**3. Nano-composite solders**

As electronic devices continue to become lighter and thinner, they require much smaller solder joints and fine-pitch interconnections for microelectronic packaging. For example, portable electronic devices, such as portable computers and mobile phones, have become thinner and smaller while adding more complicated functions. The miniaturization of these electronic devices demands better solder-joint reliability. Hence, in all chip connection and ball grid array (BGA) technologies, solder interconnection through flip-chip assembly has been proven to offer the highest density of input/output (I/O) connections in a limited space. To meet the insatiable appetite for ever-finer I/O pitches and ever-higher I/O densities, C4 (controlled collapse chip connection) technology was developed by IBM in the mid 1960s, and this technology was applied to future microelectronic packaging. According to the International Technology Roadmap for Semiconductors (ITRS), the pad pitch may fall below 20 μm by the year 2016 [11]. In some flip chip packages, solder balls of 20μm in size are used to connect the pads on the chip and the print circuit board (Fig. 1). Furthermore, Thru-Silicon-Via (TSV) technologies are also lurking on the horizon as the next-generation higher-density chip connection technology, and they also require fine-pitch Pb-free solder interconnections.

The conventional solder technology may not guarantee the required performance at such pitches due to characteristics such as higher diffusivity and softening [12]. In order to solve these problems, efforts have been made to develop new Pb-free solders with a low melting point, good mechanical properties, better microstructure properties, and high creep resistance. Recently, Pb-free solders doped with nano-sized, nonreacting, noncoarsening oxide dispersoids have been identified as potential materials that could provide higher microstructure stability and better mechanical properties than the conventional solders [13- 24]. Tsao et al. [14-16] studied the influence of reinforcing TiO2 and Al2O3 nanoparticles on microstructural development and hardness of eutectic Sn-Ag-Cu solders. In their work, microhardness measurements revealed that the addition of TiO2 and Al2O3 nanoparticles is helpful in enhancing the overall strength of the eutectic solder. Shen et al. [17] controlled the formation of bulk Ag3Sn plate in Sn-Ag-Cu solder by adding ZrO2 nanoparticles to reduce the amount of undercooling during solidification and thereby suppress the growth of bulk Ag3Sn plates. Zhong and Gupta [18] successfully prepared a nano-Al2O3 reinforced nano-

**All system Eutectic composition ( wt.%) Melting point or range (C)** 

Table 1. Data showing the enhancement of the mechanical properties of Pb-free solders[9, 10].

As electronic devices continue to become lighter and thinner, they require much smaller solder joints and fine-pitch interconnections for microelectronic packaging. For example, portable electronic devices, such as portable computers and mobile phones, have become thinner and smaller while adding more complicated functions. The miniaturization of these electronic devices demands better solder-joint reliability. Hence, in all chip connection and ball grid array (BGA) technologies, solder interconnection through flip-chip assembly has been proven to offer the highest density of input/output (I/O) connections in a limited space. To meet the insatiable appetite for ever-finer I/O pitches and ever-higher I/O densities, C4 (controlled collapse chip connection) technology was developed by IBM in the mid 1960s, and this technology was applied to future microelectronic packaging. According to the International Technology Roadmap for Semiconductors (ITRS), the pad pitch may fall below 20 μm by the year 2016 [11]. In some flip chip packages, solder balls of 20μm in size are used to connect the pads on the chip and the print circuit board (Fig. 1). Furthermore, Thru-Silicon-Via (TSV) technologies are also lurking on the horizon as the next-generation higher-density chip connection technology, and they also require fine-pitch Pb-free solder

The conventional solder technology may not guarantee the required performance at such pitches due to characteristics such as higher diffusivity and softening [12]. In order to solve these problems, efforts have been made to develop new Pb-free solders with a low melting point, good mechanical properties, better microstructure properties, and high creep resistance. Recently, Pb-free solders doped with nano-sized, nonreacting, noncoarsening oxide dispersoids have been identified as potential materials that could provide higher microstructure stability and better mechanical properties than the conventional solders [13- 24]. Tsao et al. [14-16] studied the influence of reinforcing TiO2 and Al2O3 nanoparticles on microstructural development and hardness of eutectic Sn-Ag-Cu solders. In their work, microhardness measurements revealed that the addition of TiO2 and Al2O3 nanoparticles is helpful in enhancing the overall strength of the eutectic solder. Shen et al. [17] controlled the formation of bulk Ag3Sn plate in Sn-Ag-Cu solder by adding ZrO2 nanoparticles to reduce the amount of undercooling during solidification and thereby suppress the growth of bulk Ag3Sn plates. Zhong and Gupta [18] successfully prepared a nano-Al2O3 reinforced nano-

**3. Nano-composite solders** 

interconnections.

Sn-In Sn52In 118(e) Sn-Bi Sn58Bi 138(e) Sn-Zn Sn9Zn 198.5(e) Sn-Ag Sn3.5Ag 221(e) Sn-Cu Sn0.7Cu 227(e) Sn-Ag-Bi Sn3.5Ag3Bi 206-213 Sn-Ag-Cu Sn3.8Ag0.7Cu 217(e) Sn3.50.5Cu 218 Sn-Zn-Bi Sn8Zn3Bi 189-199 composite solder by mechanically intermixing nano-Al2O3 particles into Sn0.7Cu Pb-free solder, and this composite solder shows improved mechanical properties. The best tensile strength realized for the composite, which contains 1.5 wt.% alumina, far exceeds the strength of the eutectic Sn–Pb solder. Many authors have studied the effect of adding singlewalled carbon nanotubes [19] or multi-walled carbon nanotubes [20, 21] on the mechanical properties of nano-composite solders. The data on the enhancement of the mechanical properties of nano-composite solders collected from some of the literature are listed in Table 2 [13, 14, 16, 22, 23]. Here, it should be stressed that although the addition of nanoparticles into solder matrices can improve the creep behavior[24], the effects on the corrosion resistance and mechanical properties of the nano-composite solders cannot be ignored.

Fig. 1. Micro bump and pillar bump structures for highly reliable chip-to-substrate interconnects: (a) SnAg microbump (20 μm diameter), and (b) Cu pillarbump (height: 80 μm) [11]

Corrosion Resistance of Pb-Free and

Novel Nano-Composite Solders in Electronic Packaging 111

Fig. 2. Top view of the IMC at the interfaces of the nano-composite solder joints on Cu

The diversity of materials, drive toward miniaturization, and globalization have significantly contributed to the corrosion of microelectronic devices [37]. However, the key point is that solder joints are often exposed to corrosive environments that can accelerate the corrosion process. Although corrosion resistance is an important parameter in choosing solder alloys, the corrosion behavior of Sn-Pb solder joints was rarely of interest because the oxide that forms on the tin-lead alloy is relatively stable. Mori et al. showed that both Pbrich and Sn-rich phases dissolve when the Sn-Pb solder alloy is immersed in corrosive solution, and the corrosion rate is slower than that of the Sn-Ag solder [38, 39]. Compared to traditional Sn-Pb solders, Sn-Ag-Cu solders are easily corroded in corrosive environments due to their special structures (as shown in Fig. 3). The presence of Ag3Sn in Sn-Ag-Cu solders accelerates the dissolution of tin from the solder matrix into a corrosive medium

substrate after aging for 7 days at 175oC: (a) SAC and SAC- TiO2 [31].

**5. Corrosion behavior of Pb-free solder joints** 


Table 2. The data showing the enhancement of the mechanical properties of nano-composite solders[13, 14, 16, 22, 23].
