Normally stated in terms of °C/W, it is the indicator of the package's ability to dissipate the heat generated by the chip during operation. ?JC is the indicator of the chip's ability to pass the heat generated by the semiconductor junctions to the package, that is the thermal resistance between the semiconductor junction and the case; ?JA is the thermal resistance between the semiconductor junction and the ambient environment, or the indicator of the case's ability to pass chip heat into the ambient air.