Influence of Ni on the refinement and twinning of primary Cu6Sn5 in Sn-0.7Cu-0.05Ni.

Abstract The influence of Ni on the size and twinning of primary Cu 6 Sn 5 crystals in Sn-0.7Cu-0.05Ni and Sn-xCu (x = 0.7, 0.9, 1.1) (mass%) solder joints is studied using synchrotron radiography and SEM-based EBSD. It is shown that the Ni addition does not cause significant refinement of primary Cu 6 Sn 5 if the alloy is fully melted. However, for peak temperatures ≤250 °C relevant to industrial soldering, primary Cu 6 Sn 5 are not completely melted in Sn-0.7Cu-0.05Ni and there are 10–100 times more numerous and smaller crystals than in Sn-0.7Cu. X-shaped Cu 6 Sn 5 crystals with an angle of ∼70° commonly formed in Sn-0.7Cu-0.05Ni/Cu joints and are shown to be penetration twins. This type of growth twinning was only found in partially melted samples, both in Sn-0.7Cu-0.05Ni/Cu joints and binary Sn-1.1Cu alloy. The frequency of twinned crystals was significantly higher in Ni-containing solders. The results are discussed in terms of the influence of Ni on the Cu 6 Sn 5 liquidus slope and on the lattice parameters of (Cu,Ni) 6 Sn 5. Graphical abstract Image 1 Highlights • Ni solute itself is shown to be ineffective in refining primary Cu 6 Sn 5. • The size of primary Cu 6 Sn 5 after solidification strongly depends on the original size in the original solder ball/paste before melting. • Growth twinning occurs in primary Cu 6 Sn 5 and is promoted by Ni additions. • The changing lattice parameters of Cu 6 Sn 5 by Ni additions facilitate coexisting { 10 1 ¯ 2 } and { 10 1 ¯ 1 } twin planes. [ABSTRACT FROM AUTHOR]