Abstract
Sn-Zn system and Sn-Ag-Cu system solders have been supposed as promising lead-free solders. In this work, the shear strength and fracture mechanism of Sn-9Zn-2.5Bi-1.5In and Sn-3Ag-0.5Cu pastes joints with Cu substrates under the conditions of aging treatment was studied. It is indicated that the intermetallic compounds (IMC) in SnAgCu/Cu joints surfaces are scalloped Cu6Sn5 and planar Cu3Sn. And the IMC in SnZn/Cu joints are Cu5Zn8 layers. In both solder joints, the cracks tend to transform from solder-fracture model to the solder/IMC interface-fracture model with aging treatment. In SnAgCu joints, Cu6Sn5 scallops grew larger and Cu3Sn layer thickened owing to the continuous interdiffusions of Cu and Sn atoms. More Kirkendall-voids appeared in the Cu3Sn layer, and cracks expanded at the interface. Then, the joints would tend to fracture at the solder/IMC interface or in the IMC layer, and the joint strength decreased. However, the situation is different for SnZn joints with aging treatment. In the as-reflowed joints, fracture mainly occurred in the solder matrix. As aging time increased, little zinc could react with copper, which brought out a slow growth rate of Cu5Zn8 IMC. When the aging time increased to 168h, obvious diffusion channels between Cu5Zn8 IMC interface layer and Zn-phase in solder can be observed, and therefore, many Zn-phases reacted with Cu to form Cu5Zn8 IMC in the solder matrix. The different diffusion velocities of Cu and Zn could result in voids at the interface, and then fractures tend to expand along the interface. In summary, both solder joints as reflowed mainly showed ductile fracture mode in the solder, while the shear strength of SnZn joints is 64MPa, higher than the 56MPa of SnAgCu joints. As aging time increased, the shear strength of SnZn joints reduced slightly to 60MPa when the time is 48h, then dropped 30% to 45MPa when the time is 168h, and finally stayed above 40MPa until 720h. As for SnAgCu joints, the shear strength decreased from 56MPa to 45MPa until 168h, and finally dropped about 40% to 35MPa until 720h.
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