Effect of additions of ZrO₂ nano-particles on the microstructure and shear strength of Sn–Ag–Cu solder on Au/Ni metallized Cu pads

Abstract

Nano-sized, nonreacting, noncoarsening ZrO₂ particles reinforced Sn–3.0 wt%Ag–0.5 wt%Cu composite solders were prepared by mechanically dispersing ZrO₂ nano-particles into Sn–Ag–Cu solder. The interfacial morphology of unreinforced Sn–Ag–Cu solder and solder joints containing ZrO₂ nano-particles with Au/Ni metallized Cu pads on ball grid array (BGA) substrates and the distribution of reinforcing particles were characterized metallographically. At their interfaces, a Sn–Ni–Cu intermetallic compound (IMC) layer was found in both unreinforced Sn–Ag–Cu and Sn–Ag–Cu solder joints containing ZrO₂ nano-particles and the IMC layer thickness increased with the number of reflow cycles. In the solder ball region, AuSn₄, Ag₃Sn, Cu₆Sn₅ IMC particles and ZrO₂ nano-particles were found to be uniformly distributed in the β-Sn matrix of Sn–Ag–Cu solder joints containing ZrO₂ nano-particles, which resulted in an increase in the shear strength, due to a second phase dispersion strengthening mechanism. The fracture surface of unreinforced Sn–Ag–Cu solder joints exhibited a brittle fracture mode with a smooth surface while Sn–Ag–Cu solder joints containing ZrO₂ nano-particles ductile failure characteristics with rough dimpled surfaces.

Introduction

Environmental and health concerns with lead and lead-containing compounds in microelectronic devices attract more and more attentions in academia and industry [1], [2], [3], [4]. Rapid switching to lead-free solder has come to replace lead-based solders in the packaging process of electronic devices and components. The development of new solders and their composites is also driven by the continual miniaturization of integrated circuits and the quest for better performance and reliability from interconnection joints [5]. Now, several types of Sn-based lead-free solders such as Sn–Ag, Sn–Cu, Sn–Au, Sn–Ag–Cu and Sn–Zn have been developed and applied in the electronic packaging industry [6], [7], [8]. Among them the Sn–Ag–Cu solder has been proposed as one of the most promising substitutes for lead-containing solder because of its good basket of properties such as superior solderability as well as good compatibility with current components, and is regarded as the most promising substitute for conventional Sn–Pb solder [9], [10], [11], [12].

Microelectronic components evolved to become smaller, lighter and more functional. There are also strict performance requirements for solder materials. In general, it must fulfill the expected level of electrical and mechanical performance and have a low melting temperature. Reliability of the solder joints is mainly dependent on the yield strength, elastic modulus, shear strength, fatigue and creep behavior [13], [14]. Studies have shown that a potentially viable and economically affordable approach to improve the mechanical properties of a solder is to add appropriate second phase particles, of a ceramic, metallic or intermetallic, to a solder matrix so as to form a composite [15], [16], [17]. The foreign dispersoid second phase particles which are induced as a reinforcement with in the solder matrix, should not coarsen easily. In addition to strengthening the solder against creep deformation, the dispersed particles can serve as obstacles to grain growth and coarsening of the solder microstructure. Mavoori and Jin used nano-sized TiO₂ and Al₂O₃ particles as reinforcements for a conventional Sn–Pb solder and reported significant improvements of creep and mechanical properties [18]. Gao et al. [19], [20] studied the enhanced creep resistance of a Sn3.5Ag solder by introducing micro-sized Ag, Ni, or Cu particles. It was found that solder joints reinforced with Ni particles were about five times more creep resistant than composite solder joints reinforced by Cu-particles, and about 30 times more creep resistant than the plain Sn3.5Ag solder joints and those reinforced with Ag-particles. Shen et al. [21] studied eutectic Sn–Ag solder alloys with nano-sized ZrO₂ reinforcement particles and significantly improved hardness as well as refined Ag₃Sn IMC particles. Mohan et al. [22] reported that a Sn–3.8Ag–0.7Cu composite solder reinforced with single wall carbon nano tubes (SWCNTs) had significantly improved hardness and ultimate tensile strength. In the current study, ZrO₂ particles of a nanometer size were used. The main advantage of ZrO₂ nano-particles are; (a) a similar density to Sn–Ag–Cu, ρ (Sn–3.0Ag–0.5Cu) = 7.11 and ρ (ZrO₂) = 5.83 g/cm³ as compared to other ceramic particles such as ρ (Al₂O₃) = 3.97 g/cm³, ρ (SWCNT) = 1.3 g/cm³, and (b) a higher hardness as compared to a Sn–3.0Ag–0.5Cu matrix.

However, the result of a literature search revealed that no studies have been reported so far on lead-free Sn–Ag–Cu solder joints containing ZrO₂ nano-particles. Accordingly, the aim of the present study is to synthesize a Sn–Ag–Cu solder with different percentages of ZrO₂ nano-particles. Unreinforced Sn–Ag–Cu solder joints and solder joints containing ZrO₂ nano-particles were characterized in terms of interfacial microstructures and shear strengths on Au/Ni metallized Cu pads on BGA substrates as a function of the number of reflow cycles and the content of ZrO₂ nano-particles.