Thermal Resistance, Junction-to-Board 2.5 °C/W RθJA Thermal Resistance, Junction-to-Ambient (Note 1) 64 Note 1: RθJA is determined with the device mounted on one square inch of copper pad, single layer 2 oz copper on FR4 board. See https://epc-co.com/epc/documents/product-training/Appnote_Therm.