The Operational Reliability of Semiconductor Devices and the Junction-to-Case Thermal Resistance

Key Takeaways

• The thermal metric that is most commonly used to estimate junction temperature is the junction-to-case thermal resistance.

• The junction-to-case-thermal resistance can also be defined as the ratio of the temperature difference between the junction and case to the conducted thermal power between the same two points. 

• The junction-to-case thermal resistance is an important parameter in the optimization of semiconductor packaging.

When a semiconductor device is connected to a heat sink, it lowers the junction-to-case thermal resistance

Thermal stress is a serious challenge faced by semiconductor devices. And, since the downsizing of semiconductors is a prevailing trend in the electronics market, a way to address thermal stress issues is needed so the long life and reliability of these devices can be maintained.

High junction temperature affects the electrical performance of semiconductor devices and can lead to metal migration, accelerated aging, and failures. The evaluation of junction temperatures in semiconductor devices is critical to preventing these failures. One thermal metric commonly used to estimate junction temperature is junction-to-case thermal resistance. Let’s take a closer look at this metric and how engineers use it to prevent thermal stress and device failure.  

Junction-to-Case Thermal Resistance

Junction-to-case thermal resistance (JC) is the resistance to the flow of heat from the internal operating area of a semiconductor device to the external case, which is nearest to the mounting area. The case can either be the top surface or the bottom surface.

Junction-to-case-thermal resistance can also be defined as the ratio of the temperature difference between the junction and case to the conducted thermal power between the same two points, where TJ is the temperature at the junction, TCis the temperature at the casing, and P is the conducted thermal power:

Note that the junction-to-case thermal resistance is not an intrinsic property of a semiconductor device. It depends on the cooling mechanism used along with the semiconductor package. The temperature difference between the junction and case for a liquid-cooled semiconductor device is different from an air-cooled counterpart. With different thermal management methods, the temperature difference between the junction and the case changes, and, therefore, the conducted thermal power also varies. 

Thermal Management and Junction-to-Case Thermal Resistance

The junction temperature is a critical thermal parameter in semiconductor devices. When the temperature crosses a specified limit, the semiconductor device is destroyed or thermal management shuts down the device’s operation. Luckily, the junction-to-case thermal resistance provides information about junction temperature, helping engineers avoid this type of failure.

Heat sinks are one frequently used heat removal technique to address this issue. They remove heat from the semiconductor case to the ambient. When the semiconductor device is connected to a heat sink, it lowers the junction-to-case thermal resistance. Thermal compounds—such as thermal interface materials (TIM), thermal putty, thermal paste, etc.—are applied between the semiconductor package and the heat sink. These thermal interfaces between the casing and heat sink further reduce the junction-to-case thermal resistance.

The Applications of Junction-to-Case Thermal Resistance

Semiconductor Packaging

The junction-to-case thermal resistance is a critical parameter in the optimization of semiconductor packaging; semiconductor manufacturers utilize junction-to-case thermal resistance values for package design. Additionally, the junction-to-case thermal resistance is helpful in evaluating the thermal performance of semiconductor device packages.   

One Resistor and Two Resistor Compact Models

Junction-to-case thermal resistance is also an important parameter for developing one or two resistor compact models of power semiconductor devices in thermal simulations. For thermal performance evaluations, the junction-to-case thermal resistance values for the same boundary conditions are compared.

Understanding junction-to-case thermal resistance helps engineers prevent thermal stress and failures in semiconductor devices. Cadence software, including the Celsius Thermal Solver in the video below, aids engineers in the thermal analysis of semiconductor devices, helping them ensure their device’s operational reliability.

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