Study of Nucleation and Solidification of Sn-based Lead-Free Solders

The aims of the project are to study and understand the nucleation of BSn in lead free solders and the influence on the nucleation undercooling of reaction layers and phases present in solder joints. Microstructures of solders will also be studied with the aim of developing an understanding of the influence of BSn undercooling on the resulting solidification microstructures in solder joints.

Nucleation of BSn in lead-free solder joints commonly exhibits large undercooling of the solid from liquid. Undercooling values of as large as 80K can be obtained in high purity samples, whereas adding certain alloying elements like Ti, Ni, and Co can reduce the nucleation undercooling of BSn to around 10-5K. The nucleation undercooling of BSn, due to the small size of each solder joint, has huge influence on the final properties of the joint, and the solidification microstructure of them. Better in-service lifetimes can be linked to the prevalence of certain microstructures that are determined by the nucleation undercooling of BSn.
The approach used here is to combine differential scanning calorimetry (DSC) with novel analytical microscopy techniques (3D imaging of microstructure linked with crystallographic measurements) to extract new insights into microstructure formation in solder joints. The research has the potential to improve the reliability of electronics through controlling solder joint microstructures.

The project aims to answer the following questions:

-Nucleation of BSn in the presence of Ni3Sn4 intermetallic compound (IMC)

Ni3Sn4 is a common IMC present as a reaction layer between Sn and Ni substrates, its effect on nucleation undercooling is known, however the mechanism of BSn nucleation on Ni3Sn4 IMC particles is not known. Electron Backscatter Diffraction (EBSD) study of BSn solidified on Ni joints, and Ni3Sn4 primary IMC crystals will reveal if this nucleation catalysis can be explained crystallorgaphically through an orientation relationship (OR), or any other mechanism.

Nucleation of BSn can be studied with the aid of a Differential Scanning Calorimeter (DSC) to observe how the undercooling changes with varying processing parameters, which will be linked to the morphology and features of the Ni3Sn4 reaction layer.

The above research will be used, along with past research on nucleation of BSn on Cu6Sn4, to explain why BSn solidifies with a lower undercooling on Ni substrates than on Cu substrates, both of which are used in industry.

- Influence of BSn microstructure on the nucleation intercooling

Microstructures of Sn-3Ag-0.5Cu (SAC305) solder alloy will be studied at different nucleation undercooling values to report the relationship in great detail. The use of optical microscopy combined with EBSD and Focused Ion Beam (FIB) milling on SAC305 samples will reveal the 3D microstructure of the various solidification morphologies.

The morphologies will be used to better understand the nucleation and growth of BSn from liquid, and hopefully allow better control of BSn microstructures in industrial electronic joints.

- Nucleation kinetics of BSn

DSC studies of BSn nucleation in various solder alloys, and solder joints can be used to develop a quantitative framework on BSn nucleation, which will hopefully be useful in quickly assessing the mechanism of nucleation on different alloys.

The research will aim to apply theoretical nucleation models to real industrial systems, with the aim of developing the framework.