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The Influence of Thermal Resistance on Heat Transfer and Thermal Management in Electronic Packages

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • During the cooling of electronic devices, temperatures must be kept below the maximum allowable limits in worst-case operating conditions. 

  • The total thermal resistance of an electronic package can be classified into three levels: component level, package level, and system level. 

  • To improve cooling in electronic packages, it is advisable to reduce the internal thermal resistance, which enhances the heat flow within the package. 

Integrated circuit chip

In an attempt to improve the performance, reliability, and cost of electronics, it is advisable to incorporate a greater number of components or circuits into a single enclosure or package. As circuits are limited to smaller spaces with higher power density, heat generation and dissipation are major concerns for designers. The integration of several circuits into a single package such as a PCB challenges thermal management techniques and heat transfer in electronic devices. 

In this article, we analyze the influence of thermal resistance on heat transfer and thermal management to better protect electronic devices. 

Defining Thermal Resistance in Heat Transfer

For the thermal analysis or heat transfer analysis of integrated electronic packages, thermal resistance is an important parameter, as it plays a significant role in implementing a cooling mechanism. One key criterion that must be satisfied during the cooling of electronic devices is that temperatures must be kept below the maximum allowable limits in worst-case operating conditions such as thermal resistances, coolant flow rate, and module power. 

All electronic circuit packages can be characterized by thermal resistance. As we can relate the diffusion of heat flow to the movement of electric charges, thermal resistance is analogous to electrical resistance. The equation of thermal resistance is below, where T is the temperature difference  in ℃, q is the heat transfer rate in watts, and R is the thermal resistance in  ℃/W: 

R=Tq(1)

The value of thermal resistance in heat transfer differs in the range and application of devices. Low-powered packages for telephones, personal computers, and consumer electronics can be characterized by very high thermal resistance. Computer mainframes and supercomputers have higher component power dissipation and power densities, and are characterized by a lower thermal resistance value. 

The Different Levels of Thermal Resistance

In electronic packages, heat gets transferred from the component junction to the ultimate heat sink. Total thermal resistance determines the junction temperatures in the heat transfer path. 

The total thermal resistance of an electronic package can be classified into three levels according to the heat transfer path. Classification levels are:

  • Component level—At the component level, there is an internal thermal resistance, denoted by Rint. It is the resistance to the flow of heat from the junction or any other circuit element to the outside surface of the component case. 

  • Package level—External thermal resistance, expressed by Rext, is at the package level and offers resistance to the heat transfer from the surface of the case to some reference point. This reference point can be ambient temperature, the edge of a PCB, or a liquid-cooled cold plate. 

  • System level—System level thermal resistance is the final stage of thermal resistance. This stage focuses on heat transfer from the coolant to the heat sink. 

The Role of Internal and External Thermal Resistance in Heat Transfer

Internal and external thermal resistances govern device junction temperature in electronics packaging. Total thermal resistance is given by the sum of Rint and Rext and its value decreases with package power density. 

For proper thermal management and heat transfer, combinations of internal and external thermal resistances should be selected in such a way that it allows specified values of power and junction temperatures. Internal and external thermal resistances are influenced by cooling mechanisms and package designs. This indicates the importance of thermal management and cooling system selection in controlling the temperature of electronic packages. 

To improve cooling in electronic packages, it is advisable to reduce internal thermal resistance, which enhances heat flow within the package. A reduction of external thermal resistance can improve the heat flow from the outside surface of the package to the cooling medium. 

The influence of thermal resistance on heat transfer in electronic packages is significant. The method of cooling, the package design, and the material chosen are all crucial in establishing the internal and external thermal resistance. If the selection of thermal management is incorrect, the combination of internal and external thermal resistance will fail to satisfy the temperature constraints of the electronic package. When designing electronic packages with high power density, carefully select the thermal management and cooling mechanisms to ensure a reliable and efficient performance. 

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