Effects of Ni thickness and reflow times on interfacial reactions between Ni/Cu under-bump metallization and eutectic Sn-Pb solder in flip-chip technology

Flip-chip interconnection technology plays a key role in today's electronics packaging. Understanding the interfacial reactions between the solder and under-bump metallization (UBM) is, thus, essential. In this study, different thicknesses of electroplated Ni were used to evaluate the phase transformation between Ni/Cu under-bump metallurgy and eutectic Sn-Pb solder in the 63Sn-37Pb/Ni/Cu/Ti/Si3N4/Si multilayer structure for the flip-chip technology. Interfacial reaction products varied with reflow times. After the first reflow, layered (Ni1-x,Cu-x)(3)Sn-4 was found between solder and Ni. However, there were two interfacial reaction products formed between solders and the UBM after three or more times reflow. The layered (Ni1-xCux)(3)Sn-4 was next to the Ni/Cu UBM. The islandlike (Cu1-y,Ni-y)(6)Sn-5 was formed between (Ni,Cu)(3)Sn-4 and solders. The amounts of (Cu1-y,Ni-y)(6)Sn-5 intermetallic compound (IMC) could be related to the Ni thickness and reflow times. In addition, the influence of Cu contents on phase transformation during reflow was also studied.