Formation And Mechanical Properties Of IMC Micro-Solder Joints In Nano-Ni/Sn/Micro-Ni Structure

With the development of high integration of 3D chip packaging,the density of micro-solder joints in the package body has rapidly increased,requiring a significant reduction in the scale of micro-solder joints.This has led to an increase in the proportion of intermetallic compound（IMC）between interface metals,resulting in difficult problems such as thermal fatigue,electromigration,and reflow collapse.This thesis proposes a Temperature Gradient Transient Liquid Phase（TG-TLP）bonding method based on a Nano-Ni/Sn/Micro-Ni structure to rapidly prepare mechanically reliable IMC micro-solder joints.The growth and morphological evolution of interface IMC in Ni coatings across grain boundaries were studied,and the effects of grain size were analyzed to design a Nano-Ni/Sn/Micro-Ni bonding structure.The growth and micro-morphological evolution of IMC micro-solder joints under TG-TLP conditions were studied,and the growth kinetic mechanism of interface IMC was discussed to characterize the mechanical properties of IMC micro-solder joints.The research results are as follows:（1）Using direct current electroplating,two types of Ni coatings with different grain sizes,Nano-Ni and Micro-Ni,were prepared.The grain diameters were approximately 21nm and1.2μm,respectively.In the solid-liquid reaction,the Nano-Ni/Sn interface Ni₃Sn₄had a very fast growth rate,nearly four times faster than that of the Micro-Ni/Sn interface Ni₃Sn₄.This was mainly due to the Nano-Ni coating providing more nucleation sites.The morphology of the Nano-Ni/Sn interface Ni₃Sn₄ evolved from the initial rod-like shape to a mixed morphology of rod-like and block-like,while the morphology of the Micro-Ni/Sn interface Ni₃Sn₄ evolved from small blocks to larger blocks.The block-like morphology of micro-Ni interface Ni₃Sn₄ and the rapid growth of Nano-Ni interface Ni₃Sn₄ were respectively conducive to the mechanical reliability and rapid formation of IMC micro-solder joints.（2）Based on the growth and morphological characteristics of Ni₃Sn₄ at two different grain sizes,a TG-TLP bonding method based on a Nano-Ni/Sn/Micro-Ni structure was proposed,which could rapidly prepare defect-free Ni₃Sn₄ micro-solder joints within 12minutes.During the reaction process,the Ni₃Sn₄ at the cold and hot ends of the micro-solder joints interface grew non-uniformly.The growth rate and morphology of the micro-Ni interface Ni₃Sn₄ were block-like,while the Nano-Ni interface Ni₃Sn₄ had almost no growth and had a rod-like morphology.As time reached 12 minutes,the large block-like Ni₃Sn₄ at the cold end penetrated the entire solder joint,rapidly forming a defect-free Ni₃Sn₄ micro-solder joints.The rapid diffusion of Ni atoms driven by the temperature gradient,the rapid dissolution of the Nano-Ni layer at the hot end,and the large number of rod-like IMC grains at the interface were the main reasons for the rapid formation of Ni₃Sn₄ micro-solder joints.（3）In the TLP reaction,the growth rate of micro-solder joints Nano-Ni interface fine rod Ni₃Sn₄ is significantly faster than that of micron Ni interface block Ni₃Sn₄.After 15 min of reaction,the Ni₃Sn₄ micro-solder joints was formed,which had obvious hole defects,which was related to the fact that the Nano-Ni interface thin rod-like Ni₃Sn₄ dominated the IMC micro-solder joints formation.The Ni₃Sn₄ micro-solder joints formed under TG-TLP conditions has a shear strength of 69.3MPa,which shows good mechanical reliability.