The heat sink is a passive heat transfer device. When transferring heat from the IC package to the surrounding environment, its thermal resistance is much smaller than the parallel thermal resistance from the package to the environment caused by thermal convection and thermal radiation. In order for the heat sink to work, its equivalent thermal resistance must satisfy the following equation:
among themIs the effective thermal resistance of the heat sink,Is caused by thermal convection on the top of the package
Thermal resistance,It is the thermal resistance at the top of the package caused by thermal radiation.
Figure 1. Thermal resistance model of an N-fin heat sink with the TIM connected to the top of the package. Figure 1 shows the thermal resistance model of the N-fin heat sink (N is the number of Fin), where the thermal interface material (TIM) is connected to the top of the package. We need TIM to improve the contact between the package and the heat sink, so the effective thermal resistance of the heat sink needs to include the thermal resistance of the TIM. From the content of the third chapter of thermal management foundation, we can know:
This shows that the effective resistance is equal to the resistance of the TIM plus the resistance at the bottom of the heat sink and the parallel resistance of the N-fin. If we assume that the resistance of each heat sink is equal, the equation can be further simplified as:
The equivalent resistance of the heat sink is approximately equal to the resistance of the TIM plus the resistance at the bottom of the heat sink, and the resistance of the heat sink divided by the number N. Since the area of the heat sink can be larger than the top surface area of the package, its heat convection and heat radiation resistance can be smaller than the heat convection and heat radiation resistance of the top surface of the package. In addition, if the resistance is divided by the number of heat sink Fin, an improvement of N times can be achieved. However, for a given heat sink substrate area, when the increase in Fin is higher than a certain amount, it will eventually lead to an increase in the thermal resistance of each Fin: this is because the heat sinks begin to approach each other and reduce the effective heat transfer coefficient. . And because these thermal resistances directly increase the effective thermal resistance of the heat sink, it is very important to choose high thermal conductivity materials for the heat sink and TIM in order to improve the overall performance of the heat sink.
Post time: Nov-09-2020