Phase transformation and grain orientation of Cu–Sn intermetallic compounds during low temperature bonding process

The interconnection lengths between the stacked chips in three-dimensional (3D) package are a few of microns, hence the solder joints for the stacked chips joining are mainly composed by intermetallic compounds (IMCs) after reflow processes. To evaluate the phase transformation of Cu–Sn IMCs in the small interconnection joints, the Cu/Sn/Cu structures were bonded with different bonding times at various temperatures in argon gas atmosphere in this study. Scanning electron microscope and energy-dispersive X-ray were used to observe the joint interfacial microstructures and electron back scattering diffraction was used to identify the grain orientations in the joints. Scalloped Cu₆Sn₅ grains were found to be initially formed on the Cu substrates at the early stage. A lot of small Cu₆Sn₅ grains formed on the surfaces of the big scallop Cu₆Sn₅ grains. Those small grains gradually grew up to merge into the big Cu₆Sn₅ grains. With longer reflow time, the Cu₆Sn₅ grains initiated at both side of Cu substrate continued to grow up and started to contact with each other. Meantime, the different Cu₆Sn₅ grains with different grain orientations have merged into some bigger grains. The Cu₃Sn grains formed between Cu₆Sn₅ layers and Cu substrates have further developed at the expense of the depletion of Cu₆Sn₅. Most of columnar Cu₃Sn grains were vertical to Cu substrate surface and their grain sizes were 1–5 μm. With 960 min at 300 °C, the pure Cu₃Sn IMC joint has formed. The Cu₃Sn grains in IMC joint had different grain orientations and a contact line was observed in the middle of the Cu₃Sn IMC joint.