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2021.03.10 Thermal Design

Fundamentals of Thermal Resistance and Heat Dissipation: Heat Transmission and Heat Dissipation Paths

Thermal Design of Semiconductor Components in Electronic Equipment

Heat is transmitted through objects and through space. "Transmission" means that heat moves from the source generating the heat.

Three Modes of Heat Transmission

There are three modes of heat transmission: conduction, convection, and emission (radiation).

・Conduction: The motion of molecules due to thermal energy propagates to neighboring molecules.
・Convection: Movement of heat by fluids, such as air or water
・Emission (radiation): The emission of thermal energy through electromagnetic waves

Heat Dissipation Paths

Heat that has been generated escapes to the outside air over various paths, by means of conduction, convection, or emission. Here, because our theme is "Thermal Design of Semiconductor Components", we explain an example of an IC mounted on a printed circuit board.

The source of heat generation is the IC chip. The heat is transmitted by conduction to the package, lead frame, die attach pads, and the printed circuit board. The heat is transmitted from the surfaces of the printed circuit board and the IC package to the ambient atmosphere by convection and emission. When expressed using the concept of thermal resistance, we have the following.

The colors of the parts in the IC cross-sectional diagram on the upper right correspond to the colors in the circuit network; for example, the color for the chip is red throughout. From the chip temperature TJ, the circuit network extends through the thermal resistances indicated to reach the ambient (surrounding environment) temperature TA.

The path surrounded by the dashed red line is the main path for heat dissipation in the case of surface mounting on a printed circuit board (PCB). Specifically, heat is transmitted from the chip, via the die bond (adhesive between the chip and the die pad), to the exposed pad, and then, via solder on a land of the printed circuit board, to the printed circuit board. This heat is then conveyed from the printed circuit board to the atmosphere (TA) by convection and emission.

As another transmission path, heat from the chip passes through bonding wires to the lead frame and then to the printed circuit board, followed by convection and emission; and in a further path, heat from the chip passes through the package and is dissipated by convection and emission.

If the thermal resistances of these paths and the power dissipation of the IC are known, then the thermal Ohm's law described in the previous article can be used to calculate the temperature difference, which in this case is the difference between TA and TJ.

Thermal design involves reducing various thermal resistances such as those described above, that is, reducing thermal resistances of heat dissipation paths from a chip to the ambient atmosphere. As a result, TJ is lowered, and the chip reliability is improved.

Key Points:

・There are three modes of heat transmission: conduction, convection, and emission (radiation).

・In an example of an IC mounted on a printed circuit board, the source of heat generation is the IC chip. And the heat is transmitted by conduction to the package, lead frame, die attach pads, and the printed circuit board. Then the heat is transmitted from the surfaces of the printed circuit board and the IC package to the ambient atmosphere by convection and emission.

・If the thermal resistances of these paths and the power dissipation of the IC are known, then the thermal Ohm's law can be used to calculate the temperature difference between TA and TJ.

・Thermal design involves reducing thermal resistances of heat dissipation paths from a chip to the atmosphere.

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