Of course, pushing the edge on speed, as may be required in some video applications, most times means higher dissipation. At the same time, there is a need for smaller and smaller packages. This brings about a dilemma in how to remove the heat generated from within a small package. Most small packages do not lend themselves to using external heat sink, though this is possible in some cases. One of the ways to get the heat out of the package is to use the copper area on the PC board as heat sink. Here you see the effective thermal resistance of the board as a function of the copper area used. An example is worked out here where knowing the device dissipation, the designer would want to know how much copper area is required to keep the junction temperature safe. The graph shown is consulted to arrive at the result. Keep in mind that this plot assumes fairly good thermal coupling between the device and the board.  Some data sheets may show the recommended land pattern to allow good heat transfer. When this information is not available, however, layout the board for thick connections to the ground plane and power plane if used using minimum via holes. Keep in mind that heat flow and current flow are essentially analogous in that they both require minimum resistance for easier flow. The designer may have to avert to using multi-layer boards if the requirements for heat dissipation interfere with the need to place the external components as close as possible to the leads where they are used. In the example worked out here, the power dissipation, maximum operating temperature, and the device package type are known. These known parameters are then used to come up with how much board area needs to be devoted to heat sinking. First the ThetaJA of the PC board heat sink is calculated to be about 35 degrees C per watt and then the plot shown is consulted to determine the approximate board area necessary. This is a very effective way of reducing junction temperature and thereby increasing reliability, especially for miniature packages in high speed applications such as video which generally tend to burn more power in the device.