This property is a measure of how a material of a specific thickness resists the flow of heat. The relationship between kand R is shown by substituting Equation (2) into (1) and rearranging to form (3)
Equation 3 shows that for homogeneous materials, thermal resistance is directly proportional to thickness. For non-homogeneous materials, the resistance generally increases with thickness but the relationship may not be linear.
Thermal conductivity and thermal resistance describe heat transfer within a material once heat has entered the material. Because real surfaces are never truly flat or smooth, the contact plane between a surface and a material can also produce a resistance to the flow of heat. This contact plane is depicted in Figure 1. Actual contact occurs at the high points, leaving air-filled voids where the valleys align. The air voids resist the flow of heat and force more of the heat to flow through the contact points. This constriction resistance is referred to as surface contact resistance and can be a factor at all contacting surfaces.
The thermal impedance, θ, of a material is defined as the sum of its thermal resistance and any contact resistance between it and the contacting surfaces as defined in Equation (4).
Surface flatness, surface roughness, clamping pressure, material thickness and compressive modulus have a major impact on contact resistance. Because these surface conditions can vary from application to application, the thermal impedance of a material will also be application dependent.