The key factor is the thermal path from the LED junction to the outside of the package. Heat should be conducted away from the LED in an efficient manner, and then removed from the area by convection. This latter process can be passive, involving convection from the outside of the package or from a finned heatsink with a large surface area, while higher-power arrays may require active convection using forced air cooling or water cooling. Each interface produces a resistance to heat transfer, which means the number of interfaces should be reduced, and the thermal resistance between mating surfaces should be minimized. Surfaces are rarely smooth, for example a metal heat sink might have grooves where it has been milled. If two rough surfaces are mated, most of the heat transfer takes place via point contacts, since air is a poor conductor of heat. This problem can be overcome by filling the gap with a soft, thermally conductive material.