Thermal resistance simply is the opposition offered by the materials involved to the flow of heat energy. Heat flows when there is a temperature difference from a hot junction to a cold one. It is conceptually similar to electrical resistance with current analogous to heat and temperature difference analogous to voltage difference. The relationship between thermal resistance, heat flow (power dissipation) and temperature difference is shown in Equation 1.
In Equation 1, ΔT defines the type of thermal resistance based on whether the resistance in question is between the chip junction to ambient location or between chip junction to package case and so on. One of the commonly used thermal resistance terms, Junction to Ambient Thermal Resistance (Θja), is described in detail below.
Θja measures the ability of the package to dissipate heat from the chip junction to the ambient environment. The methodology to calculate Θja is based on JEDEC standard environment (JESD 51-2 for natural convection and JESD 51-6 for forced convection environments). It is used to rate and compare thermal performance of packages but has little relevance in an actual application environment if the environment differs from JEDEC standards. Equation to calculate Θja is shown in Equation 2 and the respective temperature monitor points in the following figure.