Study of nanoparticle doped lead-free solder alloys for electronic packaging applications

Abstract

The advancement towards enhanced mobility, portability and greater performance in micro/nano-electronic devices has led a constant downsizing in interconnection bump size, indirectly contributing to a significant rise in heat and current density. It is very important to understand the reaction phenomena at the interfaces of solder joints, since these reactions are very crucial from the manufacturing point-of-view. Increased current density and the Joule’s effect of heating in these micro/nano-electronic interconnects pose a serious threat to the reliability of the electronic devices. Also, electromigration (EM) in solder joints has become one of the most discussed reliability issue in the electronic packaging industry, due to continuous miniaturization of interconnects and never ending pursuit for high performance devices. Lead (Pb)-based solders have been widely replaced by the Pb-free solders in the electronic industry, in the recent years. The era of Pb-free solders is continuously pushing the researchers towards investigating the reliability for practical applications. This dissertation is aimed at investigating the interfacial reactions at the solder joints under multiple reflow, aging, EM behaviour and failures in nanoparticles doped Pb-free composite solder joints due to atomic transport. In the first study, different weight percentages of antimony (Sb) nanoparticles (100-120 nm) were added to Sn-9Zn eutectic solder alloy to investigate the effect of third element addition on the microstructure, mechanical properties as well as thermal behaviour of the newly developed composite solder alloys. The results indicate that the Sb nano-particle based intermetallic compounds (IMC) were found uniformly distributed, refined the microstructure and formed IMC particles in the eutectic solder alloy. After the addition of nano Sb particles in Sn-9Zn solder, fine α-Zn phase and Ɛ-Sb3Zn4 IMC particles were clearly observed in the β-Sn matrix. The Ɛ-Sb3Zn4 IMC particles were uniformly distributed in the β-Sn phase, which resulted in an increase in the tensile strength, due to the second phase dispersion strengthening mechanism. The average tensile strength and micro-hardness of the Sb doped Sn-9Zn solder alloys were consistently higher than the plain Sn-9Zn solder. The tensile strength and the microhardness increased with increasing Sb nano-particle content, up to 1.0 wt. % of Sb content, and then decreased beyond that threshold value. Consequently the percentage (%) elongation of the Sb nanoparticle doped Sn-9Zn solder decreased with increasing Sb nano-particle content, up to 1.0 wt. % of Sb content, and then increased beyond that threshold value. The doped solder alloy was then studied under isothermal aging at different temperatures with Cu Ball Grid Array (BGA) substrates and the shear strength of the joints was characterized. The addition of trace amounts of Sb nanoparticles to the lead free solder shows retardation effect on the IMC growth at lower aging temperatures. However, at higher temperatures the IMC growths were not retarded. Secondly, non-reacting and non-coarsening, nano-sized SrTiO3 particles has been added into Sn-3Ag-0.5Cu solder alloys and the interfacial microstructure and shear strength on Au/Ni metalized Cu pads ball grid array substrates were investigated as a function of the number of reflow cycles and aging time. In Digital Scanning Calorie meter (DSC) analysis. the melting point of SrTiO3 containing composite solders was slightly higher due to change in surface stability of the eutectic solders and the physical properties of the grain boundary/interfacial characteristics. At their interfaces, a scallop-shaped ternary Sn-Ni-Cu intermetallic compound layer was found in both plain Sn-Ag-Cu solder joints and solder joints containing SrTiO3 nano-particles, and the intermetallic compound layer thickness increased with the number of reflow cycles and aging time. After the addition of SrTiO3 nano-particles, a fine microstructure of AuSn4, Ag3Sn, Cu6Sn5 intermetallic compounds appeared in the β-Sn matrix, in the solder ball region. In addition, the shear strength of solder joints containing SrTiO3 nano-particles exhibited a consistently higher value than that of plain Sn-Ag-Cu solder joints due to a second phase dispersion strengthening mechanism as well as a refinement of the intermetallic compounds. The fracture surface of plain Sn-Ag-Cu solder joints exhibited a brittle fracture mode with a smooth surface while Sn-Ag-Cu solder joints containing SrTiO3 nano-particles showed ductile failure characteristics with rough dimpled surfaces. Trace amounts of nano diamond particles were added to Sn-3Ag-0.5Cu eutectic solder to investigate the effects of nano-particles on the formation, growth retardation and evolution of the IMC layer structure at the BGA pad/solder interfaces. The physical and mechanical properties of solder joints were also characterised. The IMC growth behaviour is comparatively slower than that of plain solder joint due to change in the diffusivity of the constituent atoms and the thermodynamic parameters of elemental affinity. The solder joints containing nano diamond particles consistently showed higher hardness and strength than that of plain Sn-Ag-Cu solder joints, after a numbers of reflow cycles. It is reasonable to suggest that the hardness of the bulk solder was enhanced by the addition of nano particles due to the homogeneous dispersion of nano particles and well refined and fine IMC phases, which acted as reinforcement on the solder matrix and as barrier to the movement of dislocations with increased dislocation densities. The fracture surface of plain solder exhibited a brittle fracture mode with a relatively smooth surface while doped solder joints showed typical ductile failures with very rough dimpled surfaces. Damping characteristics of the doped and plain solder joints were also studied to find out the vibration reliability of the solders under harsh condition. The damping capacity of the doped solder was better than the plain solder at a wide range of temperatures and the doped solder exhibited a less internal friction. Lastly, the study on the effect of addition of Ag nanoparticles on the EM influenced microstructure evolution in Sn-58Bi solders has been reported. Doped-Sn-58Bi solders were prepared by mechanically dispersing Ag nanoparticles additive in Sn-58Bi solders. The interfacial morphologies of the plain solder and doped Sn-58Bi solders under a direct current (DC) of 2.5 A (5x103 A/cm2) at 75°C temperature with Cu pads on daisy chain type ball grid array (BGA) substrates for up to 20 days, were analyzed. Unlike the plain solder, there is no obvious formation of Bi-rich IMC growth on the anode side in the doped solder alloys. The Cu-Sn IMC phases were formed in the Cu, near the cathode and anode interface after the first-reflow. Ag3Sn particles were found dispersed in the solder matrix and it behaved like barriers, blocking the movement of metal atoms during current stressing. In addition, during the shear testing of FC package, fracture occurred at the IMC interfacial region with a ductile fracture mode. In the solder ball region β-Sn matrix of Sn-58Bi solder joints with a refined microstructure and Ag based inter-metallic compound particles were observed, which resulted in an increase in the reliability and strength, due to a second phase dispersion strengthening mechanism.

Date of Award	16 Jul 2012
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yan Cheong CHAN (Supervisor)