Progress in Measurement and Analysis Technology of LED Thermal Resistance Structure

LED solid-state light source has the advantages of high efficiency, long life, flexible application and no pollution, and has been widely used in the field of illumination. However, most of the electrical energy consumed by LEDs is converted into thermal energy, which causes the chip temperature to rise significantly, and temperature has an important influence on LED performance, including color temperature change, efficiency degradation, life reduction and reliability. Therefore, improving the thermal management performance of LEDs has become a key technical link in the design of high-power LED structures.

Commonly used LED thermal management analysis techniques include the use of thermal design software simulation and measurement using thermal resistance analysis equipment. The former is often used for thermal management design of LEDs; the latter focuses on thermal resistance measurement and analysis of actual samples to verify the actual effects and product quality of the design, and to improve the manufacturing process or to guide the secondary design.

2. Basic principle of thermal resistance

The heat dissipation of the LED is carried out in three ways: heat conduction, convection, and heat radiation. Inside the LED, heat conduction is the main heat dissipation path, and its thermal conductivity depends on the thermal impedance of the medium. Thermal impedance is determined by both thermal resistance and heat capacity. The definition of thermal resistance is:

. Where ΔT is the temperature difference, Rth is the thermal resistance, and P is the thermal power.

As shown in Fig. 1, the heat flow corresponds to the current, and the potential corresponds to the temperature, the thermal resistance corresponds to the resistance, and the heat capacity corresponds to the capacitance. For any thermal medium, it can be simplified as an RC parallel circuit:

When heat flows through the media unit, a temperature difference is formed at both ends. Similar to the circuit, the initial heat will accumulate in the heat capacity, and the temperature difference between the two ends will gradually increase until the heat balance is reached. The thermal resistance at this time is usually called the "steady-state thermal resistance". Before the device reaches thermal equilibrium, the junction temperature of the device changes continuously according to the heat capacity and the thermal resistance, and the corresponding thermal resistance also changes with time. The thermal resistance is called “transient thermal resistance”. Measurement of transient thermal resistance is the basis for thermal resistance structure measurements.

3. Thermal resistance measurement and thermal resistance structure function

3.1 The necessity of thermal resistance structure measurement

In theory, when we determine the material, shape, size and other information of a device, its heat capacity and thermal resistance can be determined. However, in LEDs, in addition to the thermal characteristics of the individual devices themselves, there is contact thermal resistance at the interface that contacts each other. There are many factors that determine thermal contact resistance, such as the flatness of the contact surface, positive pressure, finish, temperature, or tie layer process. These factors tend to be nonlinear with the contact thermal resistance, and the actual situation is difficult to determine and may change with environmental changes. Therefore, it is impossible to accurately understand the thermal management situation inside an actual product only through simulation. To more accurately describe the thermal management of the actual product, thermal resistance structure measurements must be made.

As shown in Figure 2 and Figure 3, the typical thermal resistance measurement can only give the thermal resistance of the device as a whole, and does not reflect the internal heat distribution relationship. The thermal resistance structure measurement can give the layered thermal resistance inside the device. Information has an important guiding role in actual design or process improvement.

3.2 Thermal resistance structure measurement technology

In the thermal resistance measurement, the first step to obtain the characteristics of the thermal resistance structure is to measure the transient thermal resistance curve, that is, the junction temperature variation curve, as shown in FIG.

It is not difficult to see that the information on temperature changes is mainly included in the initial period of testing. To distinguish the tiny structure such as the LED chip , it is necessary to be able to sample and measure the junction temperature in the microsecond time. At the same time, because the measurement analysis is quite sensitive to noise, the determined boundary conditions and the stable power supply are also crucial. Importantly, that is, the test equipment must achieve high-precision measurement sampling at the MHz level to meet the needs of LED thermal resistance structural analysis, which is quite challenging.

In addition, the electric power consumed by the LED is generally converted into two parts: thermal power and optical radiation power. By definition, the thermal resistance is the ratio of the temperature difference to the thermal power on the heat conduction path, and to accurately obtain the thermal power, the optical irradiance of the LED must be performed. Measurements, in fact, some existing thermal resistance measurement systems do not consider the proposed optical radiation power, they can only be called "reference thermal resistance" [4].

3.3 Thermal resistance structure function analysis

The analysis of the thermal resistance structure function is the key to obtaining the LED thermal resistance structure, and it is also difficult.

In reality, it is difficult to obtain a continuous thermal resistance structure function. Instead, the finite element analysis method is usually used to decompose the heat conduction path into a finite number of cells, and calculate the heat capacity and thermal resistance of each element to obtain a discrete form of the thermal resistance structure function. . As we have already mentioned in the basic principle, the heat transfer medium can be described in the form of a circuit, making the analysis process more intuitive. The finite element model for the one-dimensional heat transfer medium can be expressed as a Cauer model loop in the form of an RC circuit, see Figure 5.

The model describes the heat capacity and thermal resistance of the individual cells that pass through the PN junction to the heat sink process. The more units of the Cauer model used in the analysis, the more detailed the description of the medium and the closer it is to the actual situation.