Effect of (111) oriented nanotwinned Cu substrate on the electromigration of Sn58Bi solder joint at high current density

(111) oriented nanotwinned copper ((111)nt-Cu) has attracted much attention and shown a good application prospect as Under Bump Metallurgy (UBM) in advanced packaging technology, because of its excellent interconnect performance. However, the research on the microstructure evolution of (111)nt-Cu/Sn58Bi solder joint during the Electromigration (EM) process remains to be enriched and in-depth. In this paper, the linear Cu/Sn58Bi/Cu sandwich solder joint structure was adopted to study the Liquid–solid (L-S) EM behaviors of (111)nt-Cu and polycrystalline Cu (C-Cu) substrates under condition of 25 ℃ for 10, 30, 60 and 90 min with the current density of 2 × 10⁴ A/cm² by the scanning electron microscopy (SEM). The results showed that a polarity effect on the growth of interfacial intermetallic compound (IMC) was observed on both solder joints. The formation of Cu₃Sn phase on the anodic side of (111)nt-Cu solder joint could be seen after 60 min energization while only Cu₆Sn₅ existed on C-Cu solder joint during the whole EM process. The growth rate of interfacial IMCs on the anodic side of (111)nt-Cu solder joint was faster than that on C-Cu solder joint. For example, the average thickness of IMCs on (111)nt-Cu was 164.4 μm at 90 min, which was about 100 μm thicker than that on C-Cu. This phenomenon was related with the grown behavior of Cu₃Sn on cathode side. Comparing to C-Cu joint, Cu₃Sn on (111)nt-Cu formed rapidly and replaced Cu₆Sn₅ completely after 30 min EM, but grew slowly after 60 min EM. The diffusion of Cu atoms on (111)nt-Cu was promoted because of the faster diffusion rate of Cu in Cu₃Sn or the shorter diffusion distance. More importantly, as the conductance time prolonged, Sn atoms were rapidly consumed, while copper atoms still maintained at a higher concentration level due to the unique grain structure of (111)nt-Cu. Additionally, the faster formation of IMCs accelerated the release of Bi owing to the lower solubility and promoted the occurrence of Bi-rich phase either on the cathode side or the anodic side of (111)nt-Cu solder joint. The above results will provide a deeper understanding and guidance for (111)nt-Cu as UBM in advanced packaging.