Ag-Pd纳米合金低温连接及其抗电化学迁移性能

Low-Temperature Bonding of Ag-Pd Nanoalloy and Its Resistance to Electrochemical-Migration

纳米银焊膏能够实现低温连接、高温服役,同时具有优异的导电、导热性能,其缺点是非常容易发生电化学迁移。本文采用脉冲激光沉积成功制备了完全互溶的Ag-10%Pd纳米合金,并将其用于SiC芯片的封装互连,旨在提高纳米颗粒烧结层的抗电化学迁移能力,同时保持纳米颗粒的低温烧结特性。研究表明,采用Ag-10%Pd纳米合金烧结连接SiC芯片及镀银的直接覆铜基板(DBC),在250℃的温度下可以实现剪切强度为21.89 MPa的接头,达到了美国军标MIL-STD-883K的要求(7.8 MPa)。Ag-Pd纳米合金抗电化学迁移能力是同等条件下纯银的4.3倍。在Ag-Pd纳米合金中,银离子的析出受到PdO的阻碍,迁移产物呈云雾状分布,有效延长了电极的短路时间。脉冲激光沉积Ag-Pd纳米合金的烧结避免了传统银、钯颗粒直接混合方法后续高达850℃的合金化过程。Ag-Pd纳米合金作为封装互连材料是实现低温连接的有效保证,并有望为功率电子器件的高可靠性封装提供解决方案。

Objective There is an increasing demand for die attach materials with the rapid development of SiC devices,which can be bonded at low-temperature and function at high temperature.Nano-Ag sintering has been extensively investigated for application in high-temperature power electronics.However,the electrochemical-migration of Ag ions is the main drawback.Pd is famous for its chemical stability,and various studies have focused on the influence of Pd content on the effectiveness and its mechanism.Recently,researchers have been trying to mix Pd and Ag nanoparticles(NPs)to improve the resistance to electrochemical-migration of the sintered layer.However,Pd has a melting point higher than that of Ag,whereas the alloying process needs high temperature(~850℃)to form Ag-Pd alloy.Pulsed laser deposition(PLD)is a physical method feasible for fabricationg Ag-Pd nanoalloy without using organic additives such as polyvinylpyrrolidone,which is required in the chemical method.In this work,Ag-10%Pd nanoalloy was fabricated by the PLD method,which can be used to connect SiC and Ag-coated direct bonding copper(DBC)substrates.The sintered layer enhances resistance to electrochemical-migration with low-temperature bonding characteristics.The microstructure of the bonding,shear properties,and its electrochemical-migration resistance are studied.Methods Ag-10%Pd NPs were fabricated using PLD with a pressure of 750 Pa of Ar atmosphere.The Ag-Pd target was fabricated by powder sintering with weight ratio of 90∶10.A picosecond laser with a pulse width of 10 ps was employed to ablate the target.Ag-Pd NPs were deposited on the back side of SiC chip(G.P.Tech,Ti/Ni/Ag metallization),then the SiC chip was removed from the substrate and placed on the Ag-coated DBC(HuaSemi Electronics,Ni/Au metallization).The interconnecting process is performed at a temperature range of 200℃-350℃assisted with a pressure of 5 MPa for 30 min in air.The shear test is conducted using Dage 4000.The electrochemical-migration test is conducted using a water drop test.Results and Discussions The microstructure of as-deposited Ag-Pd film comprises various NPs with diameters less than 1μm(Fig.3).Element results indicate that these deposited NPs are in alloy state with a uniform composition distribution.The sintered joint comprises SiC chip,bondline and Ag-coated substrates(Fig.4).The bondline thickness is about 27μm,which is only 31.6%of the as-deposited state.Thus,the Ag-Pd film had excellent deformability.The bondline exhibited Ag-9.57%Pd alloy microstructure without obvious element segregation.The sintered joint achieved a shear strength of 21.89 MPa at the sintering temperature of 250℃,which is higher than the US military standard MIL-STD-883K(7.8 MPa).Therefore,Ag-Pd nanoalloy film can be used as die attach material for low-temperature bonding.The sintering temperature provides the driving force for sintering process,as a denser bondline is achieved when the temperature is increased to 300℃(Fig.6).Fracture surface reveals that the failure mainly occurred at the bondline,indicating that high bonding quality interface is realized(Fig.7).Compared with pure Ag,Ag-Pd nanoalloy exhibited a more than quadruple resistance to electrochemical-migration during the water drop test(Fig.8).For pure Ag electrode,the current reached 1 mA with only 81.4 s,while the Ag-Pd electrode required 349.7 s for the short-circuit process.The dissolution of Ag ion was blocked by PdO formation on the anode,which played a paramount role in extending the short-circuit time,whereas the migration product was cloud-like instead of dendritic growth.This work proposed a method for fabricating Ag-Pd nanoalloy films as die attach material without the high alloying temperature.It should be noted that,Pd has a higher melting point(1554℃)than Ag(961.7℃),and Ag-Pd nanoalloy sintering requires higher sintering temperature than pure Ag NPs.Moreover,adding Pd is costly.Consequently,the sintering temperature,demand of electrochemical-migration resistance and its cost should be balanced when applying Ag-Pd nanoalloy in electronic packaging.Conclusions Ag-10%Pd nanoalloy was successfully fabricated as die attach material using PLD.The sintered joint achieved a shear strength of 21.89 MPa at the sintering temperature of 250℃,which was higher than the US military standard MIL-STD-883K(7.8 MPa).Compared with pure Ag,Ag-Pd nanoalloy exhibited a more than quadruple electrochemical-migration resistance.The dissolution of Ag ion was blocked by PdO formation on the anode with obviously extended short-circuit time,whereas the migration product was cloud-like.Compared with conventional direct sintering of Ag and Pb nanoparticles,pulsed laser deposited Ag-Pd nanoalloy sintering avoids high-temperature alloying process(850℃),which is promising for Ag-Pd low-temperature bonding and is expected to provide a solution for the high-reliability power electronic packaging.