Thermal conductivity test methods fall into two families, steady-state and transient, each based on a different principle. In the thermally conductive silicone industry the usual choice is the steady-state hot-plate (heat-flow) method, following ASTM D5470, the standard test method for thermal transmission properties of thin thermally conductive solid electrical insulation.

Principle

A sample of known thickness is clamped between an upper and a lower plate under a set heat flow and pressure. A heat-flux sensor reads the heat passing through the sample, and the instrument records the thickness and the temperature difference between the hot and cold plates, from which thermal resistance is calculated. The measurement is repeated at several thicknesses, and a straight-line fit of resistance against thickness gives the conductivity.

Why several thicknesses

The measured total resistance is the material's own resistance plus the contact resistance: R = d/λ + Rc. Plot R against thickness d: the slope is 1/λ and the intercept is the contact resistance. For example, a 1 mm sample measuring 3.8×10^-4 m²·K/W and a 2 mm sample measuring 7.2×10^-4 m²·K/W give a slope of (7.2−3.8)×10^-4 ÷ 0.001 = 0.34, so λ ≈ 1/0.34 ≈ 2.9 W/(m·K). These are illustrative numbers, not measurements of a specific product.

Advantages and limits

• Advantages: it yields both thermal resistance and conductivity, and the applied pressure resembles real service conditions.
• Limits: the sample must meet thickness requirements, contact resistance affects the result, and reaching steady state takes a long time.

When comparing data from different suppliers, confirm that the same method and the same pressure were used; otherwise the numbers may not be comparable.