Study of grain size effect of Cu metallization on interfacial microstructures of solder joints

Cu films owing different grain size and hardness in terms of fine copper (similar to 1.3 mu m, similar to 0.85 GPa) and nano copper (100 nm, similar to 1.94 GPa) were fabricated through direct current (DC) electroplating. The film microstructures were investigated using focused ion beam (FIB) and electron backscattered diffraction (EBSD). Texture and hardness of the Cu films were studied by X-ray diffraction (XRD) and nanoindentation. The results indicated a growing (200) texture and higher hardness along with the increase of current density. To study the interfacial reactions of the solder joints based on the two Cu films, the Sn-3 wt% Ag-0.5 wt% Cu/Cu and Sn-3.5 wt% Ag/Cu joints were prepared and thermally aged at 150 degrees C. The fine-Cu exhibited an excellent compact interface, while nano-Cu suffered from void formation at the joint interface. The secondary ion mass spectrometer (SIMS) analysis correlated the results with higher impurity level in nano-Cu. Thus, the strong dependence between interfacial morphology of solder joints and Cu grain size indicated an important role of grain boundary in controlling the hardness and impurities of Cu films along with its reliability of the solder joints.