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 Cu ₆Sn₅ 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 Cu ₆Sn₅ grains. With longer reflow time, the Cu ₆Sn₅ 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 Cu ₃Sn IMC joint.