Microcosmic Mechanism and Rule of the Interfaces in Ultrasonic Bonding
|School||Central South University|
|Keywords||Ultrasonic bonding Interface Microcosmic mechanism HRTEM Ultrasonic flip chip|
Ultrasonic bonding interface is chief concern, in this paper, the interface mechanism of wire bonding and ultrasonic flip chip bonding is analyzed systemically. Sample of TEM of bonding joints is produced successfully, the growth of the dislocation density in bonded interface after ultrasonic treatment is observed, then microcosmic mechanism of ultrasonic fast bonding is discussed. Lift-off characteristic of bonding interface is researched by using SEM. Driving voltage and current signals of piezoceramie transducer are measured, the features of the input power and impedance are obtained. Atom diffusion of two interfaces of ultrasonic flip chip is studied, multi-pattern process of flip chip bonding are tested, then a innovated process is suggested in order to improve the performance of two interface, and an experiment-based mode of ultrasonic energy conversion is found.The study is organized as follows:Firstly, the mechanism of fast formation at bonding interface: the ultrasonic vibration in flip chip FC bonding results in the generation of dislocations, and the atomic diffusion can be activated more easily along the dislocation lines which perform the fast diffusion channels, thus the dislocation diffusion is probably more prominent than the body diffusion during ultrasonic bonding. The growth of the dislocation density proved also that atom diffusion was happened below the annealed temperature. Based on the dislocation diffusion, the mechanism of fast diffusion at bonding interface is formed at lower temperature.Secondly, the depth of atomic diffusion was about 200-500 nm. Lift-off characteristic of interface shows that the peeling underdeveloped bonds simulate a torus （or doughnut） with an unbonded central region and ridged peripheral region is bonded hardly. The atomic diffusion at the bond interface enhances the micro-structural strength, which increases beyond that of the base materials. The fracture surfaces of bonded interfaces separated by pull-testing were characterized by dimpled rupture. The tensile fractures occurred not in the bond interface, despite the presence of an inter-metallic compound, but in the weaker base material. So bonding strength consists of ’wire material - diffusion layer - pad material’ .Thirdly, microstructure evolutions of interface are associated with the normal bond parameters-load, power, and time. For constant other machine variables, with increasing load, the total area of bond pattern increases in size, and minor axis of torus extends major axis; with increasing time, the ridged periphery spreads a whole torus, and the ridged location of the bonded region moves closer to the bond center; the sliding trace and the ridge-like of the bond pattern strengthen when more power applied. Moreover, the machine variables have an optimal range for microstructure characteristics. Driving voltage and current signals of piezoceramic transducer were measured directly by using digital storage oscilloscope; the power curves of bonding have been calculated. Results show such a trend that power curves of badly bonding were much lower than that of hard bonding, and indicated a monitoring system of ultrasonic bonding reliability.Fourthly, it is discovered that the depth of atom diffusion at up-interface is much thicker than that of down-interface, Furthermore, an experiment-based mode of ultrasonic energy conversion was found that the ratio of up interface to down interface in ultrasonic FC bonding was about,D1:D2=(sum from k=1 to k=N D1（k）/N):(sum from k=1 to k=N D2（k）/N)≈2.28:1 , and it may be aninter-metallic compound （e.g. Au4Al is major） at up-interface. To minimize the inter-metallic compound layer, the effectiveness of a different bonding approach is confirmed.The study done in this paper can provide reference for the ultrasonic bonding technology, the design of process, and on-time monitoring of ultrasonic bonding reliability. It is expected that the study can be benefit to the development of equipment and technology of high-density and high-performance microelectronics packaging.