Preparation And Properties Of High-temerature Resistant Bondline Based On Cu@Sn Core-shell Solder Paste

In recent years,as electronic devices are frequently applied in high-power,high-frequency,and high-temperature environments,whether they can work stably at high-temperature for a long time has become an urgent problem to be solved.With outstanding performance based on third-generation semiconductor materials,Si C and Ga N are available in high temperature and harsh environments.In order to fully exploit their advantages,it is imperative to develop new lead-free high-temperature solders to ensure that the interconnect structure between the chip and the substrate can resist higher working temperatures.Based on transient liquid phase bonding?TLP?,we proposed to apply the core-shell structure to the TLP process,because the core-shell material increases the effective reaction area between the metals,accelerates the soldering process.Cu was adopted due to its high electrical and thermal conductivity,and Sn was used as the most common soldering material in electronic packaging,so that the materials can be bonded by reflow.The Cu-Sn intermetallic compound formed by solder has a high melting point and meets the requirements of "Interconnecting at low temperature but operating at high temperature".In the thesis,we optimized the existing powder preparation process of the Cu@Sn core-shell structure firstly,and conducted detailed experimental research on various important parameters in the process.Finally,a uniform Sn layer with a thickness of 1 ?m was coated on the outer layer of 5 ?m Cu core.To better adapt to actual production,we combined Cu@Sn core-shell structure powder with various paste flux to prepare solder paste,which was coated on Cu substrate to form "Sandwich" structure between the substrate and the joints.In this experiment,the paste flux of the model MK-504 L was mixed with the powder at a mass ratio of 5:2,and the joints quality was the best when the coating thickness on the substrate was 0.2 mm.Moreover,higher temperature leads to the faster formation of the Cu-Sn IMC inside the joints,and joints under the greater pressure condition could have fewer voids.In order to improve the wetting and spreading ability of the joints,the mass ratio of Sn powder to Cu@Sn powder was 3:5,the shear strength reaches 24.8 MPa and fracture of the joints was categorized into two types: brittle fracture and plastic fracture?the fracture shape of the overall joints was like "Z"?.In the aging test for the Cu@Sn core-shell solder paste joints,the Cu₆Sn₅ inside the joints was gradually converted to Cu₃ Sn with aging.When the aging time was 10 days,the Cu₆Sn₅ inside the joints was completely converted into Cu₃ Sn.On the 5th day,the shear strength reached as the highest as 28.1 MPa and the high temperature shear strength was 25.2 MPa at 300 ?.The Vickers hardness was up to 124.4 HV.Analysis of joints fracture appearance and EDX indicated that the fracture mode was still like “Z”,the resistivity was 6.87 ??·cm and the thermal conductivity was 109.67 Wm^-1K^-1.Compared with the thermal conductivity of the traditional interconnect materials for packaging,the conductivity and thermal conductivity of the Cu@Sn core-shell solder paste prepared by the optimized process in this experiment are significantly improved.