HMC441LM1 thermal resistance

The thermal resistance specified in the datasheet (118.2 degC/W) is from the die to the metal slug (or paddle) on the underside of the device.  We do not specify the thermal resistance from the die to the (top of the) plastic case because this path has high thermal resistance and we don't want to suggest that you add a heat sink on the top of the package (a waste of space and money)

We have to be careful with language here because there seems to be inconsistency in how people understand junction-to-case and junction-to-board. I understand junction-to-board to be from junction to the metal on the underside of the pakcage (assuming the part has a metal slug on its underside). So this is the number we provide in our datasheets. And I understand junction-to-case to be from die to the top or side of the package. 

The thermal resistance specified in the datasheet (118.2 degC/W) is from the die to the metal slug (or paddle) on the underside of the device.  We do not specify the thermal resistance from the die to the (top of the) plastic case because this path has high thermal resistance and we don't want to suggest that you add a heat sink on the top of the package (a waste of space and money)

We have to be careful with language here because there seems to be inconsistency in how people understand junction-to-case and junction-to-board. I understand junction-to-board to be from junction to the metal on the underside of the pakcage (assuming the part has a metal slug on its underside). So this is the number we provide in our datasheets. And I understand junction-to-case to be from die to the top or side of the package.