The Thermal Resistance of Integrated Circuits

Key Takeaways

• In an IC package, thermal resistance is defined as the rise in temperature when a power of 1 watt is dissipated in the IC. 

• The thermal resistance from the junction to the ambient (JA) is the measure of an IC‘s ability to dissipate heat energy from the surface of the die to the ambient via all paths.

• The thermal resistance from the junction to the case (package)(JC) is dependent on packing materials such as lead frames, mold compounds, and die-attach adhesive as well as package design parameters such as die thickness, internal thermal vias, exposed pads, and the thermal conductivity of metals. 

Thermal resistance is a significant parameter in the thermal management of semiconductor ICs

The junction temperature of semiconductor chips greatly influences the life span of devices. Any integrated circuit comes with a maximum junction temperature value specified in its datasheet. If the junction temperature exceeds the maximum value, say for example a rise by 10℃, then life expectancy shrinks to half with a doubled rate of failure. The higher the junction temperature, the more probability there is for the device to break and burn off. It is essential to keep the junction temperature in a safe range for the reliable operation and performance of the IC.

To maintain junction temperature, thermal management techniques are employed to dissipate heat from the IC junction to the ambient through the package. Thermal resistance is a significant parameter that is a key part of the thermal management of semiconductor ICs. 

The Thermal Resistance of Integrated Circuits

In an IC package, thermal resistance is the rise in temperature when a power of 1 watt is dissipated in an IC. Typically, thermal resistance is expressed in ℃/W. When an IC datasheet specifies its thermal resistance as X ℃/W, then for 1 W power dissipation in the IC, the junction temperature rises by X ℃ above the ambient. The thermal resistance is usually denoted by theta (θ) and can be given by equation:

Note that T is the temperature difference between two points and P is the power dissipated in Watts.

From the above equation, it is clear that if the temperature difference between two points in the IC package is known, the heat flow between the points can be determined using the thermal resistance value.  

Defining Thermal Resistance 

Thermal resistance is the measure of the ability of the IC package to transfer heat from the junction to the package and then to the ambient. From this description of thermal resistance, it is clear that there is heat flow from the junction to the package and from the package to the ambient. The junction, package, and ambient are three points of different temperatures, which is why there are three thermal resistance values that can be defined for an IC package. 

Junction to Ambient Thermal Resistance,

The thermal resistance from the junction to ambient is denoted by and expressed in ℃/W. The is the measure of the IC‘s ability to dissipate heat energy from the surface of the die to the ambient via all paths. In the thermal resistance definition, the ambient is regarded as the thermal ground. The values are influenced by various parameters such as IC package, airflow, board, system characteristics, etc. The junction to ambient thermal resistance is useful in planning thermal management systems utilizing natural convection.

Junction to Case Thermal Resistance,

The thermal resistance from the junction to the case (package) is given by . The outside surface of the package is specified as the case. The represents the resistance to the flow of thermal energy to the IC package surface through a specific conductive thermal path, whereas represents conductive, convective, and radiative heat transfer paths. The difference in the heat path consideration makes greater than .

The value is dependent on packing materials such as lead frames, mold compounds, and die-attach adhesive as well as package design parameters such as die thickness, internal thermal vias, exposed pads, and the thermal conductivity of metals. 

With IC packages, the reference point on the case changes. The reference point in leaded packages is the point where pin 1 emerges from the plastic. In standard plastic packages, the reference is at the corner of pin 1. The reference is at the center of the exposed pad for ICs utilizing exposed pad packages.

The measurement of thermal resistance from the junction to the case is determined by attaching the IC package directly to a liquid-cooled copper block. The liquid-cooled copper block can absorb an infinite amount of thermal energy, making it an infinite heat sink with zero thermal resistance. The heat transfer mechanism from the semiconductor junction to the IC package during this measurement is purely conduction. Increased heat conduction in IC packages can be inferred from the low  value and a high value shows decreased heat conduction.

Case to Ambient Thermal Resistance,

The thermal resistance from the case to the ambient is denoted by . It takes into account all thermal paths from the external package surface to the ambient. Knowing the values of and , the thermal resistance from junction to ambient can be determined using the equation:

= +

Let’s see the measurement of when other thermal resistance values are unknown.

Measurement of

The junction to ambient thermal resistance  is the thermal metric used to measure the thermal performance of an IC package. The values of ICs from different manufacturers measured under the same test conditions are commonly used for comparing their thermal performances. IC manufacturers are advised to follow the standardized test procedures set by  the Joint Electron Device Engineering Council (JEDEC) to measure  .

The measurement steps for can be summarized as:

1. Mount the IC package containing the thermal test chip on the test board. The thermal test chip can dissipate power and measure the IC temperature. 

2. Calibrate the temperature sensing component of the thermal test chip.

3. Place the IC package and test board in either still air or moving air to measure or , respectively. 

4. Dissipate a known power (P_D) in the test chip.

5. Measure the ambient temperature (T_A) and junction temperature (T_J) after attaining the steady-state. 

6. Substitute the obtained values into the equation below for calculating the thermal resistance .

The thermal resistance is useful for comparing the thermal performance of IC packages and plays a significant role in implementing thermal management techniques in semiconductor chips. Cadence software offers simulation tools that can assist in the thermal evaluation of integrated circuits. 

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