Nucleation and grain refinement of tin alloys: the role of nucleant particles and solute
Creators: name: Shang, Hao
Publisher: name: Imperial College London
Year: 2017

description: This thesis develops the understanding of heterogeneous nucleation and grain refinement during the solidification of Pb-free solder alloys, and explores the potential of grain refining ball grid array (BGA)-scale solder joints by catalysing nucleation during solidification in the reflow process. A review of the literature revealed that numerous past studies have identified dilute alloying additions to Sn-Ag-Cu solders that suppress the nucleation undercooling, but no past study has explored in detail whether these additions can be used to grain refine solder alloys. Of the additions that suppress undercooling, the mechanisms of nucleation catalysis have been proved in past work only for Co, Pd and Pt alloying. In this thesis, the mechanisms of βSn heterogeneous nucleation in alloys containing Zn and Ti are explored. Zn additions are shown to introduce a variety of phases by reaction of the Zn with impurities such as Cu, Fe and the atmosphere. A lattice matching analysis shows that there is a reasonable lattice match between βSn and ZnO and it is suggested that ZnO is the heterogeneous nucleant in Zn-microalloyed solders. In Ti-microalloyed SAC305 solder, past research has found that Ti-microalloying introduces the Ti2Sn3 IMC (Intermetallic Compound) phase but, in this thesis, it has been shown that Ti-microalloying introduces two IMC phases: Ti2Sn3 with the V2GaSn2-structure type and (Ti,Cu,Fe)Sn2 with Mg2Ni-structure type, where the Fe is present as an impurity in commercial purity SAC305. Reproducible orientation relationships (ORs) have been measured between βSn and both primary Ti2Sn3 and (Ti,Cu,Fe)Sn2, although it is unclear whether these ORs are due to nucleation or particle pushing and engulfment. The potential of grain refining solder alloys by combining nucleant particles with solute is then explored in both large (60g) samples and 500μm solder balls. For large samples, Sn-based solders were grain refined effectively with this approach. Solute played a strong role in grain refinement and the grain size was approximately inversely proportional to the growth restriction factor, similar to past work on Mg-, Al-, and Ti-based casting alloys. For 500μm BGA balls, the key factors affecting the number of nucleation events during solidification are shown to be: (i) the addition of nucleant particles such as PtSn4 or αCoSn3, (ii) the addition of solute that generates a high growth restriction factor, and (iii) a high cooling rate. The most nucleation events were triggered when combining (i), (ii) and (iii) and up to ~12 independent βSn orientations formed in 500μm BGA balls compared with 1 independent grain in typical solder alloys. Thus, the approaches used in this thesis can grain refine solder balls, however, even if these approaches are further optimised and better nucleants are develop. At this stage it appears they are unlikely to be able to generate the hundreds of different βSn orientations in a BGA solder ball that would be required to give a near-isotropic joint.