What Is Heat Conduction?
There are three methods of heat transfer: heat conduction through solids, heat convection through fluids and gases, and heat generated by radiation. This report focuses on heat conduction as it dominates the heat transfer in PCBs and is therefore most relevant to temperature measurements. Heat conduction is defined as the transfer of heat through a volume or a body. Heat is transferred through microscopic collisions of particles; the more collisions, the hotter the object is. Heat transfer occurs when there is a temperature difference between two objects or between different areas of an object, and its rate depends on the geometry, thickness, and material of the object. Due to the law of equilibrium, heat transfers from a hotter body to a colder body until the whole system reaches final equilibrium, as shown in Figure 1. There is no net heat transfer between two objects that are equilibrium temperature. The equation for heat transfer through conduction is shown in Equation 1.
Thermal conductivity (k) is the measure of a material's capability to conduct heat. It is used to describe how heat conducts through a material. Metals are highly thermally conductive whereas materials like air, wool, paper, or plastic are poor conductors of heat. Materials with a very low thermal conductivity, such as polystyrene foam, act like a thermal insulator.
The materials that are most relevant to thermal analysis of PCBs are copper, FR4, and solder mask. Copper is an excellent conductor of heat; it conducts heat significantly faster than FR4. Table 1 lists the thermal conductivities found in PCBs. The higher the value, the more efficient the material is in transferring heat, which results in a shorter thermal response time. For low k values, the temperature gradient between the source and the sensor can be significantly large and must be considered carefully during layout.