Research on transient temperature field and thermal stress distribution of power LED

LED has become the most promising lighting source due to its advantages of no pollution, high efficiency, long life and small size. With the continuous development of power LED applications in the field of lighting, the requirements for miniaturization and high power of LED are becoming more and more urgent. The packaging structure with low thermal resistance, good heat dissipation and low stress is the technical key of power LED devices. Existing research results show that the bonding material has the greatest impact on the thermal resistance of LED packaging. The key to improving the heat dissipation capacity of power LED is to reduce the thermal resistance of the bonding layer. The thermal conductivity of the bonding material is low, and the contact thermal resistance between the materials after curing is very high, resulting in a large temperature gradient, which will generate a lot of thermal stress; in addition, the thermal expansion coefficient (CTE) between the bonding material and the chip and the heat sink is quite different, and when the expansion is subject to external constraints, it will also generate a large thermal stress. The thermal stress generated during the packaging process not only affects the physical stability of the LED device, but also changes the refractive index of the packaging silicone lens, thereby affecting the light output efficiency and light field distribution of the LED. The magnitude of thermal stress has become one of the main indicators of the reliability of low-cost power LEDs.

At present, relevant research on the thermal stress distribution of LEDs has been conducted at home and abroad. In 2006, Jianzhen Hu et al. conducted a finite element simulation of the thermal stress distribution of Ga-N-based LEDs. The results showed that the maximum thermal stress of the LED package was concentrated at the edge where the chip and the bonding layer contacted. In 2007, Yu Xingang et al. analyzed the influence of the thermal conductivity of the substrate material on the junction temperature and maximum thermal stress of the LED. In 2008, Dai Weifeng et al. used finite elements to simulate the changes in the transient temperature field and stress field of high-power LEDs. However, in the above studies, the temperature field and stress field of the LED were simulated and analyzed separately, without analyzing the corresponding change relationship between the temperature field and the stress field, nor analyzing the change trend of stress and strain. Moreover, from the perspective of public literature, no research has been found on the influence of the key factor of the bonding layer material on the stress field distribution of the LED.

Based on the thermal stress theory, the paper simulates the changes in the distribution of transient temperature and stress fields of LEDs, and compares them with the measured changes in the center temperature of the bottom of the LED substrate; analyzes the corresponding changes in the transient temperature field and stress field; simulates the influence of the thermal conductivity of the bonding layer material on the LED junction temperature and maximum equivalent stress; calculates the changing trends of thermal stress, strain and shear stress on the top surface of the substrate parallel to the X-axis path. The research in the paper is meaningful to the thermal design of LED packaging.

1 Theoretical model and physical model of thermal stress

According to heat transfer theory, the transient temperature field distribution of a high-power LED with an internal heat source should satisfy the following equation:

Where: T is temperature; t is time; x, y, z are three-dimensional coordinate systems; α is the thermal expansion coefficient, and α satisfies the equation:

Where: λ is the thermal conductivity, ρ is the density, and c is the specific heat capacity. According to the theory of thermoelasticity, the transient thermal stress generated when the thermal expansion caused by the temperature gradient of the LED is subject to external constraints satisfies the following equation:

Where: σ is thermal stress, α is thermal expansion coefficient, E is elastic modulus, T is temperature, and Tref is reference temperature. It can be seen from formula (3) that the temperature field inside the LED is the premise for determining the magnitude of thermal stress, and the temperature distribution is determined by the heat conduction differential equation (1). As long as the corresponding boundary conditions are given, the temperature field and stress field distribution can be obtained.

The Lumileds 1 W power LED device (as shown in Figure 1) is used as the research object. The LED consists of a lens, a chip, a bonding layer, a heat sink, a substrate, and a plastic package. The heat is conducted from the chip to the heat sink through the bonding layer, and finally dissipated by convection between the substrate and the air. The thermal performance parameters of various LED packaging materials are shown in Table 1.

Figure 1 Lumidleds 1 W LED model

Table 1 Thermodynamic parameters of LED packaging materials

2 Experimental, simulation results and analysis

The LED finite element model is established using free mesh, the heat source and bonding layer use the first-level mesh, and the rest use the sixth-level mesh. The chip input thermal power is calculated as 0.9 W at 90%, the ambient temperature is 25℃, the heat generation rate is 4.0×109 W/m3, the convection coefficient is loaded on the contact surface between the LED model and the air is 10 W/m2.℃, and the contact thermal resistance in each layer of material is ignored. The solution time is set to 600 s, and the time substep is 20 s. The finite element software ANSYS is used to solve equations (1) to (3) to obtain the transient temperature field distribution of Lumidleds 1 W LED.

2.1 LED transient temperature test experiment and simulation

In order to verify the reliability of finite element simulation, a set of experiments was designed to test the temperature of Lumidleds 1 W LED. The measuring point was the center of the bottom surface of the aluminum substrate. The given current was 350 mA, the voltage was 3 V, the temperature test time was 10 min, and the data was recorded every 10 s. The experimental results showed that after lighting for 8 minutes, the LED was basically in a thermal equilibrium state, and the center temperature of the substrate was 56°C. The simulation results showed that the LED junction temperature was 76.1°C at this time (as shown in Figure 2).

The temperature change curve of the substrate measurement point and the simulation results from the start of LED operation to steady state are shown in Figure 3. During the heating process, the measured results are slightly lower than the simulation results. After reaching steady state, the difference between the two is 2.9°C, which verifies the reliability of finite element analysis. The main reasons for the error are the error in material parameters, the neglect of thermal radiation in the simulation process, and the application of convection as a simple boundary condition.