It is measured at the center of the exposed-pad surface for exposed-pad packages. For standard plastic packages, Θ JC is measured at the corner of pin 1. Θ JC depends on the package materials (the lead frame, mold compound, die attach adhesive) and on the specific package design (die thickness, exposed pad, internal thermal vias, and thermal conductivity of the metals used).įor leaded packages, the Θ JC reference point on the case is where pin 1 emerges from the plastic. Case is a specified point on the outside surface of the package. Θ JC is the thermal resistance from junction to case. Θ JA values are listed for natural convention conditions (no forced air) only.
Generally, the effects of radiation are negligible. Ambient is regarded as thermal "ground." Θ JA depends on the package, board, airflow, radiation, and system characteristics. Θ JA is the thermal resistance from junction to ambient, measured as ☌/W. The following section defines Theta (Θ) and Psi (Ψ), standard terms used in thermal characterization of IC packages. The Maxim website ( Manufacturing, Layout, Production, QA/Reliability, Procurement) provides information about commonly used thermal-resistance values for ICs. By knowing the thermal resistance of a package, one can calculate the IC's junction temperature for a given power dissipation and its reference temperature. Given the temperatures at two points, the amount of heat flow from one point to the other is completely determined by the thermal resistance. The thermal resistance of an IC package is the measure of the package's ability to transfer heat generated by the IC (die) to the circuit board or the ambient. In calculations, thermal resistance is identified as "Theta," derived from the Greek word for heat, "thermos." It is thermal resistance that particularly interests us. Thermal management of semiconductors involves thermal resistance, which is an important figure of merit describing the heat transfer properties of material. The article also provides thermal calculations and data to ensure proper junction (die), case (package), and board temperature. In discussing package heat transfer, it defines important terms for thermal characterization, which begin with thermal resistance and its various "theta" representations.
This article helps designers and customers understand basic IC thermal-management concepts. Therefore, to maintain the device's junction temperature below the maximum allowed, effective heat flow from the IC through the package to the ambient is essential.
All ICs generate heat when power is applied to them. Thermal management should be considered during package selection to ensure high product reliability.
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