Nucleation of tin on the Cu₆Sn₅ layer in electronic interconnections

A Cu₆Sn₅ layer is an integral part of many electronic interconnections. Here we show that, although primary Cu₆Sn₅ is not a potent nucleant for Sn, the Cu₆Sn₅ layer plays a key role in Sn nucleation and microstructure formation in solder joints. Combining thermal analysis, FIB-tomography and EBSD, we show that conical cavities are present between the scallops of the Cu₆Sn₅ layer that act as geometric nucleation sites for Sn, that Sn grows from the Cu₆Sn₅ layer, and that reproducible nucleation orientation relationships (ORs) exist between Cu₆Sn₅ and Sn. With these ORs, a near-random distribution of Sn orientations is predicted from joint to joint even for Cu₆Sn₅ layers with a strong [0001] fibre texture. It is shown that the nucleation undercooling is strongly affected by manipulation of the Cu₆Sn₅ layer shape, and that it is possible to prevent nucleation on the Cu₆Sn₅ layer by adding more potent nucleants.