In the field of semiconductor device usage, understanding and controlling junction temperature is essential. For many years, users have relied on thermal resistance values provided by manufacturers to estimate junction temperature. By measuring the temperature at specific points and applying these thermal resistance values, they calculate the junction temperature. This practice has been followed for decades, especially in the LED industry. However, the limitations and potential inaccuracies of this method have often gone unnoticed.
This paper explores the use of simulation software to analyze the thermal resistance involved in measuring junction temperature. Many of the issues with traditional methods remain undetected unless visualized through simulation results. While it's theoretically possible to manually solve complex equations to generate isothermal surface maps, the effort is immense and time-consuming. Simulation software can produce similar results in just half an hour, making it a far more practical approach.
These challenges are particularly difficult to address through experimental testing, such as achieving uniform isothermal conditions across the entire device. As a result, the improper application of the thermal resistance method has often led to significant errors that have remained unrecognized.
The paper begins by introducing the fundamental principles of heat transfer and then uses simulations to validate the problems discussed. It introduces the concept of "intrinsic equivalent thermal resistance," emphasizing that only this value should be provided as a characteristic parameter of the device. It also warns that not all devices' intrinsic equivalent thermal resistance test points are suitable for real-world applications.
It becomes clear that the traditional thermal resistance method is not reliable for measuring junction temperature. The general equivalent thermal resistance values calculated from extrinsic temperature measurement points are often incorrect. These errors are not just numerical discrepancies but stem from theoretical misunderstandings.
The content of this article primarily focuses on the measurement of junction temperature in semiconductor devices. The introduction to the basic theory of heat transfer and its discussion are explored, along with verification through simulation. The paper discusses the limitations of using standard thermal resistance formulas and highlights the importance of intrinsic equivalent thermal resistance in accurate calculations.
The basic theory of heat transfer includes conduction, convection, and radiation, each governed by specific mathematical expressions. The thermal resistance (R) is defined as the ratio of temperature difference (ΔT) to heat flow (Φ), or R = ΔT/Φ. In practical applications, these formulas are often oversimplified, leading to inaccuracies.
For example, when considering thermal radiation, the shape factor between objects must be accounted for, which is often omitted in standard formulas. Additionally, actual heat transfer processes are typically three-dimensional, requiring the solution of heat transfer differential equations rather than relying solely on thermal resistance parameters.
The paper also discusses the application of the formula R = ΔT/Φ in various scenarios, highlighting cases where it may not hold true, especially when radiative effects cannot be ignored. It emphasizes that the formula is only valid under certain conditions and should not be applied universally.
The concept of intrinsic equivalent thermal resistance is introduced as a more accurate and reliable measure. Unlike general equivalent thermal resistance, intrinsic equivalent thermal resistance refers to the thermal resistance between an internal heat source and an isothermal surface that does not extend beyond the package boundary. This makes it less affected by external environmental factors, providing a more stable and meaningful parameter for device characterization.
Simulation tools play a crucial role in identifying and validating intrinsic isothermal surfaces. They allow engineers to visualize heat distribution and determine suitable test points for accurate junction temperature measurements. However, for some devices, such as the 3528 package, there may be no ideal intrinsic isothermal surface, making it impossible to use the thermal resistance method effectively.
Through simulations of different LED packages, the paper demonstrates how changes in the external structure significantly affect the thermal resistance values calculated from extrinsic points. This underscores the need to focus on intrinsic equivalent thermal resistance when characterizing semiconductor devices.
In conclusion, the paper stresses that arbitrary selection of test points for thermal resistance calculations is misleading. Only intrinsic equivalent thermal resistance values provide reliable data for junction temperature estimation. Extraneous factors, such as PCB layout or external structures, can distort thermal resistance measurements, making them unsuitable as universal parameters.
Finally, the paper highlights that for smaller devices, even if intrinsic isothermal surfaces exist, they may not be practically accessible for temperature measurement after packaging. This further complicates the use of traditional thermal resistance methods in real-world applications.
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