Series Resistance from Package to Air is usually measured using the Kelvin per Milliwatt[K/mW] for Thermal Resistance. Kelvin per Watt[K/mW], Degree Fahrenheit hour per Btu (IT)[K/mW], Degree Fahrenheit Hour per Btu (th)[K/mW] are the few other units in which Series Resistance from Package to Air can be measured.