This property is a measure of how a material of a specific thickness resists the flow of heat. 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. Schematic representation of two surfaces in contact and heat flow across the interface. Interface material compressed between two contacting surfaces. Surface flatness, surface roughness, clamping pressure, material thickness and compressive modulus have a major impact on contact resistance.