Thermal Design of Semiconductor Components: Heat Transfer and Thermal Paths

“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.

“

Heat is transferred through objects and spaces. Transfer is the transfer of heat from a heat source to something else.

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 adjacent molecules.
・Convection: Heat transfer through fluids such as air and water
・Radiation: release heat energy through electromagnetic waves

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 the thermal Ohm’s law given in the previous article.

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.

Key takeaways:

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