Thermal Resistance calculation
1 Thermal Resistance
The heat caused by the power loss Ptot in the active semiconductor region during operation results in an increased temperature of the component. The heat is dissipated from its source (junction J or channel Ch) via the chip, the case and the substrate (pc
board) to the heat sink (ambient A). The junction temperature TJ at an ambient temperature TA is determined by the thermal resistance RthJA and the power dissipation
2 RF and AF Transistors and Diodes in SMD Packages
In SMD packages the heat is primarily dissipated via the pins. The total thermal resistance in this case is made up of the following components:
RthJA thermal resistance between junction and ambient (total thermal resistance)
RthJS thermal resistance between junction and soldering point
RthJT thermal resistance between junction and chip base (chip thermal resistance)
RthTS thermal resistance between chip base and soldering point (package/alloy layer)
RthSA thermal resistance between soldering point and ambient (substrate thermal resistance)