Effect of reflow temperature and time on the interfacial reaction of Sn-xCu/(001) Cu

The trend toward miniaturization of electronic products has led to the shrinking of solder micro-bumps, resulting in the number of grains in under bump metallization (UBM) continuing to decrease; therefore, the anisotropic behavior will be more prominent. For a limited case, the Cu UBM has only one grain, i.e., single crystal Cu. In this paper, single crystal (001) Cu was used as UBM, and pure Sn, Sn-0.7Cu, Sn-1.5Cu, and Sn-3.0Cu (wt.%) were used as lead-free solder. The morphology and thickness of Cu 6 Sn 5 IMC grains formed between (001) Cu and lead-free solder at 250 °C, 275 °C and 300 °C for different durations times from 10 s to 300 s were investigated. The results showed that the reflow temperature significantly affected the morphology and alignment of Cu 6 Sn 5 grains, which exhibited scallop-type and random growth at low temperature (250 °C), while roof-type and perpendicular to each other at high temperature (300 °C). The morphology of the Cu 6 Sn 5 grains changed from roof-type to scalloped-type with increasing soldering time at 275 °C, while that did not change with time at 300 °C. With the increase of Cu concentration in the solders, the morphology of Cu 6 Sn 5 grains changed from roof-type to scalloped-type at 300 °C for 60 s. It showed that the formation of roof-type Cu 6 Sn 5 grains strongly depended on the reaction temperature and reaction time, and the concentration of Cu in solder. These results have important implications for the orientation and size control of Cu 6 Sn 5 grains in interfacial reactions.