Mechanism For Rapid Formation Of Intermetallic Joint With Ultrasonic-assisted Tin-based Soldering

With the development of electronic science and techn ology,the microelectronic devices trend tends to be minimized size,lightness,excellent electrical-mechanical performance and low cost with harsh working environments,such as high temperature,large current,and humid ambient,leading many challenges for the devices application.At the same time,the wide band-gap semiconductor materials,such as Si C and Ga N have emerged with potential materials to replace Si used in electronic devices,especially in high temperature power electronics.Therefore,the study of novel interconnection materials and packaging methods to take full advantage of excellent properties of Si C at high temperature has an extremely important practical signigicance for application in electrified vehicles,aerospace,military,light-emitting diode（LED）and other power electronics.Based on the transient liquid-phase（TLP）method,the Sn/Ni compound particles were chosen to replace pure Sn and ultrasonic was applied as assisted measures during bonding process.This can not only realize the bonding interface rapidly formed,but also ensure the good resistance against phase transformation.The effects of Ni content,Ni particle size and ultrasonic power on the microstructure and the properties of the joint were systematically investigated.It is found that the size of Ni particles directly affected the wettability of the solder and determines the morphology of the joint.In addition,the amount of Ni₃Sn₄ in the joint increases with the increase of Ni in the solde r.When the welding temperature,ultrasonic power,solder time were set at 250°C,500 W and 10 s,a nearly sole Ni₃Sn₄ joint with high-performance was achieved using Sn-24%Ni solder,the shear strength of 43.4MPa and the resistivity of 24.09μΩ·cm.After aging at 300°C for 72 h in air,the microstructures of the joints did not change,but the shear strength decreased to 33.4MPa due to the grains coarsening of Ni₃Sn₄.When the size and the content of Ni particles in the solder were increased to 20μm and 40%,a joint consisted of Ni and Ni₃Sn₄ with lower Young’s modulus was obtained by reflowing at 250°C for 90 min.The growth mechanism of intermetallic compound（IMC）and the mechanism of ultrasonic were also studied.The results show that Ni₃Sn₄ is the first phase formed in the Ni-Sn system,and Ni₃Sn₄ growth is mainly dominated by the diffusion of Ni atoms of parabolic type when the soldering process with ultrasounic.By calculation,we found that the growth rate constant of Ni₃Sn₄ is only 4.643×10-11cm2/s at 250°C,so the process to produce full IMC is time-consuming.To solve the problem of time-consuming of the TLP result by low atom diffusion velocity,the ultrasonic was adopted dur ing bonding process.Ultrasonic cavitation and streaming effect can produce the Ni₃Sn₄ breaked and stripped from the Ni surface and exposed highly reactive surfaces of Ni.The exposed highly reactive Ni surface will continuously react with molten Sn in a short time so that the high temperature IMC joint can be formed rapidly.