"In many cases, mounting of a heat sink over the full board area is not necessary or practical, for example, when the RF circuit board has yet to be mounted on a metal holder or in a metal housing. A local heat sink integrated in the RF circuit board will then suffice to transfer heat away from the component to the underside of the circuit board and then to an external heat sink.
For example, areas with thermal vias can dissipate the heat through the circuit board. If their thermal conductivity is not high enough, pieces of solid copper (Cu coins) are inserted in the circuit board. Copper has high electrical and thermal conductivity properties and can be readily integrated into circuit board designs.
method is to embed the copper pieces in an RF multilayer circuit board. High-frequency circuits are often stacked up to form a so-called hybrid design."