Abstract
Flip chip technology (FCT) with ball grid array interconnection has attracted a great deal of attention in today’s microelectronic packaging. In flip chip technology, one of the challenging issues is the material selection for under bump metallization (UBM). The Ni-based UBM is of interest in FCT owing to the lower growth rate of the Ni-Sn compound and limited spalling effect. In this study, the elemental distribution and related phase transformation between lead-free and eutectic solders and Cu/Ni(V)/Ti UBMs were investigated with the aid of quantitative analysis with an electron probe microanalyzer. Various types of IMC, such as (Cu1-x,Agx)6Sn5, (Cu1-y,Agy)3Sn and (Ag1-z,Cuz)3Sn were observed. The major task of this study is to evaluate the electrical characteristics in the IMC for the solder joint. Conventionally, the two-point method could only derive the resistance data, which failed to provide effective reference in the industry. To accurately determine the resistivity performance of IMC, a novel method incorporated with SELA-EM2 and FIB was developed to prepare the joint sample. It was demonstrated that more accurate evaluation of resistivity in IMCs could be achieved by this innovative technique. The SEM nano-probing with the four-points probing method to measure resistivity of a lead free solder ball was proposed. In this study, more accurate IMC/solder joint resistivity can be measured by utilizing the reliable system which integrates the SEM nano-probing technique, the four-points probing method and the precise cutting method (SELA and FIB) together, which was introduced to evaluate the electric characteristics in the intermetallic compounds of solder joints after various test conditions. The electric data would be correlated to microstructural evolution due to the interfacial reaction between solder and UBM. The electrical properties of FC-BGA lead-free solder joints after various environmental tests among temperature cycle tests (TCT) from 200 to 1000 cycles, high temperature storage tests (HTST) from 500 to 2000 hrs and highly accelerate stress test (HAST) were evaluated. The resistivity comparison among different environment test conditions, such as TCT, HAST and HTST, was studied. The results conclusively showed that the resistivity of every component in the solder joint could be precisely measured. Moreover, results of resistivity, which were constants in repeated measurements, (6 points/location) indicated the reliability of the data. By comparing electrical resistivities of data for overall resistivity in solder joint, the order was solder ball [Sn-3.5Ag-0.5Cu] (51.3 %) >Cu6Sn5 (36.2 %,)> Cu3Sn (6.6 %) >Ag3Sn (5.9 %) among all environmental tests. In addition, the effect of sample thickness on the electrical properties for thin IMC layers (Cu6Sn5 and Cu3Sn) formed within lead free solder was investigated by using the in situ resistance measurement equipped in TEM, along with treated samples ion milled through precision etching and Selected Area Diffusion Pattern (SADP) technique. It is demonstrated that the SEM/TEM combined with 4pp method display the capability to measure straight resistivity in tiny areas of the IMC layer. This advanced technique provides the receptivity information of solder joint material in the assembly technology.