Thermal Stress Analysis of LED Bonding Wire Based on Numerical Simulation
LED is a kind of semiconductor light source that directly converts electrical energy into light energy. It has the characteristics of energy saving, environmental protection, safety, long life and low power consumption. It is widely used in the fields of indication, display, decoration, backlight, general illumination, etc. [1] . The chip and the substrate are usually electrically connected by wire bonding, that is, the metal wire and the soldered pad are diffused and mutually dissolved by energy such as heat, pressure, ultrasonic wave, etc., so that the chip electrode-bonding wire-substrate is mutually Bonding connection.
In the production and manufacture of LEDs, in order to understand, evaluate, analyze and improve the environmental adaptability of LEDs, LEDs are often tested for reliability [2], and thermal shock tests are one of them. The experiment tests the chemical or physical damage caused by thermal expansion and contraction by applying a periodic transient cold and hot temperature cycle to the LED. In this test, the LED bonding wire often becomes a weak part, and its disconnection in the test plays a key role in LED reliability.
In order to understand the fracture mechanism of LED bonding wire under the thermal shock test, this paper constructs the LED bonding wire model under the condition of thermal shock under the thermal stress basic theory of the material, and simulates the bonding wire through finite element numerical simulation. The thermal stress is calculated and analyzed to confirm the thermal stress distribution of the bonding wire and the relevant parameters affecting the thermal stress.
1. Basic theory of thermal stress
Thermal stress is also called temperature change stress. The necessary condition for generating thermal stress is that there is a temperature difference, and thermal stress can be generated when the structural deformation caused by the temperature difference is restrained. There are three forms of constraints, namely external rigid constraints, deformation constraints between internal parts, and mutual deformation constraints between different materials. For LEDs, under the condition of cold and thermal shock, the LED is subjected to periodic thermal expansion and contraction, and the thermal expansion coefficients of the materials are different and mutually constrained. Therefore, stress concentration is easily generated at the interface of each material.
During the thermal shock process, due to the different thermal expansion coefficients of the components of the LED package , periodic expansion and contraction will occur. The bonding wire is subjected to different degrees of shearing and stretching. The stress is distributed in a multi-axis state in the three-dimensional structure of the bonding wire. Therefore, when analyzing the mechanical behavior of the bonding wire under the conditions of thermal shock temperature cycling, the expression synthesis is adopted. The equivalent stress of the stress intensity describes the stress distribution state of the bond wire.
Based on the Von Mises criterion of the fourth strength theory, the component of the equivalent stress stress tensor is expressed as:
It can be seen from the above analysis that for LEDs, the greater the ambient temperature temperature difference, the greater the difference in thermal expansion coefficient between the packaging materials, and the greater the elastic modulus of the material, the greater the thermal stress of the LED, and the increase of the interface stress with time. Concentration is prone to fatigue fracture.
When the thermal encapsulation material of the LED is given the thermodynamic properties and the thermal boundary conditions of the material are applied, the equivalent thermal stress of the LED three-dimensional model can be analyzed and solved by the above formulas, and the positions of the LED packaging materials under the thermal load condition can be obtained. Equivalent stress condition. Among them, the analytical solution process can be solved by finite element numerical simulation [3-5].
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