I realize that electronics naturally emit heat and that this heat can be harmful to them, but by what mechanism do electronics overheat? Is it changes in resistance of components? What is it?



There are two main ways to think about why electronics generate heat. The first is an "I^2 R" loss (I-squared R). This relates to the power dissipation in a resistor, which is current squared (I^2) times resistance. This type of heat describes resistive devices like MOSFET's and resistors themselves. The second is a "VI" (voltage/current) loss in which the power dissipated is voltage drop across the component times the current through it. This type of heat describes semiconductors whose voltage is more-or-less constant with current, like diodes.

Either way, electrical power is dissipated as heat. That heat has to escape through the mechanical packaging of the component. The more surface area there is on the component, and the more thermally conductive the material, the better the heat can escape. So a small plastic component dissipating 1W of power will get much hotter than a large metal component (like a TO-3 transistor package).

Overheating, then, is caused by dissipating too much power in an electronics package that cannot get rid of that power (converted to heat) fast enough.

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But what is the actual mechanism? i.e. Once the devices have heated themselves up, now what? Why do they stop working?

Simply because what's left inside the casing after overheating isn't what was put in to do the job. ie they're cooked. Consider it a bit like baking a cake. Once it's been in the oven it is no longer eggs, flour, water etc

And think of fuses - they get too much current through them - heat up and the wire inside them breaks. Inside every chip there are lots of wires and they're feckin tiny so with too much current - they heat up and break.

With overheated semiconductors the initial damage can be diffusion of the dopant atoms so that the device behaviour changes to something less useful (this kind of damage can be partial and cumulative). Generally this starts to happen in silicon at 175C and above. Some new silicon carbide devices are available that withstand significantly higher temperatures, but these are expensive and are used in hot environments.

Also the packaging and chip expand at different rates upon heating, so that repeated heating/cooling cycles to high temperatures can mechanically damage the unit.

Plastic packages are also limited in the temperature they can endure - ceramic packaging is more expensive but commonly used for extended-temperature-range versions.

Some semiconductor devices carry more and more current as the temperature increases, leading to 'thermal runaway' where the power dissipation rises rapidly leading to the packaging vaporizing/melting and a 'magic smoke' escape ;)