Abstract Scope |
Introduction: Low-silver Sn-Ag-Cu (SAC) solder was used extensively in lead-free soldering because of its competitive prices. Nevertheless, with the decrease of Ag content, a series of problems with low-silver SAC solders have been discovered, which will lead to an expansion of the melting point range and a decrease in solderability. To solve these problems, some scholars have done a lot of research. However, these researches have not significantly changed solder properties, especially with regard to solderability and melting temperature.Therefore, new soldering techniques are needed to solve the problems of lead-free solder applications.In early experiments, it was found that nano nickel particles could improve the wettability of solder, significantly inhibit the growth of intermetallic compounds (IMC), and improve the mechanical properties of joints. SAC0307 lead-free solder (or pastes) were used to realize Cu/Cu joint by ultrasonicassisted and adding nano-nickel particles at 210~220℃.With the 5 seconds of ultrasonic-assisted, the shear strength of SACPC joint reached the highest about 41.20MPa. It was easy to found slag and porosity, which affected the improvement of joint during stirring. Under ultrasonicassisted, the solder seam was filled and the shear strength was significantly improved, but slag inclusion and pores will accumulate and grow, which will lead to a decrease in shear strength. In this paper, Cu/SAC0307 power+10wt.%Zn/Cu joints with ultrasonic-assisted at low-temperature was successfully achieved, and effect of aging time on Cu/SAC/Cu joints were studied.
Experimental Procedures: 99.99 mass% Cu with the size of 5mm×5mm×3mm Smooth with 400# sandpaper, and clean it with alcohol, and the to-be-soldered surface of the copper substrate was placed in the fixture of the ultrasonic-assisted soldering device (Fig.1), and the width of the soldering seam is controlled to about 0.3 mm by the fixture fixed with screws. 45um Znpowder and SAC0307 No.4 solder powder were mixed to fill the Cu-Cu joint. The ultrasonic assisted soldering machine is composed of three parts: ultrasonic machine, lifting device and heating device. The ultrasonic power is 1KW, whose the frequency is 20KHZ. During the soldering process under ambient atmosphere, the heating platform provided a constant 220℃ and the ultrasonic vibrator applied a constant pressure of 4MPa to the copper substrate. After 10s, the ultrasonic assist was turned on for 10s, then the ultrasonic assist was turned off for 10s, followed by the ultrasound assist time was 10s, and the above process was repeated 2 times. Finish the soldering process after keeping 70s. In order to analyze the microstructure of the interface, optical metallurgical microscope and scanning electron microscope were used. Adopt Adobe Photoshop software and Image-pro Plus software to calculate the interfacial IMC layer thickness. The mean shear strength of several joints were identified by the PTR-1101 bonding strength tester. Aging time were 12h, aging temperature was 75、100 、125℃ respectively.
Results and Discussion: Cu/SAC0307 power+10wt.%Zn/Cu joints with ultrasonic-assisted at 220℃ was successfully achieved, the microstructure of soldering seam. Under the condition of ultrasonic vibration, not only Softening material was easy to fill the soldering seam, but also the slag inclusion and foreign gases were easy to wrap into the soldering seam. The quality of soldering seam are better. Particles between solder ball and solder ball has reduced the trend, but the size tended to be larger with increasing of the holding time. After adding micrometer Zn particles, the size of soldering seam was the narrowing. Interfacial IMCs were typical scallop-type in Cu/SAC0307 power/Cu joints, but IMCs were serrated and became smoother in Cu/SAC0307 power+10wt.%Zn/Cu joints. The thickness of interfacial IMC was 2.28μm in Cu/SAC0307 power+10wt.%Zn/Cu joints. IMCs were serrated and became smoother with increasing of aging temperature in Cu/SAC0307 power+10wt.%Zn/Cu joints. The thickness of interfacial IMC were 2.76μm and 2.65μm, and 4.75μm at 75, 100, 125℃ for 12h respectively.
Conclusion: 45um Zn-powder and SAC0307 No.4 solder powder were mixed to fill the joint and was successfully achieved. The results show that Cu6Sn5-type IMC layer change into Cu5Zn8 after adding Zn particles. The thickness of interfacial IMC was 2.28μm in Cu/SAC0307 power+10wt.%Zn/Cu joints. The shear strength of Cu/SAC0307 power+10wt.%Zn/Cu joint reached 34.2MPa, but the shear strength of Cu/SAC0307 power/Cu joint was only 12.24MPa. The thickness of interfacial IMC were 2.76μm and 2.65μm, and 4.75μm at 75, 100, 125℃ for 12h respectively. The shear strength of Cu/SAC0307 power+10wt.%Zn/Cu joint reached 34.2MPa, but the shear strength of Cu/SAC0307 power/Cu joint was only 12.24MPa. The shear strength of Cu/SAC0307 power+10wt.%Zn/Cu joint reached 23.82 MPa at 75℃ for 12h, and then drop down to 21~22 MPa at 100~125℃. The fracture presented the parabolic dimple, but there were no significant differences. When aging time was 12h, interfacial IMC of Zn8Cu5 in Cu/SAC0307 power +10wt.% Zn/Cu joint accumulated and grew with the increase of aging temperature. At the same time, particles in soldered seam would gather and grow up with the increase of aging temperature. It was the above two reasons cause the toughness crack of joint.
Keywords: Sn0.3Ag0.7Cu, Thermal ageing, Shear strength, Ultrasonic-assisted, Interfacial IMCs |