**2. Lead-free solder systems**

Although several commercial and experimental Pb-free solder alloys are available as replacements for Sn-Pb solders, the following families of solders are of particular interest and are the prevailing choices of industry [4]: eutectic Sn-Ag, eutectic Sn-Cu, eutectic Sn-Zn, eutectic Bi-Sn, and Sn–In, as shown in Table 1. Since the properties of the binary Pb-free solders cannot fully meet the requirements for applications in electronic packaging, additional alloying elements are added to improve the performance of these alloys. Thus, ternary and even quaternary Pb-free solders have been developed [5-7], such as Sn-Ag-Cu, Sn-Ag-Bi, and Sn-Zn-Bi solder. However, the knowledge base on Sn-Pb solders gained by experience is not directly applicable to lead-free solders. In other words, the reliability of Pbfree solder joints in consumer products is attracting more interest and concern from both academia and technologists[8-10].

Corrosion Resistance of Pb-Free and

Novel Nano-Composite Solders in Electronic Packaging 109

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]


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