How Junction-to-Ambient Thermal Resistance of an IC Package Affects Thermal Performance

Thermal performance of an IC (integrated circuit) package can be specified by several different parameters such as θ_JA, θ_JB, θ_JC, Ψ_JT, and Ψ_JB。更深入地理解这些参数可以help us have a more accurate estimation of the thermal performance of our product. Armed with this knowledge, we can avoid imposing unnecessary limitations on our design or predict potential pitfalls for a design that is marginal from a thermal standpoint.

In this article, we’ll take a look at the junction-to-ambient thermal resistance θ_JA。We’ll see that θ_JAcan be used to compare thermal performance of packages from different manufacturers.

结温Calculation with Junction-to-Ambient Thermal Resistance (θ_JA)

Thermal resistance specifies how hard it is for heat to flow from one place to another. Perhaps the most familiar (and most misapplied) thermal parameter for an IC package is the junction-to-ambient thermal resistance, commonly represented by θ_JA。

If we have the θ_JAparameter foran IC employed in a given system, we can easily calculate the junction temperature using the following equation:

$$T_J = T_A + P \times θ_{JA}$$

where

• T_Jis the junction temperature (defined as the maximum temperature on the die)
• T_Ais the ambient temperature, and
• P is the “total power” dissipated in the chip.

Using the above equation, we can determine the maximum allowed power dissipation of an IC. For example, if the maximum allowed junction temperature is 150°C, and T_A=25°C, θ_JA=17 °C/W, we obtain the maximum allowed power as:

$$P_{max} = \frac{T_J - T_A}{θ_{JA}} = \frac{150 - 25}{17} = 7.35 W$$

It seems like a straightforward task to evaluate the design thermal performance while having the θ_JAparameter. However, there are intricacies that must be fully understood before applying the above equation to estimate the junction temperature of an application-specific board.

θ_JA: A System Characteristic

The key point is that the thermal resistance from junction to ambient is not a characteristic of only the package. There are several different parameters that affect θ_JA。

The figure below shows the typical heat flow patterns for an exposed pad package on a PCB in a natural convection environment.

A representation of how heat moves through an IC and PCB, where the red arrows indicate the direction of heat flow. Image courtesy ofON Semiconductor

The heat flows through both the PCB and the package case. The test board on which the IC is mounted can actually play the role of a heat sink soldered to the leads of the device.

Changing the characteristics of the test board, such as the trace density and the number of power/ground planes, can give us different θ_JAvalues. The JEDEC specifications define two test board types for θ_JAmeasurements:

• 1(一个信号层)
• 2s2p (two signal layers and two power layers) test boards

The measurement result can be very different as we change the board type from a 1s board to a 2s2p board type. The following figure compares the θ_JAvariation with the board type for several different packages.

Comparing junction-to-ambient thermal resistance for 17 package types on 1s vs 2s2p PCBs.Image courtesy ofTexas Instruments。

As you can see, the board type can change θ_JAby as much as 50%.

There are several other factors that can affect the junction-to-ambient thermal resistance. For example, thermal resistance increases with altitude. A device operating at 8000 ft experiences smaller air pressure and can run 20% hotter compared to the same device operating at sea level. Interestingly, major system design companies employ pressure chambers to test the thermal performance of their design at different altitudes.

Thermal resistance is also a function of the ambient temperature. A 20% reduction in the overall θ_JAcan occur by going from an ambient temperature of 0 °C to 100 °C. Additionally, θ_JAdecreases as the power dissipated in a given IC increases.

Next: Test Conditions for θ_JA

In the next article, we'll continue this discussion by talking about the standardized test conditions that manufacturers useto measure theθ_JAparameter of an IC package。These details allow us to understand the limitations of this thermal performance metric.

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