# What does junction mean in junction to ambient thermal resistance?

1. ## What does junction mean in junction to ambient thermal resistance?

There are multiple terms used when talking about thermal resistance. This includes the case, sink, die, board, package, ambient and junction. The thermal resistance between these different things is used to calculate the temperature of the device for a given power dissipation.

Junction is basically a place where two things meet. What does junction in junction to ambient temperature refer to? Does it have practical uses as is?

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2. ## Re: What does junction mean in junction to ambient thermal resistance?

Junction refers to semiconductor junction (e.g. of a transistor), in other words the chip temperature.

The "junction" or chip is the heat source and obviously the hottest point in chain. It's also the most sensitive point.

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3. ## Re: What does junction mean in junction to ambient thermal resistance?

Junction is the point where two semiconductor materials meet. The Junction to ambient temperature basically conveys how much this semiconductor junction heats up based on the ambient temperature of the device. Now this junction temperature is basically depends on the material property and the surface area. It is known as thermal resistance in other words.

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4. ## Re: What does junction mean in junction to ambient thermal resistance?

You could ask "which junction?"; often the power dissipation
is localized, not uniform, and die-scale calculations from uniform
heating are not accurate or useful in predicting reliability or
electrical effects of thermal rise at the hot spot. While a power
device of the simplest sort may have only one, two junctions
closely coupled, ICs have many and not necessarily related in
thermal or electrical conditions. But as a rule this is indeed the
rating's basis (a standardized ambient airflow, and a diffuse
thermal input to the chip).

Using thermal imaging it's not unheard of to see a lot of
of spread in an IC that's got nonuniform power dissipation.

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