Study On Microstructure And Mechanical Property Of Cu-foam Reinforced Sn3.0Ag0.5Cu Solder Joints Under Ultrasonic Vibration

With the advent of the 5G era,the trend of volume miniaturization and functional integration has put forward higher requirements on the packaging technology of electronic devices and the reliability of interconnect solder joints.Sn-Ag-Cu lead-free solders have been widely applied in the field of electronic packaging due to their excellent comprehensive properties.However,the melting point of Sn-Ag-Cu solder is relatively high,and the addition of Ag will cause the production of brittle intermetallic compounds(IMC),which will reduce the reliability of solder joints.In this paper,Cu-foam was used for reinforcing Sn3.0Ag0.5Cu(SAC305)solder.The modified Cu-foam/SAC305 composite solder was used for ultrasonic-assisted soldering to prepare solder joints.The interfacial microstructure and mechanical properties of the solder joints were tested by field emission environmental scanning electron microscopy(SEM),X-ray diffractometry(XRD)and uniaxial micro-force test machine.And the aim is to obtain highly reliable solder joints.In order to study the effects of different thickness of Cu-foam and ultrasonic vibration time on the interfacial microstructure and mechanical properties of the solder joints,SAC305 solder reinforced by 0.15 mm Cu-foam and 0.5mm Cu-foam were used as the intermediate layer,and different ultrasonic assisted time was used to prepare solder joints.The results show that:excellent metallurgical bonding generated between the Cu-foam/SAC305 composite solder and the copper substrate.The Cu₆Sn₅IMC appeared at the Cu-foam skeleton and the copper substrate.The application of ultrasonic vibration would refine the Cu₆Sn₅grains.The average thickness of interfacial IMC layer decreased with the extension of the ultrasonic soldering time.Compared with the solder joints prepared with 0.5 mm Cu-foam,more obvious grain refinement was observed in the solder joints prepared with 0.15mm Cu-foam.The average shear strength of joints soldered with Cu-foam/SAC305composite solder was higher than that of the joints soldered with pure SAC305 solder.In addition,the grain refinement caused by ultrasonic cavitation significantly improved the shear strength of solder joints.The fracture failure mode of the joints soldered without ultrasonic vibration was ductile brittle mixed type.When ultrasonic vibration was applied,the fracture type of the solder joints changed from ductile brittle mixed type to ductile type.In order to further study the influence of Cu-foamed with different porosity and ultrasonic power on the interfacial microstructure and mechanical properties of solder joints,Cu-foam with a porosity of 75%and 98%were used for reinforcing SAC305 solder,and different ultrasonic power was used for preparing solder joints.The results shows that the excessive solid phase in the SAC305 solder reinforced by Cu-foam with 75%porosity limited the blasting of ultrasonic cavitation bubbles,resulting in weak cavitation effect.Therefore,the Cu₆Sn₅IMC thickness of the solder joint prepared by using 75%porosity Cu-foam was thicker.The average shear strength of solder joints prepared by different porosities of Cu-foam increased firstly and then decreased with the increment of ultrasonic vibration power.Excessive ultrasonic vibration power would cause micro-cracks in the IMC grains of the solder joints,resulting in the decreased of the average shear strength of solder joints.The use of Cu-foam with 75%porosity had a stronger absorption effect on ultrasonic energy,so the shear strength of Cu-foam solder joints with low porosity would decrease under higher ultrasonic power.The fracture failure mode of solder joints prepared by low-power ultrasonic vibration was mainly ductile fracture.When an excessively high ultrasonic power was applied,the fracture type of the solder joint changed from ductile type to ductile brittle mixed type.