Tin bumping for wafer level lead-free packaging

A pure tin bumping technology is described in this paper. TiW/Cu was used as under bump metallurgy. The pure tin was electroplated and formed solder bumps by reflow. The height of the solder bump is about 50 micron. SEM was used for observing the microstructure of interface, and the EDAX was used for investigating the composition of the intermetallic compounds. Tin and copper interreacted with each other to form intermetallic compounds, Cu₆Sn₅. Shear tests and thermal aging were used for evaluation of the tin solder bumps. The pure tin bumps were stored at 150°C for 1, 4, 9 and 16 days respectively. The composition of intermetallic compound changed as time going on. After several days of thermal aging, C₃Sn formed between Cu and Cu₆Sn₅. The total thickness of intermetallic compound increased after thermal aging. The relationship between the thickness of intermetallic compound and the square root of aging time is linearity, implying that the growth of the IMC layer was diffusion controlled. Comparing the strength of tin solder bumps before and even after thermal aging, it was found that after 16 days of thermal aging, the shear strength of solder bump did not change obviously.