Microstructure formation and evolution in Pb-free solder alloys and solder joints on Cu substrates depend on the nucleation and growth of primary Cu6Sn5 and beta-Sn during solidification and thermal cycling in service. This thesis explores the mechanisms responsible for microstructure evolution at different stages during the lifetime of a solder joint.
Cu6Sn5, a common intermetallic in Pb-free soldering, is usually first to nucleate and past work showed that aluminium additions can cause significant refinement of primary Cu6Sn5. In this work, it is showed that the mechanism of refinement is heterogeneous nucleation of Cu6Sn5 on either deltaCu33Al17 or gamma1Cu9Al4 coupled with significant constitutional supercooling ahead of growing Cu6Sn5 crystals. Cu-Al particles are shown to be effective catalytic nucleant particles in both hyper-eutectic Sn-4Cu-0.02Al and hypo-eutectic Sn-0.7Cu-0.05Al/Cu joints and share reproducible orientation relationships with Cu6Sn5.
The growth of primary Cu6Sn5 also plays a role in determining the final microstructure. A deeper understanding of crystal growth mechanisms and transitions between different Cu6Sn5 morphologies is developed. It is shown that, for different composition and cooling rate combinations, Cu6Sn5 crystals undergo a faceted to non-faceted growth transition as a result of a kinetic interface roughening transition and a gradual change in mechanism from lateral growth governed by anisotropic attachment kinetics to continuous growth governed by diffusion and curvature.
As the majority phase in most solder joints, betaSn nucleates at a later stage of solidification after Cu6Sn5 has nucleated. The nucleation of betaSn on the Cu6Sn5 layer in solder joints is studied in detail. It is shown that primary Cu6Sn5 is not a potent nucleant for Sn, but the Cu6Sn5 layer plays a key role in betaSn nucleation and microstructure formation in solder joints.
Thermal contraction of Cu6Sn5, betaSn and other common phases in soldering is another important phenomenon that affects the performance of solder joints in service. Directional data on the anisotropic coefficient of thermal expansion (CTE) of Cu6Sn5 and other non-cubic intermetallics are measured and correlated with the directional Young’s modulus.