Dramatic morphological reservation of prism-type Cu₆Sn₅ formed on single crystal Cu substrates under temperature gradient

The morphology evolution of Cu₆Sn₅ grains in (001)Cu/Sn/Cu and (011)Cu/Sn/Cu micro solder joints reflowing with and without a temperature gradient was studied. In the as-soldered state, regular and perpendicular prism-type Cu₆Sn₅ grains formed on (001)Cu substrate, while a mixture of stubby prism-type and scallop-type Cu₆Sn₅ grains formed on (011)Cu substrate. The prism-type morphology gradually transferred into scallop-type after isothermally reflowed at 270 °C for 10 min. However, with a temperature gradient, the initial perpendicular small prismatic Cu₆Sn₅ grains on (001)Cu substrate and mixed-type Cu₆Sn₅ grains on (011)Cu continued to grow into large prism-type. All the prism-type Cu₆Sn₅ grains grew along same direction after long time reflow under temperature gradient due to the merging of adjacent Cu₆Sn₅ grain pair.