Effect Of Nickel Metallization Thickness On Microstructure And Mechanical Properties In Pb-free Micro-sized Solder Joints

Au/Ni/Cu is a classical and common metallization on pad of PCB.During reflowing,the Au layer quickly dissolves in solder.Ni layer is the actual bonding layer between pad and solder ball.It's thickness significantly affects the formation and evolution of the microstructure in solder joint and affects the mechanical properties of solder joint further.This work focuses on the effect of nickel metallization thickness on microstructure evolution and mechanical properties in Sn3.0Ag0.5Cu/Au/Ni/Cu solder joints.There are two kinds of nickel metallization thickness:one is denoted as S1 with 3?m nickel layer thickness and the other is denoted as S2 with 6?m nickel layer thickness.This work clarified the evolution of Au-Sn compounds and interfacial IMC during reflowing and solid-state aging,compared the mechanical properties of solder joints with different Ni metallization thickness,and explained the fracture behavior of the Au-Sn compounds and interfacial IMC during the fracture process of solder joints.Firstly,A large amount of finely dispersed Au(X,Sn)₄(X is one element or two elements of Cu and Ag)is formed in both S1 and S2 after reflow.During solid-state aging,Cu and Ag gradually precipitated from Au(X,Sn)₄.Cu participated in interfacial reaction and formed interfacial IMC.Ag formed granular Ag₃Sn with Sn attaching to the interfacial IMC.The component of Au(X,Sn)₄fixed as Au Sn₄.The morphology of Au(X,Sn)₄changed from needle-like to lamellar-shaped,and finally to lath-shaped during solid-state aging.Secondly,a single layer(Cu,Ni,Au,Ag)₆Sn₅IMC formed at the interface after reflow and then differentiated into double layers during solid-state aging in both S1and S2.The interfacial IMC which closes to solder was recorded as(Cu_1-x-yAu_xNi_y)₆Sn₅(U-IMC)and the other which closes to Ni metallization was recorded as(Cu_1-a-bNi_aAu_b)₆Sn₅(D-IMC).U-IMC is irregular-shaped,and D-IMC is flat-shaped.The growth of the interfacial IMCs is controlled by diffusion process.They grow as a whole.The total thickness of the interfacial IMC(T-IMC)in S1 is higher than that of S2 at all stages,and the final relative increasing is 17.58%.The differentiation of the bilayer IMCs is caused by the redistribution of Ni and Au during solid-state aging.Thirdly,S1 exhibits higher average shear force and lower brittle fracture tendency during reflowing and solid-state aging,indicating that the solder joints with thinner Ni metallization thickness have more excellent mechanical properties.Finally,gold embrittlement is caused by the mosaic structure composed of lamellar-shaped Au(X,Sn)₄and the interfacial IMC.This structure hindered the movement of dislocations in the solder joint,making it easy to form voids and stress concentration in this area,and transferred fracture from solder ball to the interface of IMC/Ni.