Study On Microstructure And Mechanical Property Of T2 Copper Joint With Ni/Sn Solder Under Ultrasonic Vibration

Copper is widely used in the electronics industry for its excellent electrical conductivity and low price and one of its uses is to fabricate solder joints by connecting pads and solder.With the rapid development of volume miniaturization and functional integration of electronic products,microelectronic systems have imposed higher requirements on the reliability of interconnection solder joints,while some severe service environments such as thermal cycling and sustained vibration have highlighted the importance of a reliable joint.Although the traditional Sn-based solders are widely used,there still exist some disadvantages,such as the poor strength of pure Sn and easy oxidization inclination and poor wettability of Sn-Zn.In this paper,Ni-Sn composite solders are prepared by using porous Ni to plate Sn in order to effectively enhance the strength of solder.At the same time,employing ultrasonic vibration to assist the soldering process can not only promote the wetting of filler metal on the surface of Cu,but also play a significant role in grain refinement.In the experiment,two kinds of porous Ni with different porosity are used to enhance Sn and different joint forms are used for soldering.When 98%porosity Ni/Sn solders are used,solder joints are fabricated with and without controlling the thickness of solder seam.When 60%porosity Ni/Sn solders are used,the ultrasonic welding rods are applied to the upper base metal with controlling the thickness of solder seam during the soldering process.After the experiment,the microstructure of the joints is analyzed and the microstructure change process and metallurgical reaction mechanism under different ultrasonic vibration time are explored.The shear strength of joints is tested and the failure mechanism is investigated.The experimental results show that when 98%porosity Ni/Sn composite solders are used,the products in the joint are mainly（Cu,Ni）6Sn5 and the shape of them are granular in the thickness controlled joints.The number of these particles increases with the elongating of ultrasonic vibration time.The product next to Cu is（Cu,Ni）6Sn5 and that next to porous Ni is（Ni,Cu）3Sn4 in the joints which doesn’t control thickness of solder seam.The product shape in solder seam undergoes a transition from lamellar to lamellar and particulate to particulate.The highest shear strength is only 36.9 MPa in the thickness controlled joints and the highest shear strength is 64.9 MPa in the joints without controlling the thickness of solder seam.When using 60%porosity Ni/Sn solder,good wetting is acquired and the product next to the base material is continuous layered（Cu,Ni）₆Sn₅.The product beside Ni is（Ni,Cu）₃Sn₄ and they gradually change into fine particles from layer shape with the prolongation of the soldering time.The maximum shear strength of the joint is 86.9MPa and it is obtained at the vibration time of 60 s.When different filler metal is employed,the difference between the microstructure and the phase composition of solder joint is obvious.This is mainly because of the change in the contents of Cu,Sn and Ni in soldering seam.The reaction products in the joints are significantly affected by the elemental content.When the thickness of soldering joint and the porosity of porous Ni are altered,the element content in soldering seam changes accordingly,which leads to the change of reaction products.