Thermal Design of Semiconductor Components: Heat Transfer and Heat Dissipation Paths

btty038

Thermal Design of Semiconductor Components: Heat Transfer and Heat Dissipation Paths [Copy link]

Heat is transferred through objects and space. Transfer is the movement of heat from a source to another location.

Three forms of heat transfer

There are three main forms of heat transfer: conduction, convection and radiation.

Conduction: Molecular motion caused by heat energy is propagated to neighboring molecules.
Convection: Heat transfer through fluids such as air and water
Radiation: Release of heat energy through electromagnetic waves

Heat dissipation path

The generated heat escapes to the atmosphere through various paths, such as conduction, convection, and radiation. Since our topic is "thermal design of semiconductor components," we will explain here using 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. This can be expressed using thermal resistance as follows:

In the cross-sectional view of the IC in the upper right of the figure above, the color of each section matches the color of the circuit net circle (for example, the chip is red). The chip temperature TJ reaches the ambient temperature TA through the thermal resistance shown in the circuit net.

When mounted on a printed circuit board (PCB) using a surface mount method, the path surrounded by the red dotted line is the main heat dissipation path. Specifically, heat is conducted from the chip to the back frame (pad) via the bonding material (adhesive between the chip and the back exposed frame), 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 circuit board.

Other pathways include a path from the chip through the bonding wires to the lead frame and then to the printed substrate for convection and radiation, and a path from the chip through the package for 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 using Ohm's law given in the previous article.

As described in this article, the so-called "thermal design" is to strive to reduce the thermal resistance everywhere, that is, to reduce the thermal resistance of the heat dissipation path from the chip to the atmosphere, ultimately reducing TJ and improving reliability.

Key Takeaways:

Thermal resistance is a value that indicates how easy it is to transfer heat.

The symbols for thermal resistance are Rth and θ, and the unit is ℃/W (K/W).

Thermal resistance can be considered in much the same way as electrical resistance.

Source: ROHM

Author:ROHM

qwqwqw2088

IC should also have a maximum allowable case temperature

btty038

qwqwqw2088 posted on 2021-3-6 21:05 IC should also have a maximum allowable case temperature

The junction temperature plus the temperature rise should not be greater than the MAX temperature. Generally, the industrial grade is 125° and the military grade is 150°. The design is guaranteed to be OK within the lower temperature limit.

alan000345

Good learning.