Thermal Interface Materials: Types & China Sourcing

A thermal interface material (TIM) fills the microscopic air gaps between a heat-generating component — a CPU, power MOSFET, GaN device, or LED — and its heatsink or enclosure. Air conducts heat at roughly 0.025 W/m·K; a 6 W/m·K gap pad is about 240× better, which is why a bare metal-to-metal contact (which is really metal-to-air-to-metal at the microscopic level) throttles or fails. Choosing the wrong TIM, or accepting an overstated conductivity number from a supplier, is one of the most common causes of thermal throttling and field failure in China-built electronics.

Overview

No two solid surfaces touch across more than a few percent of their nominal area — the rest is air. TIM displaces that air with a material that conducts heat far better. It appears in nearly every product with meaningful power density: power electronics (MOSFET and IGBT modules), LED lighting (COB modules bonded to aluminum), fast chargers built around GaN power ICs, EV onboard chargers, and ruggedized industrial IoT boards inside sealed enclosures.

The single most important sourcing fact: the number that determines performance is in-situ thermal resistance at your actual bond line thickness, not the bulk conductivity printed on the datasheet. A material can quote 6 W/m·K from a laser-flash bulk measurement and still perform badly because of high contact resistance and a thick bond line. Chinese suppliers routinely quote the favorable bulk number; your job is to specify the test method that reflects reality (ASTM D5470).

Key Parameters

Conductivity is necessary but not sufficient — also specify bond line thickness, hardness (Shore OO for pads), operating temperature range, dielectric strength, and whether the material is electrically insulating, before comparing prices.

TIM Type Descriptions

Thermal grease / paste — lowest resistance per dollar Silicone or non-silicone carrier loaded with zinc oxide, aluminum oxide, or silver particles. Forms the thinnest bond line and gives the best thermal performance per dollar, which is why it dominates CPU/GPU and power-transistor mounting. Downsides: messy, hard to automate at volume, and subject to pump-out (grease squeezed out by repeated thermal expansion) and dry-out (carrier evaporating) over thermal cycles — both raise thermal resistance over the product’s life. Budget Chinese grease often fails here first.

Thermal gap pad — easiest to assemble Pre-cut silicone elastomer pads loaded with ceramic filler, often with a fiberglass carrier. The default for hand assembly and for filling larger, less precise gaps (0.5–5 mm). Specified by thickness and Shore OO hardness — a softer pad (Shore 00 ≈ 25) conforms with less mounting pressure and lower PCB stress; a harder, higher-filled pad conducts better but needs more compression. Higher thermal resistance than grease, but far more practical in production.

Dispensable gap filler — for automation and uneven gaps One- or two-part material a robot dispenses onto the board, then cures (or stays soft). Fills uneven, application-specific gaps without inventorying many pad thicknesses, and suits high-volume automated lines. Common in EV power modules, automotive ECUs, and dense DC-DC converter modules. Watch for entrapped air voids during dispensing — they become hotspots.

Phase change material (PCM) — grease performance without pump-out Solid and easy to handle at room temperature; softens at the operating temperature to wet both surfaces like grease, then re-solidifies on cooldown. Delivers near-grease thermal resistance while resisting pump-out, so it is used on CPUs and power devices that see many thermal cycles. Applied as a pre-applied pad or screen-printed film.

Thermal adhesive / epoxy — bond plus conduct Conducts heat while mechanically bonding the component to the heatsink, eliminating clips or screws — common for bonding LED COB modules and small heatsinks. Trade-off: rework is destructive. Use only where you will never need to remove the part.

Graphite sheet — lateral heat spreading Very high in-plane conductivity (400–1900 W/m·K) but modest through-plane. Spreads heat sideways away from a hotspot rather than driving it into a heatsink — used in phones, tablets, and thin devices with no room for a heatsink. Electrically conductive: keep it off exposed traces.

Liquid metal — highest performance, highest risk Gallium-based alloys reach 20–80 W/m·K, far above grease. But they are electrically conductive (a spill shorts the board) and corrode aluminum, so they require nickel-plated surfaces and careful barriering. Niche use only; rarely the right call for a mass-produced product.

Selecting a TIM

• Gap size: under 0.1 mm flat-to-flat → grease or PCM; 0.5–5 mm or uneven → gap pad or dispensable gap filler.
• Automation: hand assembly → pre-cut pads; robotic line → dispensable gap filler or pre-applied PCM.
• Rework: needs removal → grease or pad, never adhesive/epoxy.
• Electrical isolation: if the heatsink must stay isolated from the device → insulating pad/filler, never graphite or liquid metal.
• Thermal budget: size conductivity to the dissipated power and allowable temperature rise — do not over-buy W/m·K you cannot use.

Sourcing from China

Western incumbents — Dow (Dowsil), Henkel (Bergquist), DuPont/Laird, Shin-Etsu, Fujipoly, Parker Chomerics — are well qualified and carry a price premium. Chinese suppliers such as Aochuan and a cluster of Shenzhen-based makers run roughly 30–60% cheaper and are entirely viable for consumer, LED, and most industrial work — provided you verify performance rather than trust the datasheet. For a sourcing buyer, the decision is rarely “domestic vs import”; it is “verified vs unverified.”

The standard you reference is ASTM D5470 (steady-state thermal transmission of thermally conductive materials), which measures thermal resistance at a defined bond line — the figure that predicts real performance. Laser-flash gives a bulk conductivity number that is easy to quote and easy to inflate. Specifying D5470 in your PCB assembly and component purchase orders is the single most effective protection against an overstated claim.

Quality Inspection

• ASTM D5470 in-situ test: require it on incoming samples, not just the supplier’s laser-flash bulk number. This is the core check — see the broader electronics quality control plan for how it fits incoming inspection.
• Thermal cycling reliability: cycle samples (for example -40 to +125°C, 1,000 cycles) and re-measure thermal resistance. Cheap grease dries out and cheap pads harden — both show up here, not on day one.
• Bond line thickness control: verify the assembled bond line matches the spec; an over-compressed pad stresses the PCB, an under-compressed one traps air.
• Void inspection (dispensed fillers): X-ray or cross-section a sample to confirm no entrapped air voids under the die.
• Batch certificate of analysis: require a COA per lot and spot-test against it; Chinese batch-to-batch filler loading can drift.

What to Specify When Ordering from China

• Conductivity and test method: e.g., “≥3 W/m·K verified by ASTM D5470 at 0.5 mm bond line” — never a bare W/m·K number.
• Bond line / thickness and tolerance: the gap the material must fill, with tolerance.
• Hardness (pads): Shore OO value, to balance conformity against mounting stress.
• Operating temperature range and dielectric strength: matched to your product environment.
• Reusability and application method: hand-placed pad, screen-printed PCM, or robot-dispensed filler — state it.
• Compliance: RoHS and REACH documentation; low-outgassing grade if the assembly is sealed.

Common Issues

Overstated conductivity number: datasheet quotes 6 W/m·K (laser-flash bulk), but in-application thermal resistance is poor due to high contact resistance and a thick bond line. Prevention: specify and verify ASTM D5470 in-situ.

Pump-out and dry-out of budget grease: after thermal cycling, grease migrates out of the joint or its carrier evaporates, raising temperature until the device throttles or fails. Prevention: PCM or a qualified grease, plus thermal-cycle verification.

Gap pad over-/under-compression: too much compression stresses solder joints and the PCB; too little leaves an air gap. Prevention: specify pad thickness against the real assembled gap and confirm mounting pressure.

Voids in dispensed gap filler: entrapped air under the die creates hotspots invisible from the outside. Prevention: X-ray or cross-section sampling on the first production run.

Liquid metal corrosion / shorting: gallium alloy attacks bare aluminum heatsinks and shorts exposed copper. Prevention: avoid liquid metal in mass production unless surfaces are nickel-plated and fully barriered.

How this shows up in our work

On any thermal review, we treat the datasheet conductivity figure as a claim to be verified, not a spec. Our standing practice is to require an ASTM D5470 in-situ report rather than a laser-flash bulk number, pull samples from the production batch for third-party measurement, and thermal-cycle them before approving a first order — because the gap between a quoted bulk conductivity and real in-application thermal resistance is exactly where thermal designs fail. We also require a certificate of analysis per batch, since filler loading can drift between lots. The principle we hold to: verify the test method, not the headline figure.

Related Resources

• PCB Substrate Materials — board thermal conductivity affects the whole heat path
• GaN Charger ICs — high power density that makes TIM selection critical
• IGBT Modules — the baseplate-to-heatsink joint where TIM choice is critical
• EV Onboard Charger — power modules relying on dispensable gap fillers
• Conformal Coating — the other PCBA material spec buyers forget to define
• Electronics Quality Control in China
• Industrial IoT Hardware Sourcing
• Quality Inspection Services
• Power Electronics Sourcing