On-Line Measurement Of Cu Fab Deformability In Thermosonic Wire Bonding And Interfacial Reaction Study Of Cu/Al

It has great siginificance to develop a new high efficient method to evaluate the deformabilities of Cu wires. Furthermore, for further application of copper wire bonding, it is well recommended to study the effects of Electrical Flame Off(EFO) process on the Cu Free Air Ball(FAB) deformability and the Cu HAZ breaking strength, and to study the reliability issues of Cu bonds on Al metallization pad. In this thesis, a new on-line measurement is firstly presented to quickly evaluate the deformabilities of Cu wires and Cu FABs with high precision. The effects of EFO process on Cu FAB deformability and Cu HAZ maximum breaking force are investigated using the on-line measurements. We also investigate the effects of EFO parameters on Cu FAB quality and the solidification process during the Cu FAB formation. The growth behaviors of IMCs and cracks in Cu/Al bonds during thermal aging are studied in details. The main phases of the IMCs in the Cu bond are determined using the micro-XRD.The on-line measurement is found to be sensitive to the bonding conditions. The effects of wire types, substrate types, or capillary types on the measurment result are great pronounced. For a wire or FAB deformability comparison study, it is well recommended that the capillary type, substrate type, and substrate temperature are kept constant and the capillary or its position with respect to the horn should not be changed during the study. It is suggested by the on-line measurement results that the Cu wires with high deformabilities will not produce the Cu FABs with high deformabilities automatically.During EFO process, the Cu FABs with higher deformabilities which can reduce the impact force for bond deformation during bonding are obtained with a higher EFO current combining a shorter firing time. Furthermore, higher EFO current combining shorter firing time can lead to a stronger and shorter Cu HAZ which is benefical for the wire looping in the application of small scale packaging. Under the deforming force of 0.6 N, the deformed Cu ball height from the FAB with the EFO current of 250 mA is 14 % lower than that from the EFO current of 45 mA. The deformability of Cu FAB from the higher EFO current increases up to 7-8 %. As a consequence, the impact force required for the ball deformation can be decreased up to 7-8 % and the underpad stress can be reduced siginificantly during ball bonding. The Cu HAZ from the EFO current of 250 mA have 7.5-9.4 % higher maximum breaking load and 22.45 % shorter HAZ length compred to that from the EFO current of 45 mA.For the Cu wire with the diameter of 50.4μm, the optimum Cu FAB can be obtained with the EFO current of 120 mA, the EWD of 0μm, and the FAB preset size of 101.6μm. The flow rate of shielding gas of 0.8 l/min is preferred during Cu FAB formation and the excessive shielding gas will lead to the decrease of the EFO efficiency.The microstrctural study reveals that the Cu FAB is composed by a few big columnar grains. During solidification, the columnar grains initiates from the unmelted wire end and then grow downwards radially to the free end of the ball. As no nucleation is required, the initial solidification simply adopts and extends the crystal structure of the adjacent solid with which it is in contact and the columnar grain comprises a subset of the solid crystal orientation. The orientation of the columnar grains are mainly controlled by the heat flow direction. During solidification, the principal way to lose heat in the molten ball is conduction through the wire.In the as-bonded Cu/Al bond, there is no obvious IMC observed at the bonding interface under SEM with the magnification of 1500 times. After 1 h aging at 250°C, the Cu/Al IMC forms at the bond periphery. The IMC grow rate decreases with the increase of the aging time. The IMCs develop laterally and vertically to the bond centre area. Furthermore, some cavities occur between the IMC layers and the Cu bond bottom. With the growth of the IMCs, the cavities start from the ball periphery and grow inward to the bond centre and keep growing to form the cracks as a result. A complete gap between the Cu bond bottom and the upper IMC layer is formed after 81 h aging. The IMCs growth stopes completely as the formation of the cracks. The Cu/Al IMC growth is controlled by diffusion control mode and obeys the parabolic law. The main phases in the Cu/Al IMC layers are confirmed to be Cu9Al4, and CuAl2. CuAl with a smaller amount is believed to be another phase.