Low alkaline solution to deposit electroless Ni-Zn-P film on al pad

The amorphous electroless NiP film is used as underbump metallization (UBM) structure of chip pad or surface finish of IC substrate (or printed circuit board) to join with different solders on microelectronics package due to low cost process, low IMC growth rate, and good solder wetting behavior. However, Au film usually induced some problems on Au/NiP films, including Au corrosion, black pad issue on Ni-P pad, Au delamination and Au embrittlement when the process step of Au film was out of control. There is another ternary electroless nickel alloys Ni-Zn-P film, which is considered as a barrier film on a galvanic Zn or Ni-Zn sacrificial layer in a multi-component corrosion protective coating on steel. Specific properties of electroless Ni-Zn-P film were reported, especially in electrochemical corrosion tests. However, little study is stressed on the formation Ni-Zn-P film under low alkaline. In this study, four kinds of ternary Ni-Zn-P films (Ni-5Zn-13P, Ni-4Zn-7P, Ni-8Zn-8P, and Ni-7Zn-10P) were deposited on sputtered Al film in alkaline electroless solution at a pH value of 8.5 controlled by NH4OH addition. The electroless Ni-8Zn-8P film consisted of typical nodule surface and was free from crack. The interfacial morphology and wetting angle of the Sn-3Ag-0.5Cu/Ni-8Zn-8P joint showed the good wetting behavior. The (Ni,Cu)₃Sn₄ intermetallic compound can attach well onto the Ni-8Zn-8P film after reflow. The line profile of Zn element showed that the Zn only distributed within the Ni-8Zn-8P film, indicating that Zn can't migrate to form (Ni,Cu,Zn)₃Sn₄ IMC, in which no Zn was traced. This study demonstrates that the ternary Ni-Zn-P film might be a potential alternative for under-bump metallization (UBM) application.