liquid-phase and vapor-phase silicone migration using indicators such as weight loss, oil spread diameter and low-molecular-weight siloxane emission.
Material compatibility: no chemical reaction and no electrochemical corrosion with chips, heat sinks and other components.
Compatibility with Operating Conditions
Long-Term Reliability
Electronic components must remain stable over a long product lifecycle, and those applications may involve demanding conditions such as extreme temperatures and high humidity. All of this must be taken into account when selecting a thermal interface material.
- Dual-85 test: 85 °C, 85% RH × 1000 h
- High-temperature aging: 150 °C × 1000 h
- Thermal cycling: -40 to 125 °C, 1000 cycles
After these tests, assess whether changes in the material's basic, thermal and electrical properties remain within an acceptable range.
Almost all electronic devices require thermal interface materials to comply with environmental regulations such as RoHS and REACH, which prohibit heavy metals and other hazardous substances.
Automotive applications must additionally comply with GB/T 30512-2014, Requirements for Prohibited Substances on Automobiles.
Laws and Regulations
Learn More About Thermal Interface Materials
We now look at TIMs from a formulation perspective, and at how the formulation determines material performance.
Formulation
TIM Formulation
Thermal interface materials consist of three parts: a polymer, thermally conductive fillers and functional additives.
Silicone materials are the mainstream polymer choice for thermal interface materials thanks to their excellent high- and low-temperature performance and good wetting behavior.
The thermally conductive filler is the component that determines a thermal interface material's thermal performance. Note in particular that fillers fall into two broad classes, insulating and non-insulating; a thermal interface material containing a non-insulating filler must be matched carefully to the application.
Functional additives include pigments, heat stabilizers and rheological additives, and further improve material performance.
From a Formulation Standpoint, Raising Thermal Conductivity and Lowering Thermal Resistance Are Competing Design Goals
For thin-bond-line products such as thermal grease, thermal conductivity cannot reach the levels achieved by thermal gels.
Taking electrically insulating thermal grease as an example, thermal conductivity above 5 W/m·K combined with thermal resistance below 0.1 °C·cm²/W already represents a leading product in the industry, and it is also close to the material limit, leaving little room for further improvement.
Thermal gels can reach relatively high thermal conductivity, but their thermal resistance is also relatively high and their BLT is relatively large.
Non-chlorinated aromatic and aliphatic solvents: toluene, xylene, hexane, mineral oil.
Polyethylene with the anti-blocking agents, antioxidants or other additives listed above.
Pigmented primers containing the compounds listed above.
100% sodium salt primers such as sodium alginate or sodium carboxymethyl cellulose; however, if these salts are used together with hydroxyethyl cellulose, no inhibition occurs.
Clay coatings using polyvinyl acetate or acrylic latex as the binder.
Coatings containing calcium carbonate.
Coatings composed of the following combinations: natural rubber latex/clay; latex/ethyl starch; styrene/acrylic; polyvinyl acetate; polyvinyl acetate/acrylic.