Thermal Pad, Gel or Grease? A Comparison Guide to Thermal Interface Materials

In thermal design for power devices and battery packs, the chip or cell and its heatsink may look firmly attached, but less than 10% of two solid surfaces actually touch — the rest is air trapped in microscopic gaps, and air conducts at just 0.026 W/m·K. A thermal interface material (TIM) replaces that air with a conductive path. The catch: TIMs come as pads, gels and greases, and picking the wrong family often hurts more than picking the wrong conductivity number.

What each family looks like

Thermal pads are pre-formed silicone sheets with a fixed as-shipped thickness, compressed into the gap by assembly pressure — the simplest to use, with straightforward tolerance control.

Thermal gels are paste-like materials dispensed by equipment. Before cure they flow and conform to warped surfaces; after cure they become soft solids. Low-bleed one-part post-cure gels are well suited to volume production lines.

Thermal greases never cure and stay paste-like, offering the lowest thermal resistance — but under high temperature and pump-out they dry out and migrate over time, so they suit ultra-low-resistance, periodically maintained assemblies.

Three field rules of thumb

• Battery packs / ESS: cell-to-coldplate gaps are usually above 0.5 mm with stacked tolerances — start with pads or flowable gels; check UL94 rating when flame retardancy is required.
• Modules with poor die coplanarity (IGBT, SiC power modules): gel flow absorbs height differences far better than pads, avoiding local voids.
• SoC-to-vapor-chamber in consumer electronics: consider grease only when gaps are under 0.3 mm and ultimate resistance matters — and accept its maintenance interval.

One overlooked fact: resistance is not conductivity

Many selections fixate on the W/m·K number, but total interface resistance = bulk resistance (thickness ÷ conductivity) + contact resistance at both faces. A 1 mm pad at 5 W/m·K already carries ~0.2 cm²K/W of bulk resistance, and insufficient clamp pressure can add just as much again. So check compression ratio and hardness for pads, wettability and thixotropy for gels, and printed thickness control for greases — conductivity without assembly context is meaningless.