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Title: Controlling tin nucleation and grain orientations in Pb-free solders
Author: Ma, Zhaolong
ISNI:       0000 0004 7969 8228
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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Lead-free solder joints made with Sn-Ag-Cu or Sn-Ag solders usually contain only a single βSn grain or three twinned βSn grains which are oriented differently in every joint. Due to the anisotropy of βSn every joint therefore has unique thermomechanical properties and, in an array of numerous joints, it is likely that some will be poorly oriented and could cause early failure of a component. The problems of few grains with variable orientation can be attributed to the nucleation of βSn. This thesis explores catalyzing βSn nucleation using heterogeneous nucleation and develops methods to control βSn microstructures and orientations in Sn-3Ag-0.5Cu/Cu solder joints. The developed droplet nucleation technique in this study provides a new 'motorway' for heterogeneous nucleation study, and the introduced orientation control method paves the way to solve chronic problems, such as electromigration, thermomechanical fatigue, and shear fatigue in the electronic packaging industry. It is found that Co additions and Co substrates are effective at catalyzing βSn nucleation. This is demonstrated to be due to heterogeneous nucleation on αCoSn3 crystals. By using a 'droplet nucleation technique', in which Sn droplets are solidified directly on an intermetallic compound (IMC) particle, it is proved that αCoSn3 forms a reproducible orientation relationship (OR) with βSn that has a good lattice match. Strong grain refinement occurred in 60g Co-microalloyed Sn-3.0Ag-0.5Cu samples but only weak grain refinement occurred in 550μm solder balls and joints. When soldering Sn-3.0Ag-0.5Cu on Co substrates, βSn is always observed to grow from the interfacial αCoSn3 layer with an orientation inherited from the αCoSn3 layer texture. However, it is shown that nucleation on the αCoSn3 layer does not give useful βSn orientation control. The droplet nucleation technique was then applied to gain a deeper understanding of βSn nucleation on a range of IMC phases. A family of transition metal stannides, PtSn4, PdSn4, and βIrSn4 that have similar crystal structures to αCoSn3, were identified as potent nucleants for βSn. The common solder IMCs, Cu6Sn5, Ag3Sn, Ni3Sn4, were also investigated. It was found that reproducible ORs formed on all IMCs studied and the nucleation mechanisms were explored by combining nucleation undercooling measurements with measured ORs. The nucleation potency of all studied intermetallics is: αCoSn3>βIrSn4 >PdSn4>PtSn4 >Ni3Sn4> Ag3Sn, Cu6Sn5. The droplet nucleation technique also generated new insights into solidification twinning in solder joints. It was found that cyclic twins formed in droplets when the undercooling was sufficiently high and the liquid contained Ag, Cu and/or Ni. Complex interrelated cyclic twins were found in droplets on Cu6Sn5, Ag3Sn, and Ni3Sn4 where up to five rings of cyclic twins formed each related by a common <100>. The twinning mechanisms in these cases were explored and discussed. The thesis then applies the new understanding developed in the previous chapters to develop a technique to reliably control the orientation of βSn in solder joints. Ball grid array (BGA) joints were fabricated reproducibly by introducing an extra step into the manufacturing process: bonding a nucleant IMC 'seed crystal' onto each Cu pad so as to control the nucleation location, nucleation undercooling and crystallographic orientation of βSn at the moment of nucleation. Each joint made by this technique had a uniform single-grain microstructure with the c-axis of βSn parallel with the substrate plane. This orientation is reported in the literature to give the best resistance to electromigration and shear fatigue.
Supervisor: Gourlay, Christopher Sponsor: Imperial College London
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral