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This paper investigates the effect of 0.05 wt.% Ni on the formation and growth of primary Cu6Sn5 in Sn-0.7 wt.%Cu solder paste soldered on a Cu substrate, using a real-time synchrotron imaging technique. It was found that small additions of Ni significantly alter the formation and growth of the primary Cu6Sn5 intermetallics, making them small. In contrast, without Ni, primary Cu6Sn5 intermetallics tend to continue growth throughout solidification and end up much larger and coarser. The primary effect of the Ni addition appears to be in promoting the nucleation of a larger amount of small Cu6Sn5. The results provide direct evidence of the sequence of events in the reaction of Nicontaining Sn-0.7 wt.%Cu solder paste with a Cu substrate, and in particular the formation and growth of the primary Cu6Sn5 intermetallic 2

The authors found inhibition of cracking in the interfacial Cu 6 Sn 5 intermetallic when Ni containing Sn-0.7Cu alloy was used for soldering. It is thought that this crack inhibition occurred due to the stabilisation of the high temperature hexagonal Cu 6 Sn 5 phase through the presence of Ni from a Sn-0.7Cu-0.05Ni solder alloy. To explore the mechanisms associated with the differences in joint integrity, in-situ synchrotron X-ray diffraction (XRD) at the Australian Synchrotron was conducted in the temperature range of 25 to 200°C. The results show that Ni stabilises a high-temperature allotrope of the Cu 6 Sn 5 phase, avoiding stresses induced by a volumetric change that would otherwise occur on phase transformation.

Trace elements are increasingly being incorporated into lead-free solder compositions. This paper analyses the distribution of trace elements in solder joints when commercial purity Sn-based alloys are soldered onto Cu substrates. Analysis techniques include μ-XRF (X-ray fluorescence) mapping performed at the SPring-8 synchrotron radiation facility. The mapping results indicate that Ni is present in the Cu 6 Sn 5 intermetallic reaction layer, and is distributed in a relatively homogeneous fashion as (Cu,Ni) 6 Sn 5 . In alloys containing trace levels of Ge (60 ppm), this element is comparatively concentrated within the oxide at the solder surface, and a lower concentration is distributed homogeneously in the solder matrix and the intermetallic reaction layer. In Sn-Pb alloys, the Pb was found to segregate to the boundaries between adjacent Cu 6 Sn 5 grains.