Abstract
In electronic packaging, rapid growth of intermetallic compound (IMC), fast consumption rate of UBM, dual-phase IMC and spalling phenomenon are concerned in the Sn-Ag-Cu (SAC) solder system. Many studies have aimed to improve the characteristics of the SAC solder system, including adding a fourth element to SAC solder or changing to another solder material. Doping other elements to the under bump metallization (UBM) or the alternative material UBM was also employed. All above methods are material control, which would bring to some concerns, such as the issue of oxidation and the cost of the modified process due to the new material system. Beyond the material control, the method of the architecture control by modifying the microstructure of UBM was demonstrated to affect the interfacial reaction in solder joint. The microstructural control of UBM is a potential way to solve the critical issues in the SAC solder system. With the structural difference of Ni UBM, it is expected that the dense structure of Ni UBM may inhibit its consumption rate of UBM and suppress the growth of IMC during reflow. While the bias voltage applied during coating process, the microstructure of film can be shaped and modulated to the dense structure. When the bias voltage increased to 200 volt, the (111) nanotwinned Ni film was produced due to the high mobility of adatoms and the high energy of ion bombardment during the growing process. By using the nanotwinned structure of Ni film, slow interfacial reactions in solder joint were observed because of the attractive properties of nanotwinned structure. Nanotwinned structure was regarded as a perfect symmetric crystal, which possessed the lower boundary energy and higher activation energy along boundary. As a result, the stable structure of nanotwinned Ni film significantly suppress the formation of interfacial IMC and can be employed as thinner Ni-based UBM in electronic packaging.