“The heat generated escapes to the atmosphere through various paths by conduction, convection and radiation. Since our topic is “Thermal Design of semiconductor components“, we will use an IC mounted on a printed circuit board as an example.

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Thermal Design of Semiconductor Components in Electronic Equipment
Heat is transferred through objects and spaces. Transfer is the transfer of heat from a heat source to something else.

01
Three forms of heat transfer

There are three main forms of heat transfer: conduction, convection and radiation.
Conduction: Molecular motion caused by thermal energy is propagated to neighboring molecules.
Convection: Heat transfer through fluids such as air and water.
Radiation: The release of thermal energy through electromagnetic waves.

02
heat dissipation path

The heat generated escapes to the atmosphere through various paths by conduction, convection and radiation. Since our topic is “Thermal Design of Semiconductor Components”, we will use an IC mounted on a printed circuit board as an example.

The heat source is the IC chip. This heat is conducted to the package, lead frame, pads and printed circuit board. Heat is transferred from the printed circuit board and IC package surfaces to the atmosphere by convection and radiation. Thermal resistance can be expressed as follows:

In the cross-sectional view of the IC at the top right of the image above, the color of each section matches the color of the circuit mesh circle (e.g. the chip is red). The chip temperature TJ reaches the ambient temperature TA through the thermal resistance shown in the circuit net.

When surface-mounted on a printed circuit board (PCB), the path enclosed by the red dotted line is the primary heat dissipation path.

Specifically, heat is conducted from the chip via the bonding material (adhesive between the chip and the backside exposed frame) to the backside frame (pad), and then to the printed circuit board through the solder on the printed circuit board. This heat is then transferred to the atmosphere (TA) by convection and radiation from the printed substrate.

Other paths include the path from the chip to the lead frame through the bond wire, and then to the printed substrate to achieve convection and radiation, and from the chip through the package to achieve convection and radiation.

If the thermal resistance of this path and the power loss of the IC are known, the temperature difference (here the difference between TA and TJ) can be calculated by thermal Ohm’s law.

As mentioned in this article, the so-called “thermal design” is to try to reduce the thermal resistance everywhere, that is, reduce the thermal resistance of the heat dissipation path from the chip to the atmosphere, and finally reduce the TJ and improve the reliability.

03
What is thermal resistance

Thermal resistance is a numerical value that expresses the ease of heat transfer. is the value obtained by dividing the temperature difference between any two points by the heat flow (heat flow per unit time) flowing between the two points. A high thermal resistance value means that heat is difficult to transfer, while a low thermal resistance value means that heat is easy to transfer.

The symbols for thermal resistance are Rth and θ. Rth comes from the English expression “thermal resistance” of thermal resistance.
The unit is °C/W (K/W).

04
Thermo Ohm’s Law

Thermal resistance can be thought of in much the same way as resistance, and the basic formula for thermal calculations can be treated in the same way as Ohm’s Law.

Therefore, just as the potential difference SV can be obtained by R×I, the temperature difference ST can be obtained by Rth×P.

Key takeaways:

Thermal resistance is a numerical value that expresses the ease of heat transfer.
The symbols for thermal resistance are Rth and θ, in °C/W (K/W).
Thermal resistance can be considered in much the same way as resistance.