More and more applications today contain LED components. LEDs have the property that they create and suffer from heat. So it is imperative to lead the heat away from the component. This can be done by using IMS boards (ALU) and mount it on an heat sink. But when a more complex layout for the board is needed, one can no longer rely on the single sided IMS but will need a double sided board. Heat sink paste allows to do that. Vias are placed under the specific component and filled with heat sink paste or thermal conductive and electrical insulated paste. The paste in the vias will lead the heat through the vias to the other side of the board where it can again be brought in contact with a heat sink. This type of technique is often used for pulsing LED (examples are warning lights on emergency vehicles).
Our Design Guidelines have a section on heat sink on page 21.