Research On Mechanical And Electromigration Behavior Of Ni₃Sn₄ Particle Enhanced SnBi Lead-Free Solder

The reliability of micro-interconnect structure plays a vital role in electronic packaging,and the failure of electronic products and devices are often related to the micro-solder joint failure in the micro-interconnect structure.Because of the decreasing size of microsolder joint,micro-solder joint failure has become one of the main causes of electronic products and devices failure.With the rapid development of new lead-free solder materials,Sn58Bi solder material has attracted much attention because of its low melting point and cost,good wettability and excellent mechanical properties.However,due to the brittleness of bismuth（Bi）element itself,conventional Sn58Bi solder material has disadvantages such as poor ductility and impact performance.At the same time,the microstructure of Sn58Bi solder material is usually coarse,and the Bi-rich phase is prone to coarsening at high temperatures,which further leads to the deterioration of plasticity of SnBi micro-solder joints.In addition,as the package size continues to decrease,the current density in the solder joints increases,resulting in the reliability problem of electromigration in micro-solder joints.Therefore,in order to improve the microstructure of Sn58Bi solder material,enhance the mechanical performance and anti-electromigration ability of solder joints,this paper prepared Ni₃Sn₄ particle reinforced SnBi lead-free solder material,and studied its mechanical and electromigration behavior.The main research contents of this paper include the following four parts:1.The Ni-Sn powder was melted by vacuum arc melting,and then Ni₃Sn₄ micro-nano particles were obtained by ball milling.The Ni₃Sn₄ micro-nano particles were mechanically dispersed into Sn58Bi solder paste at a mass ratio of 0.3%,0.5%and 1.0%.Using a stainless steel mesh with a thickness of 300μm and an aperture of 600μm,the mixed solder paste was printed on the PCB substrate,and Cu/Sn58Bi-xNi₃Sn₄（x=0.3,0.5and 1.0 wt.%）single interface and Cu/Sn58Bi-xNi₃Sn₄（x=0.3,0.5 and 1.0 wt.%）/Cu double interface micro-solder joints were prepared by reflow soldering.2.The microstructure morphology and element distribution of the single interface micro-solder joints were studied by scanning electron microscopy and energy dispersive spectroscopy.The results showed that Ni₃Sn₄ micro-nano particles were uniformly distributed in Sn58Bi-xNi₃Sn₄（x=0.3,0.5 and 1.0 wt.%）solder material,and the solder material with 0.5 wt.%Ni₃Sn₄ micro-nano particles had the best microstructure refinement effect.Moreover,the addition of Ni₃Sn₄ micro-nano particles inhibited the growth of intermetallic compound（IMC）at the interface.Among them,the interface IMC of Sn58Bi solder material without IMC particles was the thickest,and the interface IMC of solder material with 0.5 wt.%Ni₃Sn₄ micro-nano particles was the thinnest.The ball-pull test results showed that the addition of Ni₃Sn₄ micro-nano particles improved the shear strength and displacement to failure of the joints,and the fracture location changed from the interface IMC layer to the solder bulk with the increase of Ni₃Sn₄ micro-nano particle content.3.The elastic-plastic constitutive equation of Sn58Bi-xNi₃Sn₄（x=0,0.3,0.5 and 1.0wt.%）solder material was obtained by combining nanoindentation test,finite element simulation and dimensional analysis.The nanoindentation results showed that the elastic modulus decreased first and then increased with the increase of Ni₃Sn₄ particle content,and the elastic modulus of Sn58Bi-1.0 wt.%Ni₃Sn₄ solder material was the largest,while that of Sn58Bi-0.3 wt.%Ni₃Sn₄ solder material was the smallest.The hardness also showed a trend of decreasing first and then increasing with the increase of Ni₃Sn₄ content,and the hardness of Sn58Bi was the largest,while that of Sn58Bi-0.3 wt.%Ni₃Sn₄ was the smallest.For the yield strengthσ_y calculated by inverse analysis,the addition of Ni₃Sn₄ particles was higher than that without addition,and the strain hardening exponent n was lower than that without addition.4.The microstructure and IMC changes of Cu/Sn58Bi-xNi₃Sn₄（x=0,0.3,0.5 and 1.0wt.%）/Cu double interface micro-solder joints after 72 h and 240 h of electrification at30℃constant temperature oil bath were observed.It was found that Sn58Bi-0.3wt.%Ni₃Sn₄ and Sn58Bi-0.5 wt.%Ni₃Sn₄ solder materials had significantly improved anti-electromigration ability.Using image recognition method,a finite element model of Cu/Sn58Bi-xNi₃Sn₄（x=0,0.3,0.5 and 1.0 wt.%）/Cu double interface micro-solder joint with microstructure was established,and the magnitude and distribution of current density and temperature gradient were studied.