β-Sn grain orientations in Pb-free solders and their effect on the reliability of solder joints

Abstract

Eutectic Sn-Pb solders have been used the most notably to attach discrete components to PCBs. However, due to an increased awareness of the potential health hazards associated with the toxicity of lead (Pb) in humans, which can lead to disorders of the nervous and reproductive systems, and also affect neurological and physical development, actions have been taken to eliminates or reduce the use of Pb in a variety of products. Pb-free solders have been extensively developed to replace Pb-containing solders in microelectronic packaging applications. The majority of Pb-free solders are Sn-based alloys with minor alloying elements of Ag, Cu, Co, In, Ni, Zn and others. The most popular Pb-free solders include near eutectic Sn-Ag-Cu, Sn-Ag and Sn-Cu alloys. Although the near-ternary eutectic Sn-Ag-Cu solder is the leading candidate in printed circuit assembly, the reliability of the solder system has been challenged by several technical issues, particularly in chip-level interconnect applications, namely, flip-chip solder joints. Difficulty of electroplating a ternary solder composition, formation of large intermetallic phases during reflow, high modulus or stiff solder joints are just a few of the concerns. In order to overcome concerns of near-eutectic Sn-Ag-Cu, Sn-Ag or Sn-Cu systems, a low Ag and/or Cu content are proposed to produce low modulus or more compliant flip-chip interconnections. Because most Pb-free solders have a composition of more than 90% in Sn, the physical/chemical/electrical properties of Pb-free solders are predominantly affected by the properties of the $\beta$-Sn crystal. The properties of Sn are highly anisotropic owing to the unique crystal structure of $\beta$-Sn, with a lattice structure of body-centered tetragonal (bct). The coefficient of thermal expansion and Young’s modulus of $\beta$-Sn along c-axis is about two or three times larger than those along the a- or b-axis. And, the diffusivity of solute atoms in Sn matrix is very di...