Process dependence and nucleus models of β-Sn grains in SAC305 freestanding solder balls and BGA solder joints

Highlights

• Relationship between β-Sn grain morphology and process parameters was studied systematically.

• Two nucleus models were proposed to explain the formation of β-Sn morphologies.

• Relationship between spatial position of nucleus models and β-Sn orientation was established.

• Large θ angle β-Sn grains were theoretically and experimentally proved to have a formation probability higher than 77 %.

Abstract

Due to the anisotropy characteristic of β-Sn grain, the microstructure and orientation of Sn-based solder joints are closely related to the reliability of the solder joints. In this paper, the effects of cooling rate and solder ball size on β-Sn morphology and orientation in Sn-3.0Ag-0.5Cu (SAC305) freestanding solder balls and SAC305/Cu ball grid array (BGA) solder joints were firstly investigated to reveal the process dependence of β-Sn grain features. Three dominant solidified morphologies, i.e., interlaced & beach ball-like grains with three orientations, multiple twin grains with three orientations and single grain with one orientation, were found in the solder balls/joints. Solder ball size showed more significant impact than cooling rate on the morphologies that interlaced & beach ball-like grains and multiple grains were mainly presented in small sized solder balls/joints (≤ 400 μm) and single grain was mainly presented in large sized solder balls/joints (≥ 700 μm). These three solidified morphologies were nucleated and developed from two nucleus model. That is, the interlaced & beach ball-like grains and multiple twin grains morphologies were attributed to {101} nucleus model, while the single grain morphology was attributed to single grain nucleus model. Regardless of whether the solder balls/joints nucleated and solidified based on the {101} or single grain nucleus model, large θ angle β-Sn grains were proved to have a formation probability higher than 77 % both theoretically and experimentally, and this formation probability had little relationship with interfacial Cu₆Sn₅ grains, cooling rate and solder ball size.

Introduction

Sn-based solders are widely used as interconnection material for electronic packing and welding material due to their low melting point. Kotadia et al. (2014) pointed out that lead (Pb) was usually used in solder alloys for electronic packaging, but the usage of Pb has been banned in commercial electronics because of its toxicity. At present, most Sn-based lead-free solders have been widely used as substitutes, such as Sn-0.7Cu, Sn-3.5Ag and Sn-xAg-yCu (SAC). Among these lead-free solders, SAC solders have been most widely used for advanced packaging technologies because of their good overall performance. The research of SAC solders mainly focuses on improving the microstructure and mechanical properties. Skwarek et al. (2020) found that when adding TiO₂ nanoparticles into Sn-3.0Ag-0.7Cu (SAC307) solder, the TiO₂ nanoparticles could incorporate at the Sn grain boundaries which caused considerable refinement of the Sn grain structure. Skwarek et al. (2021) reported that ZnO nanoparticles could refine the Sn, Ag₃Sn and Cu₆Sn₅ grains in SAC307-xZnO solder joints. In addition, Wang et al. (2020) found that adding Ni modified multi-walled carbon nanotubes (Ni-CNTs) into Sn-3.0Ag-0.5Cu (SAC305) solder joints could significantly inhibit the coarsening of intermetallic compound (IMC) grains and reinforce the shear strength of the solder joints. Besides the success in improving the microstructure and properties of SAC solders by composition design, the efforts on process design also play an important role.

In general, the morphology and orientation of β-Sn grains are closely related to the degree of undercooling, ΔT. Powell et al. (1977) demonstrated that the growth rate of β-Sn dendrites was proportional to ΔT² in pure Sn, and it could reach ∼100 cm/s under a high undercooling as reported by Kobayashi et al. (1984). In addition, O Hara (1967) reported the growth orientation of β-Sn dendrites in pure Sn would transform from [110] to other orientations when the solidification temperature changed. Mullis et al. (2004) also found similar phenomenon in Cu-xSn alloys, where the preferred dendrite growth orientation changed from <100> to <111> as the undercooling below 90 K. Meanwhile, the undercooling for β-Sn nucleation is sensitive solder joint size and cooling rate, as reported by Zhou et al. (2011) and Zhao et al. (2013). Therefore, the morphology and orientation of β-Sn grains in Sn-based solders could be strongly affected by the solidification process.

The orientation of β-Sn grains within a solder joint has a significant effect on the reliability of the solder joint. Han and Guo (2019) reported that as the crystal structure of β-Sn is body-centered tetragonal, the solder joints with a few number of β-Sn grains will exhibit significant anisotropy in physical and mechanical properties. Huang et al. (2015) found that when the c-axis of β-Sn grain was parallel to the electron flow direction, excessive dissolution of cathode Cu occurred due to the large diffusivity of Cu along the c-axis. Chen et al. (2013) reported that the deformation and cracking of solder interconnects had a close relationship with the unique characteristics of grain orientation during thermal cycling testing. That is, the reliability of different Sn-based solder joints under same service condition may vary greatly regarding β-Sn grain orientation. Therefore, controlling the microstructure of Sn-based solders to improve the reliability of solder joints is very necessary. Many studies on the microstructure of SAC solder after different solidification processes have revealed that there are three dominant types of β-Sn morphology in SAC freestanding solder balls and ball grid array (BGA) solder joints, i.e., interlaced & beach ball-like grains, beach ball-like grains and single grain, as reported by Ma et al. (2019), Chen et al. (2014) and Han et al. (2017). However, no matter which β-Sn morphology is formed, the SAC BGA solder joints usually consist of a few orientated β-Sn grains or even only one β-Sn grain. In addition, Shang et al. (2017) found that there were no more than three β-Sn grains in SAC305 freestanding solder balls, which was also verified by Ma et al. (2016). Considering the influence of solidification process, it is necessary to investigate the effect of cooling rate on both microstructure and β-Sn grain orientation in solder joints of different sizes.

In this study, the effects of cooling rate and solder ball size on β-Sn grain microstructure and orientation in SAC305 freestanding solder balls and SAC305/Cu BGA solder joints were firstly investigated to reveal the process dependence of β-Sn grain features. This work was conducted to (i) establish the relationships between process conditions and the solidified microstructure and β-Sn grain orientation of SAC305 freestanding solder balls and SAC305/Cu BGA solder joints, and (ii) explore the mechanism of the formation and change law of β-Sn grain orientation.