Studies On Vibration Propagation And Performance Formation Of Thermosonic Flip Chip Bonding Interface

Thermsonic Flip Chip (TSFC) bonding is a new generation micro-electronic package technology with full potential. At present, the bondingmechanism is still unclear. So, it's a great challenge for microelectronicmanufacturers to construct a whole set of industry process and equipmentswith great efficiency, high precision and good qualities by optimizing process,motion and energy parameters. And it's still difficult to achieve high-qualifiedflip chip bonding interface and bonding reliability for multi I/O TSFC bonding.In this dissertation, the vibration propagation and performance formation onTSFC bonding interface were studied. The primary researches and results areas follows:1) A TSFC bonding experiment system and bonding process monitorsystem were constructed. With this bonding system, the TSFC bondingprocess was realized by applying ultrasonic, heat and force on the bondinginterface; with the monitor system, the ultrasonic power, force, vibration ontool and flip chip during bonding process were acquired. Based on therelationship among bonding parameters, bonding interface vibration, bondinginterface microstructure and bonding reliability, a bonding process—bondingquality evaluation method is developed.2) The vibration velocities of chip and tool tip were observed, and theevolution from tool tip vibration with chip to stall—chip velocity decreasedsuddenly—was found. The stall phenomenon occurs alter the bonding startedfor several milliseconds, which indicates that ultrasonic energy propagationand consumption on bonding interface is changed before and after stall.Meanwhile, initial bonding strength is formed when stall occurs, and someultrasonic energy is propagated to bonding interface through golden bumps toenhance the bonding strength alter stall; some is consumed by the frictionwear on tool and chip interface. Therefore, a time-variable bonding parameterprocess method based on stall phenomena is proposed.3) The frequency characters of chip vibration signal were analyzed. Itfound that the third harmonics can locate the stall, based on which the bondingprocess control method is proposed. 4) A ring-shaped bonding pattem of bonding interface was observed withSEM. And a FE model of bonding interface was developed to study the stressdistribution on bonding interface. The simulation results show that theperiphery of bonding interface is in a better condition for surface layerbreaking, debris removing and atom diffusion than other area, which is asignificant cause of ring-shaped bonding pattern formation.5) Some preliminary understanding about bonding mechanism wasobtained. With the bonding force and ultrasonic, the stress on bump/pad canreach several MPa, which causes dislocation proliferation, plastic flow ofbumps and pads and oxide film broken on bonding interface. During theplastic flow, the oxide film debris are expelled from bonding interface and theAu/Ag atoms are contacted directly, which results in initial bonding strengthby forming chemical bond (metallic bond) and physics bond (adsorptioncaused by van der Waals force). With the durative and cyclic stress, thedislocation density increases and contact area enlarges quickly; at the sametime, the atoms in bonding interface acquire enough energy to over the energybarrier, and inter-diffuse along short-circuit diffusion paths, which forms atomdiffusion area and reliable bonding strength in hundreds milliseconds.6) The effects of bonding parameters on vibration amplitude of tool tip andchip, bonding strength and microstructure of bonding interface were studied.And different bonding failure patterns were observed. It is found that thebonding strength should include the strength of all components in bondingarea, not only the strength of bumps/pads interface.7) A time-variable bonding parameter process method is proposed: At theinitial stage, great ultrasonic power and little bonding force are used to enlargethe relative vibration of bumps and pads interface, to break the oxide films andto form more exposed and directly contacted atoms; After stall, little ultrasonicpower and great bonding force are used to increase the stress on bondinginterface, to accelerate the atom inter-diffusion and to avoid fatigue failure ofbumps and pads caused by ultrasonic vibration. Experiments show that thisbonding process can ensure higher bonding strength and bonding reliabilitythan traditional bonding process.