Effect Of Adding Nickel Element To Substrate On The Microstructure And Mechanical Properties Of Cu/Sn/Cu Joints

With the development of electronic products towards miniaturization and multi-function,the size of interconnect solder joint is decreasing gradually,and its reliability has been concerned.The failure of electronic products is largely caused by the failure of solder joints.The research on the performance of solder joint is the premise of improving the reliability of solder joint.In the welding process,the reaction between filler metal and substrate inevitably generates Intermetallic compound（IMC）,and the presence of excessive IMC will affect the performance of welded joints.In this paper,the main research object is Cu/Sn/Cu micro interconnection structures.By adding alloy elements,the changes in the microstructure of the joints and the results of mechanical testing are analyzed.Using first principles calculations,the effects of Ni addition on the microstructure and mechanical properties of Cu/Sn/Cu welded joints are systematically investigated.The main contents and conclusions of the above research are as follows:（1）Four kinds of Cu-xNi/Sn/Cu-xNi（x=0,0.5,1.5,2.5wt.%）welded joints with different Ni contents were prepared,and the interface microstructure was observed.It is found that the interfacial IMC of Cu-2.5wt.%Ni/Sn/Cu-2.5wt.%Ni structure is（Cu,Ni）₆Sn₅and a small amount of Cu₃Sn,and this prism-like（Cu,Ni）₆Sn₅ grows close to the solder side.The other three structures of the interface IMC is Cu₆Sn₅ and Cu₃Sn.In addition,a small amount of Cu and Ni elements were observed in the filler metal matrix.The calculation of IMC thickness of four kinds of joints shows that the IMC thickness decreases first and then increases with the increase of Ni content,but the thickness of Cu₃Sn keeps decreasing.The longer the aging time,the thicker the IMC formed by the interface.（2）The shear properties of four kinds of welded joints are analyzed.The results show that the shear strength of Cu-xNi/Sn/Cu-xNi（x=0.5,1.5,2.5wt.%）welded joints is significantly higher than that of the joints without Ni added.The shear strain increases with the increase of Ni content.The crack occurs mainly between the filler metal and IMC layer,and the fracture mode is mainly tough-brittle mixed fracture.In addition,the shear properties of the joints after aging were tested,and the results showed that the shear strength decreased with the aging time.The fracture occurred mainly between the filler metal and IMC,and the fracture was mainly composed of tough-brittle mixture.Therefore,the addition of Ni element can improve the strength and plasticity of welded joints,and the shear performance decreases with the extension of aging time.（3）As the main component of the compounds generated by the four joint interface reactions is IMC dominated by Cu₆Sn₅,nanoindentation testing is mainly conducted for scallop-shaped and prismatic IMC.The results show that the addition of Ni can increase the elastic modulus and hardness value of IMC.（4）Using a first principle calculation method based on density functional theory,the effect of Ni content on the crystal structure,mechanical properties,and electronic properties of Cu₆Sn₅ phase was analyzed theoretically.The results show that the incorporation of Ni atoms does not change the crystal structure,but rather makes the structure more stable.The presence of Ni improves the elastic modulus,shear modulus,and hardness of Cu₆Sn₅ phase,which is consistent with the results of nanoindentation testing.In addition,by calculating the distribution of electron population and density of states（DOS）in the crystal structure,it is found that the hybrid gain energy of Cu-d,Ni-d,and Sn-p orbitals is the main reason for the stability of IMC performance.