WHERE DOES THE HEAT GO

A path out, or a rising temperature

junction, then case, then ambient air

Heat generated inside a working die has to travel from the junction, the active silicon itself, through the package to its case, and finally out into the surrounding air. Every step of that path resists heat flow at least a little, and that resistance is what ultimately sets how hot the chip runs.

A chip doesn't cool itself: its package and its environment do.

THERMAL RESISTANCE

Measured in one number per path

θ_JA: junction to ambient

Engineers describe how well a package sheds heat with thermal resistance, often written θ_JA for junction-to-ambient, in degrees per watt. A lower θ_JA means the same power dissipation produces a smaller temperature rise, which is why some packages add metal slugs or fins purely to bring that number down.

A smaller θ_JA lets a chip run cooler at the same power, or run more power at the same temperature.

RELIABILITY STRESS TESTS

Proving years of life in weeks

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Temperature cycling

Repeatedly swings the chip between hot and cold extremes

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HAST

Highly accelerated stress test: high heat and humidity together

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Mechanical shock

Drops and vibrates the package to catch physical weaknesses

Each test compresses years of real-world stress into days or weeks.

THE BATHTUB CURVE

Three phases of a chip's life

high, then low, then rising again

Failure rate over a chip's life traces a bathtub shape: high early on from infant mortality, low and steady through its useful life, then rising again from long-term wear-out. Burn-in exists specifically to push failures out of that first, high phase before a chip ever ships.

Burn-in and reliability testing both aim at the same curve, from opposite ends.

UNIT 41 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered getting heat out of a package, and proving it survives years of use.