Process dependence and nucleus models of β-Sn grains in SAC305 freestanding solder balls and BGA solder joints
Graphical abstract
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.
Section snippets
Experiment
Commercial SAC305 solder balls with diameters of 100 μm, 200 μm, 400 μm, 700 μm and 1200 μm were used. Fig. 1(a) and (b) show the schematics of SAC305 freestanding solder balls and SAC305/Cu BGA solder joints, respectively. The commercial SAC305 solder balls were coated with flux and separately placed on non-wetting Al substrates and printed circuit boards (PCBs), respectively. Then, solder balls and substrates were placed in a differential scanning calorimetry (DSC) together, heated up at a
Effects of cooling rate and solder ball size on the β-Sn grain morphology of the SAC305 freestanding solder balls and SAC305/Cu BGA solder joints
Fig. 3 shows the IPF-Z orientation maps of SAC305 freestanding solder balls at different cooling rates. It should be noted that although eight solder balls were examined for each combination of cooling rate and solder ball size, only one typical image was presented due to the space limitation. As shown in Fig. 3, though there were twenty combinations, only three dominant types of β-Sn morphology were found in the freestanding solder balls, i.e., interlaced & beach ball-like grains, multiple
Nucleus models of β-Sn in the freestanding solder balls and BGA solder joints
Though three types of β-Sn grain morphology were found in the freestanding solder balls and BGA solder joints, only one nucleation event occurred in each solder ball or joint, indicating that the liquid Sn should nucleate by different models during solidification. Lehman et al. (2010) reported that three β-Sn grains with twin relationship, i.e., beach ball grains, would formed in SAC305 solder when the Sn dendrites grew following the {101} nucleus model. In addition, Ren et al. (2021)
Conclusion
The effects of cooling rate and solder ball size on the morphology and orientation of β-Sn in SAC305 freestanding solder balls and SAC305/Cu BGA solder joints were investigated and the main conclusions were drawn as follows:
- (1)
All the SAC305 freestanding solder balls and SAC305/Cu solder joints were presented with three types of morphologies, i.e., interlaced & beach ball-like grains, multiple twin grains and single grain. Further, each of the solder ball/joint was composed of one of these three
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also form part of an ongoing study.
CRediT authorship contribution statement
X.L. Ren: Validation, Investigation, Methodology, Formal analysis, Writing - original draft. Y.P. Wang: Writing - review & editing, Investigation, Funding acquisition. X.Y. Liu: Data curation, Investigation, Validation. L.J. Zou: Supervision, Data curation, Resources. N. Zhao: Conceptualization, Investigation, Funding acquisition, Writing - original draft.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (grant number 52075072) and the Fundamental Research Funds for the Central Universities (grant number DUT20JC46).
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