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Power type LED transient temperature field and thermal stress distribution

In Electronic Infomation Category: P | on April 18,2011

Abstract: The thermal power type LED device characteristics, based on the theory of heat stress using the finite element software ANSYS for thermal stress calculation, has been the 1 W LED Lumileds transient temperature field and LT1789IS8-1 datasheet and stress field contours, the top substrate plane parallel to the X-axis path of the thermal stress, strain and LT1789IS8-1 price and shear stress distribution curve. The results show that the maximum stress concentration in the corner bonding layer; axial maximum displacement in the contact edge of the lens and LT1789IS8-1 suppliers and the heat sink; maximum shear stress concentration at the corner of bonding layer region. Experimental test of the LED through the bottom center of the substrate temperature, consistent with the simulation results to study the thermal conductivity of the LED layers of temperature and stress field distribution. Finally, conclusions based on the above study ways to improve the quality of LED. Results of high-power LED packaging paper have meaning.

0 Introduction

LED for a non-polluting, high efficiency, long life, small size, etc., become the most promising lighting. With the power type LED lighting applications in the continuous development of LED light, high power of more and more urgent, low thermal resistance, heat dissipation and low stress power type LED package structure is the key technical components. Existing research results show that the bonding material impact on the maximum thermal resistance LED package to improve the capacity of power type LED Thermal bonding is the key to reducing the thermal resistance layer. Thermal conductivity of bonding materials, lower the contact between the cured materials, high thermal resistance, resulting in a large temperature gradient will have a lot of thermal stress; In addition, the bonding material and the chip, heat sink thermal expansion coefficient between the (CTE) differences larger, when the expansion will be subject to external constraints have a greater thermal stress. Encapsulation process not only affects the thermal stress generated by the physical stability of the LED devices, but also the package changes the refractive index silicone lens to the optical efficiency of the LED and optical field distribution of impact. The size of thermal stress has become the price of power type LED * the main indicator of reliability.

Present, the domestic distribution of thermal stress has been on the LED do research. In 2006, Jianzhen Hu and others on the Ga-N based LED thermal stress distribution in the finite element simulation results show that the LED package, the maximum thermal stress concentration and bonding layers in the chip where the edge of the exposure; 2007, the new Gang , who analyzed the substrate material on the LED junction temperature and thermal conductivity of the maximum thermal stress; 2008, Dai Wei Feng and others using the finite element simulation of the high-power LED of the transient temperature field and stress field changes. However, the study will be LED temperature and stress fields, respectively, were simulated and analyzed, and no analysis of the corresponding temperature variation of the corresponding field, but also did not analyze the trend of stress and strain, and the view from the open literature, and The study did not find any material bonding layer on the LED of this key factor in the stress field distribution.

Paper based on the theory of heat stress simulated LED transient temperature field and stress field distribution changes, the LED substrate with the measured temperature changes at the bottom center of a comparative study; and analysis of the transient temperature field and stress The relationship between the corresponding field; simulation of the thermal conductivity of bonding material layer on the LED junction temperature of the maximum equivalent stress; calculate the top surface of the substrate path parallel to the X-axis thermal stress, strain and shear stress changes in trends, the paper Research on the LED package thermal design meaningful.

1 Theoretical model of thermal stress and physical models

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


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


Where: is thermal conductivity, is density, c is the specific heat capacity. The theory of elasticity by heat, LED temperature gradient due to thermal expansion generated by external constraints transient thermal stress, satisfy the following equation:


Where: is the thermal stress, is the thermal expansion coefficient, E is the elastic modulus, T is temperature, Tref is the reference temperature. From (3) can be seen, LED interior temperature field is a prerequisite to determine the size of thermal stress, and temperature distribution by the heat conduction differential equation (1) decision, as long as the corresponding boundary conditions given temperature can be obtained and the stress distribution.

To Lumileds of 1 W power type LED device (Figure 1) as the research object, the LED by the lens, chip, bonding layer, heat sink, substrate and plastic material composition. Heat from the chip by the conductive bonding layer to the heat sink, and finally by the substrate and air convection cooling. LED thermal properties of various packaging materials shown in Table 1.


Figure 1 Lumidleds 1 W LED model

Table 1 LED packaging materials, thermodynamic parameters


2 experiments, simulation results and analysis

Create a free LED grid finite element model, heat and bonding layer using a grid, the other by six grid. Chip at 90% thermal power input is calculated as 0.9 W, the ambient temperature is 25 , heat production rate 4.0 109 W/m3, the LED model loaded with air convection coefficient of contact surface 10 W/m2. , and ignore the materials are in the contact resistance, set the computation time is 600 s, the time sub-step is 20 s, using the finite element software ANSYS to solve equation (1) to (3) can be obtained Lumidleds 1 W LED transient temperature distribution.

2.1 LED experiment and simulation of transient temperature test

Order to verify the reliability of finite element simulation to design a set of experiments Lumidleds 1 W LED for temperature testing, measuring point for the aluminum plate bottom center, given current 350 mA, voltage of 3 V, temperature 10 min test time every 10 s recorded data, experimental results show that 8 min after the light, LED base in thermal equilibrium, when the substrate core temperature was 56 . Simulation results show that the LED junction temperature is now 76.1 (Figure 2).

LED work from the beginning to the steady-state process, the substrate temperature measuring point curve and the simulation results shown in Figure 3, heating process, the experimental results slightly lower than the simulation results, to reach steady state, two difference 2.9 to verify the finite element analysis of reliability. Material parameters of the error, the simulation ignores the process of thermal radiation and convection as a simple boundary condition will be imposed is the main reason for errors.


Figure 2 Lumileds 1 W LED temperature field contours


Figure 3 Lumileds 1W LED substrate temperature measured data and simulation center comparison of the data

2.2 LED thermal stress and thermal deformation of the simulation results and analysis

The calculated transient temperature distribution, will be converted to thermal units solid70 structural unit, with the loop command each time step the temperature reading to the stress field, and in three directions increase the bottom substrate binding, calculated are strain and stress fields at steady state as shown in Figure 4 (a), (b).

Figure 4 (a) is the Lumileds 1 W LED in the end times (600 s) after the total displacement of cloud, cloud that does not mesh with the internal deformation of the structure before, another entity that LED cloud after expansion in the heat distortion effect , where the proportion of deformation was magnified. The figure shows thermal deformation mainly concentrated in the lens and the plastic material at the particular place the lens and plastic materials contact with the maximum deformation reaches 6.3 m. The bottom of the substrate increases the X, Y, Z three directions constraint, equivalent to the bottom substrate is fixed, so the bottom of the substrate displacement is 0 m.

Figure 4 (b) is the stress distribution at steady state LED cloud. The figure shows, lenses, sealed plastic outer layer and the substrate at the top of the thermal stress is small, the stress was greater than at the top of the bottom substrate. This is due to thermal expansion of the bottom substrate by X, Y, Z three directions due to the constraints. Figure 5 (a) for the stress distribution at the bottom of the substrate, the maximum bottom corner in the substrate, to 163 MPa; Figure 5 (b) shows the substrate at the top of the largest thermal stress in the heat sink and substrate at the junction of the top substrate corner only 1.43 MPa.


Figure 4 Lumileds 1 W LED thermal deformation of cloud (a) and equivalent stress contour (b)

Figure 6 (a) a bonding layer of equivalent stress contours. The figure shows the maximum thermal stress in the bonding layer of the corners as 269 MPa, minimum stress bonding layer has reached 94.6 MPa. This is because the small thermal conductivity of bonding layer, the larger the thermal resistance, heat build up more here, leading to the corners in the bonding layer of thermal stress as the most concentrated part of the package device. Fig 6 (b) is the equivalent thermal stress contours chip, the chip at the maximum stress in the four corners to 34.1 MPa, such a high stress can lead to rupture of the chip, with particular attention.


the bottom of Figure 5 LED substrate (a) and top (b) thermal stress distribution


Figure 6 Lumileds 1 W LED bonding layer (a) and chip (b) the equivalent thermal stress distribution

Chip top central node of the displacement versus time curve shown in Figure 7, X and Z direction displacement of approximately zero, Y direction of the displacement field with time and temperature changes constantly changing (Y to the device vertical the temperature transfer direction), in the light source is about 500 s, the temperature field into the stable state, then the chip should be variable maximum 6.3 m, and the transient temperature field line.


Figure 7 Lumileds 1 W LED chip as the central node displacement-time curve

2.3 substrate, thermal stress on the path, strain and shear stress simulation and analysis

The top of the substrate X-axis parallel to the direction of a selected axis as shown in Figure 8, the path to study the strain path, stress and shear stress changes.


Figure 8, the top surface of the substrate path diagram

Figure 9 (a) that the path X, Y, Z three directions of the displacement curve. The figure shows, the path almost to zero on the UZ, Y direction, the ends of small deformation, the middle is too large, which is consistent with the temperature distribution; UX both ends of the displacement of large decreases gradually to the middle, and ends symmetric about the center, which is the shape of the substrate binding conditions. Figure 9 (b) curves for the stress path, SX and SZ stress changes the direction of the same trend, maintaining a high stress level, and SY consistently low stress level. X, Y, Z three directions displayed on both sides of the stress values ??are greater than the middle, we can see that the greatest stress occurs at the corners.



(a)



(b)

displacement path in Figure 9 (a) and stress (b) curve

Figure 10 shows the shear stress on the path changes, SYZ and SXZ almost coincide, and the shear stress is small, changes gently; SXY very dramatic change, indicating that the Y direction, that is, between the substrate and heat sink than large shear stress, and increases from the middle to both ends, indicating that the shear stress concentrated in the corner area. This is due to the substrate and heat sink for the two different materials, materials, and thermal expansion coefficient between the elastic modulus difference in the larger shear stress.


path of Figure 10 the curves of shear stress

2.4 thermal conductivity of the stress, strain and temperature

LED junction temperature Figure 11 that the device layers of thermal conductivity with the trend.

The figure shows, LED junction temperature with the heat sink and bonding layer similar to the trend of thermal conductivity, when is small, as increases, the junction temperature decreases rapidly; When is large, with the kinds of thermal conductivity changes, the junction temperature changes gently. This is because when is small, the larger the thermal resistance of the material, and when is large, the thermal resistance decreases, the heat can be spread smoothly, then the thermal conductivity of the whole system is no longer a major factor in heat transfer . LED heat does not go through the lens, so the thermal conductivity of the LED lens junction temperature change is small. Figure 12 shows the LED chip

maximum equivalent thermal stress and maximum strain, with the bonding layer of thermal conductivity changes. Strain is almost the same chip, has nothing to do with the thermal conductivity; and chips by the thermal stress decreases rapidly with increasing thermal conductivity, but increased to a certain value, the thermal stress leveled off, and the bonding layer of the thermal conductivity change trend of the temperature field is consistent. This is due to the heat transfer process, the bonding layer of low thermal conductivity, the larger the chip to heat sink thermal resistance, resulting in a higher LED junction temperature, the temperature gradient is large, making the thermal stress is relatively concentrated, so the material bonding layer Select the LED junction temperature on the change in thermal stress has a crucial role.


Figure 11 LED junction temperature with a variety of materials, thermal conductivity curve


Figure 12 LED chips maximum stress and strain as the bonding layer of the thermal conductivity curve

3 Conclusions

Power type LED devices on the temperature field and stress field simulation calculations show that: LED chip, axial strain and the change of temperature consistent, stable when in 500 s; maximum deformation in the lens and heat sink Local contacts for the 6.3 m; maximum thermal stress in the bonding layer in contact with the chip corner, as 269 MPa, the maximum stress of the chip 34.1 MPa. Through the material on the LED junction temperature and strain analysis, as the LED junction temperature and heat sink bonding layer increases the thermal conductivity first decreases sharply, but increases to a certain value, LED junction temperature changes leveled off The thermal conductivity of the junction temperature of the lens has little effect; LEDs maximum equivalent thermal conductivity with the bonding layer changes and temperature changes in the situation is entirely consistent with, the strain on the chip is almost no effect. Strain and stress concentrated in the larger temperature gradient, surface bound and prone to stress concentration of the corner regions, these regions particularly prone to damage, so LED package, must take into account the actual temperature, the requirement that materials must be able to tolerate thermal stress concentrated.

Conclusions based on analysis of the paper, LED thermal stress mainly due to the thermodynamic between the layers of packaging materials caused by the different performance parameters. In order to improve the quality of LED packages required to select the appropriate packaging materials, with the thermal conductivity is large enough to reduce the package thermal resistance between the layers, to prevent the accumulation of heat generated great stress. LED semiconductor devices in order to avoid generating large deformation, layers of packaging material between the thermal expansion coefficient is smaller. At the same time, the encapsulation layer is best not to form an acute angle corners to avoid stress concentration at the corner of the damage produced at the LED devices.

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