Influence of Composition on the Morphology of Primary Cu6Sn5 in Sn-4Cu Alloys

Alloys from the composition range Sn-(0.7 wt.% to 7.6 wt.%)Cu consist of primary Cu6Sn5 surrounded by a eutectic Sn-Cu6Sn5 mixture. The primary Cu6Sn5 intermetallics commonly adopt a coarse elongated morphology, which is not optimal for the mechanical properties of the soldered joint. This paper investigates the effect of trace elemental additions on the size and morphology of the primary Cu6Sn5 in Sn-4 wt.%Cu alloy with and without Ni additions. Elements investigated include ppm additions of Al, Ag, Ge, and Pb. It is shown that Al has a marked effect on the solder microstructure and refines the size of the primary Cu6Sn5, even at very low addition levels, in both binary Sn-Cu alloys and those containing additional Ni. The effect of Al is confirmed using real-time x-radiographic synchrotron observations of solidification.