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This article focuses on the study of voids in the solder joint interface of flip-chip LEDs. Additional flux is added to the solder paste to reduce the frequency of voids, and the mechanical properties of the solder joints are compared and analyzed. Two kinds of fluxes (NC559-TPF, TSF-6516) are selected. They are used in two kinds of solder pastes (lead and lead-free solder paste). The amount of flux added is 0 mL, 0.4 mL, and 0.8 mL. The other preparation conditions for all combinations are the same and are within the recommended scope of the manufacturer's documentation. X-rays are used to detect and analyze the distribution and quantity of voids in the solder layer. A push–pull tester is used to analyze and evaluate the mechanical strength of the solder joints. A scanning electron microscope was used to observe the microscopic morphology of the cross-section of the solder joint and the fracture morphology of the interface after the shear test. It was found that adding a proper amount of high-activity flux to the solder paste can reduce the frequency of voids and improve the mechanical reliability of the solder joints. Finally, we tested the thermal performance of the flip-chip LED filament for verification.

This paper investigates the effect of grain size on the evolution and shear behavior of lead-free solder joints. The grain size of the solder is 2-11 μm, 5-15 μm, 15-25 μm, respectively. The interface structure of the solder joint and the microstructure of the inferred surface are analyzed. The void ratio of the flip-chip solder layer is tested. In order to study the change of shear strength and fracture surface, the solder/copper joint was subjected to isothermal aging treatment (1000 h) under the environment of relative humidity of 85 °C/85%. The results show that the smaller the solder crystal grains, the larger the surface area of the crystal grains per unit mass. Because of the action of the liquid bridge force, tin powder agglomeration is prone to occur, which leads to a high void rate of the solder layer. At this time, a higher activity flux is required to remove oxides. Highly active flux (NC559-TPF) was added and verified. When the grain size is 5-15 μm, the solder paste has the best soldering performance. After the filament is aged for 1000 h, the junction temperature is the lowest. Secondly, the blue light flux is the highest.

In the actual production process, soldering is a key step in the flip-chip packaging process. The reliability of the interconnection interface of flip-chip LED chips is considered to be one of the most important reliability issues. This paper studies the influence of different soldering temperatures on the microstructure and shear behavior of the Au/Sn-3.0Ag-0.5Cu/Cu solder joints of flip-chip LED chips. The lead-free solder Sn-3.0Ag-0.5Cu was selected as the solder. The microstructure of the intermetallic compound (IMC) interface and the inferred surface of the solder joint is observed, and the microstructure evolution of the solder joint is analyzed. The void ratio of the solder joints at 250°C, 260°C, 270°C, 280°C, and 290°C soldering temperature was tested to characterize the influence of the contact area between the chip and the solder joint on the shear stress. In addition, the solder joints were aged for 1000 hours in an environment with a relative humidity of 85°C/85%, and a shear test was performed to evaluate the influence of the interface reaction on the mechanical reliability of the solder joints during the isothermal aging process. The research show that when the soldering temperature is 270°C, the Au on the bottom of the chip, and the solder and Cu on the substrate have fully reacted, and the solder joints have high shear resistance. The shear strength of the aging solder joints increases first and then decreases, because the high temperature repairs the defects in the solder layer caused by the low soldering temperature to a certain extent. With the extension of aging, cracks and voids gradually appear at the fracture interface, and the effective contact area decreases. The shear strength of the solder joint decreases, and the fracture mode becomes brittle.

This paper investigates the effect of soldering temperature on solder joint voids and reliability of flip-chip LED chips during reflow soldering. Lead-free solder SAC305 was used as solder paste. The void ratio of the flip-chip LED solder joint at 250°C, 260°C, 270°C, 280°C, and 290°C reflow soldering temperatures was detected by x-ray detector. Shear tests were conducted to evaluate the influence of interfacial reactions on the mechanical reliability of solder joints. The distribution of voids in the shear section was observed by scanning electron microscope (SEM). Next, the photoelectric and thermal properties of FC-LED filament were tested and analyzed. Finally, a high-temperature and high-humidity aging experiment was carried out to test the reliability of the LED filament. The results show that the void ratio of the LED filament soldering joint is the lowest when the soldering temperature is 270°C. The small void ratio of the solder joints results in lower steady-state voltage and junction temperature of the flip-chip LED filament. As the void density in the solder joint decreases, the shear strength of the solder joint increases. At this time, the shear resistance and mechanical reliability are the highest.

This paper investigates the effect of different soldering temperatures on the performance of the flip-chip light-emitting diode (FC-LED) filament during direct soldering. The changes in the intermetallic compound (IMC) interface, push–pull force and chip fracture surface of the chip solder joints under direct soldering temperatures of 220°C, 260°C, and 320°C for the flip-chip LED filament were explored by scanning electron microscopy (SEM). Thereby, the optimal soldering temperature of direct joining in actual production is compared. The results show that when the soldering temperature is 260°C, Cu on the substrate begins to diffuse into the solder and react with the solder in the lower layer. The Sn content is relatively uniform, and the average push–pull force of the chip increases. The fracture occurs from inside the solder. With a soldering temperature of 260°C, it is observed that the interface shear stress of the flip-chip LED chip is the largest, and the mechanical stress and residual stress are the lowest.