The objective of any thermal solution is to ensure that a devices operating temperature does not exceed the safe limits defined by its manufacturer. In the electronics industry, this operating temperature is referred to as the devices junction temperature. In a processor, for example, this term literally refers to the semiconductor junction where electrical power is converted to heat.
To maintain operation, the heat must flow out of the semiconductor at such a rate as to ensure acceptable junction temperatures. This heat flow encounters resistance as it moves from the junction throughout the device package, much like electrons face resistance when flowing through a wire. In thermodynamic terms, this resistance is known as conduction resistance and consists of several parts. From the junction, heat can flow toward the case of the component, where a heat sink may be located. This is referred to as 
or junction to case thermal resistance. Heat can also flow away from the top surface of the component and into the board. This is known as junction to board resistance, or 
Junction to board resistance is defined as the temperature difference between the junction and the board divided by the power when the heat path is from junction to board only. To measure junction to board, the top of the device is insulated and a cold plate is attached to the board edge (Figure 1). This is the true thermal resistance, which is the characteristic of the device. The only problem is that, in a real application one does not know how much power is being transmitted from different paths.
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