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https://doi.org/10.1007/s11664-020-08517-9 Ó 2020 The Minerals, Metals & Materials Society

TMS2020 MICROELECTRONIC PACKAGING, INTERCONNECT, AND PB-FREE SOLDER

Effect of Different Soldering Temperatures on the Solder Joints of Flip-Chip LED Chips XINMENG ZHAI,1 CHENGYU GUAN,1 YUEFENG LI MINGMING SHI,1 and YANG LI1,3

,1,2 JUN ZOU,1

1.—College of Science, Shanghai Institute of Technology, Shanghai 201418, People’s Republic of China. 2.—e-mail: [email protected]. 3.—e-mail: [email protected]

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. Key words: Flip-chip LED chip, SEM, soldering temperature, microstructure, push–pull force

INTRODUCTION Legislation has been established to address environmental issues, and lead-free solder is used to replace lead-containing solder globally. Among various lead-free solders, Sn-Ag-based alloys have been most widely used due to their good manufacturability, creep resistance and thermal mechanical fatigue resistance. Creep resistance can be further improved by adding a small amount of Cu to the SnAg alloy, making Sn-Ag-Cu the most popular leadfree solder alloy.1–6 The yield stress, mechanical stress and residual stress are important parameters used to characterize the reliability of flip-chip LED chip solder joints. A variety of test methods have been adopted to study the changes in the microstructure of solder

(Received July 28, 2020; accepted September 23, 2020)

joints under different conditions. At present, scanning electron microscopy (SEM) and synchrotron xray micro-diffraction (lXRD) techniques are increasingly used to detect the microstructure of solder joints. Among them, synchrotron x-ray microdiffraction (lSXRD) has proven to be a very effective method to quantitatively detect mechanical stress, and it can penetrate metal materials and monitor strain on site. Handara et al.7 were the first to use synchrotron x-ray micro-diffraction technology to probe the stress and fracture mechanism of packaged thin-film solar cells. This technology now provides us with an opportunity to understand the complete evoluti