Thermal grease

Thermal grease (also called thermal compound, paste, or TIM grease) is a viscous, thermally conductive compound applied between a heat source and a heatsink to fill the microscopic surface irregularities that would otherwise trap insulating air pockets. Unlike a thermal pad it has no defined thickness — clamp pressure squeezes it down to a bondline of typically 10–50 µm, giving the lowest contact thermal resistance of any TIM class. The trade-off is it requires careful application, can pump out under thermal cycling, and is messier to rework than a pad.

Pick grease when the bondline is thin and consistent (a flat IHS against a flat copper cold plate, sub-50 µm gap), and when achieving the absolute lowest thermal resistance matters — high-end CPUs, GPUs, RF power amplifiers. Pick a pad (TIF) when the bondline varies more than a few hundred microns, when assembly speed and reworkability matter, or when you want a dry part on the BOM. Most data-centre boards use both: pad on memory and VRMs, grease on the GPU die.

TIG7835L is a gallium-indium-tin alloy that is liquid at room temperature, with thermal conductivity ≈ 35 W/m·K — about an order of magnitude above the best silicone grease. It's used for direct die contact on the highest-flux processors. CRITICAL CAUTION: liquid metal is electrically conductive (resistivity < 10⁻⁴ Ω·m) and corrodes aluminum on contact — never use it on aluminum heat spreaders, near exposed conductors, or without a containment dam. Confirm package material (typically nickel-plated copper or nickel-plated aluminum acceptable) before specifying.

Thixotropic greases shear-thin under pump pressure (so they dispense easily) but quickly regain higher viscosity at rest (so they stay where placed and resist gravity flow or thermal-cycle pump-out). Use the -S grades for vertical-mount assemblies, vibration-prone hardware (vehicles, industrial drives, server fans), or any place where standard grease would migrate over its service life. λ is comparable to non-S equivalents at the same numeric grade.

With proper preparation (lapped surfaces, controlled clamp pressure, single thin spread or pea-method dot) silicone grease typically settles at 10–30 µm bondline, liquid metal at 1–10 µm. Measured ASTM D5470 thermal impedance follows the bondline; thinner is always better in terms of °C·in²/W until air gaps re-form. The TIG780-50HP datasheet quotes 0.008 mm (8 µm) bondline as a target for high-performance assembly.

Quality silicone-based grease (TIG780 series) typically holds performance for 7–10 years at moderate temperatures and 3–5 years above 100 °C continuous service. Pump-out is the dominant failure mode in thermally cycled assemblies — choose a thixotropic grade (-S) for vibration or cycle-heavy environments. Liquid metal does not pump out but can migrate via wetting; design a containment dam if the heatsink isn't sealed.