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Al-Cu intermetallic growth during ultrasonic ball bonding
Keywords: Ball bonding, Intermetallic, Copper Wire
Although ultrasonic wire bonding has been implemented in industrial applications for many years, from a materials science perspective, there is limited information on the mechanisms of bond formation during the process. It is not clearly understood at what stage of the process, bonding occur, and there are still debates on i) how the initial bond forms, ii) at what stage of the process intermetallic compounds (IMC) form, and iii) how IMCs evolve during the process. Understanding these phenomena not only helps design/modification of proper ultrasonic bonding processes, but also is very important in improving reliability of the bonds. This research, which is a part of a larger research being studied at Santa Clara University to provide answers to the above questions, aims to investigate intermetallic formation and growth during bonding and the following post annealing processes. Intermetallic formation at low temperatures during the very short bonding time can be related to accelerated diffusion processes. Despite the industrial importance of the phenomenon, the micro-mechanism of IMC formation is still unknown. One of the main barriers toward understanding the phenomena is the limited capability of experimental analysis for analyzing processes occurring over short period of time. Therefore, in this research, a combination of multi-scale modeling and experimental analysis is used to investigate the mechanisms that lead to diffusion enhancement and, as a result, IMCs formation. Employing an interrupted bonding set-up, wire bond samples are prepared for a range of ultrasonic time, varying from 10ms to 100ms. Intermetallic particles at the bond interface, of both as bonded as well as heat-treated (annealed) specimen are characterized using optical microscopy, microhardness measurement, and Scanning Electron Microscopy (SEM). Chemical compositions of the particles are studied using Energy Dispersive Spectroscopy (EDS). A multiscale modeling approach is developed to investigate mechanism of intermetallic formation and growth. Enhance diffusional process is linked to the high density of dislocations. Finite element simulation using ANSYS is used to predict dislocation density, while LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) molecular dynamics simulator is used to model diffusion in dislocation containing systems. The results are integrated in an analytical model to predict size of the intermetallic particles as a function of time. Modeling results are compared with experimental results. Although more comprehensive analysis is required to completely understand the system, the results obtained so far are encouraging and warrant further investigations.
Mahin Khan, Graduate Student
Santa Clara University
Santa Clara, CA

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