An LED module includes a substrate having a high thermal conductivity and at least one LED die mounted on the substrate. A wavelength conversion material, such as phosphor or quantum dots in a binder, has a very low thermal conductivity and is formed to have a relatively high volume and low concentration over the LED die so that the phosphor or quantum dots conduct little heat from the LED die. A transparent top plate, having a high thermal conductivity, is positioned over the wavelength conversion material, and a hermetic seal is formed between the top plate and the substrate surrounding the wavelength conversion material. The LED die is located in a cavity in either the substrate or the top plate. In this way, the temperature of the wavelength conversion material is kept well below the temperature of the LED die. The sealing is done in a wafer level process.