The Preliminary Study Of A New Lead-free Interconnection Method Based On Electromagnetic Stirring

The electronic packaging and assembly industry is now leading to the process of lead-free and miniature. However, many reliability problems occurred in this process, such as the microstructure of solder alloys distributed disproportionately, air holes and voids formed in the solder joints, excessive intermetallic compound (IMC) formation in solder/pad interface. The traditional way to solve this problem is adding alloy elements to reduce crystal grain size, prolonging the time spent above liquid line or developing new flux so as to eliminate voids formed in the solder joints. A new electronic packaging and assembly lead-free interconnect method based on electromagnetic stirring has been proposed in this thesis. Electromagnetic stirring can improve the reliability of lead-free solder joints by optimizing the lead-free solder's solidification and crystallization behavior.Although electromagnetic stirring is a traditional technique, and has been widely used in the fields of electromagnetic casting and magnetic controlled welding for many years, numerous specific problems will be aroused as long as the electromagnetic stirring technology is brought into the field of electronic manufacturing. The working principle and basic theories of electromagnetic stirring have been introduced briefly, and then, the feasibility of applying the electromagnetic stirring method in the fields of electronic packaging and assembly lead-free interconnect field has been profoundly discussed.Ni (nickel) is one of a ferromagnet. Based on computer numerical simulation, the influence of Ni located in the under bump metallization (UBM) layer or pad plated layer on the distribution of magnetic force line has been studied. It has been figured out how to deposit the coils in the practical electromagnetic stirring process. When the Cu was chose as solder wetting layer for UBM or pad plated layer instead of Ni, and Cu is not a magnetic material, it has no influence on the distribution of magnetic force line in the solder bulk, the simulation results has been demonstrated it. Lead-free solder is of low electrical resistivity, thus the current's skin effects may easily occur. In this thesis, the respective interconnections of the distribution of the induced current density and the influenced range of Lorentz force with the frequency of coil current were also simulated. The simulation results can serve as reference for experiments.The electromagnetic stirring was carried out in the process of solder joint formation in the lead-free solder ball and substrate by means of revolving permanent magnets. Consequently, some defects were found through analysis, causing negative impacts on solder joints reliability.In order to overcome the shortcoming of revolving permanent magnet method, a novel electromagnetic stirring device was designed based on the single chip computer. By adjusting the single chip computer's output impulse and the coil's working voltage, the intensity of magnetic field, the frequency of magnetic field and the electrified order of the coil can be regulated. It can be served as a solid preparation for the further study of the electromagnetic stirring technology.