Junhui Li

Interfacial Microstructures and Thermodynamics of Thermosonic Cu-Wire Bonding

The interfacial microstructures of the Cu-wire bonding to an Al pad are investigated first by using an X-ray microdiffractometer and high-resolution transmission electron microscopy. It was found that the intermetallic compounds hardly formed at the Cu/Al interface during the thermosonic Cu-wire bonding process. However, when heating temperature is elevated to 340°C which increases energy levels of Cu/Al, the intermetallic phases Al₂Cu and Al₄Cu₉ can form and reach to 130 nm thick within 20 ms due to atomic interdiffusion and reaction activated by ultrasonic energy and heat at the Cu/Al interface. The Al side of the interface is aluminum-rich Al₂ Cu with lattice parameters <i>a</i> = 6.067 Å and <i>c</i> = 4.864 Å, and the Cu side is copper-rich Al₄Cu₉ with lattice parameter <i>a</i> = 8.706 Å. Bonding strength and bondability increase significantly after forming the Cu/Al intermetallic phases.

Theoretical and experimental analyses of atom diffusion characteristics on wire bonding interfaces

The features of ultrasonic bonding interface were inspected by using a high resolution transmission electron microscope. Stress of ultrasonic bonding interface was analysed by the finite elements simulation. Results show that the high stress of bonding interface was caused by ultrasonic vibration, which increased the dislocation density inside the metal crystalline lattice which provides the fast diffusion channels, and provided driving force for atom inter-diffusion. Short-circuit diffusion during ultrasonic bonding is more prominent than crystal diffusion. For the given ultrasonic bonding parameters, depth of atom diffusion at Au/Al interface of ultrasonic bonding was about 100–300 nm in several ten milliseconds, which forms the bonding strength of 0.65 N, and it is an inter-metallic compound of AuAl2. These will be helpful for further analysis.

Interfacial Characteristics and Dynamic Process of Au- and Cu-Wire Bonding and Overhang Bonding in Microelectronics Packaging

Microstructure characteristics of Cu-wire bonding interface are investigated first by using the X-ray microdiffractometer, and the dynamic process of Cu- and Au-wire overhang bonding is recorded first with a high-speed camera system. It was found that the success rate of Cu-wire bonding is lower than that of Au-wire bonding, the formation of Cu-Al intermetallic compounds (IMCs) is more difficult than that of Au-Al IMCs, and the impact and deflection of Cu-wire overhang bonding process are much bigger than those of Au-wire overhang bonding process and affect bonding performance. In order to improve the bondability of Cu-wire bonding and overhang bonding, an approach with thick Al pad on the die which reduces its hardness is more compliant during Cu bonding process; another idea is to activate the formation of Cu-Al IMCs by increasing the energy. The effectiveness of the suggested approach is confirmed. In particular, Al2Cu and Al4Cu9 IMCs can form within 18 ms due to diffusion and reaction activated by high thermal (to 340 ° C) combined with ultrasonic energy at the bonding interface, and the success rate and strength of Cu-wire bonding increased significantly when Cu-Al IMCs form. For overhang bonding, the approach with thick Al layer improves dynamics features of hard Cu-wire overhang bonding process; thereby, it enhances the performance of Cu-wire overhang bonding.

Study of a dipping method for flip-chip flux coating

A dipping method for flux coating is proposed in the paper. Flow process of the flux transferred into the groove is investigated by using an optical detection means. Flow process influenced by gluing speed and viscosity is analyzed by using two indicators including reflective area ratio and flow velocity. Experimental result shows that stably reflective area ratio of glue is associated with gluing speed instead of viscosity; and it decreases with the increase of gluing speed in a same viscosity. The reason is that different gluing speed changes the amount of the glue transferred to the groove. Flow velocity of glue is related with gluing speed, a greater gluing speed leads to greater maximum flow velocity at the same viscosity, which may be due to larger gluing speed gives greater inertial force to the glue. Real chip experiments verified the feasibility of this method.

Investigation of the characteristics of overhang bonding for 3-D stacked dies in microelectronics packaging

Abstract It is indicated that the impact and deflection of overhang die during overhang bounding process results in considerable impact of performance of bonding in this paper. Specifically, the deflection is unstable and up to 27xa0μm for the given parameters, which results in low shear force strength and low success rate of overhang bonding. To improve the performance of the overhang bonding, an effective and novel approach, overhang bonding with thick Al, is presented. The approach decreases hardness of bonding die, which makes the bonding die more compliant during bonding process than that in the traditional approach, thus the shear strength and bonding success rate of overhang bonding will be improved. Finally, effects of ultrasonic and force to overhang bonding indicates that excessively low or excessively high ultrasonic power results in weak bonding performances, and so did the force.

Power and Interface Features of Thermosonic Flip-Chip Bonding

Driving voltage and current signals of piezoceramic transducer were measured directly by using digital storage oscilloscope, and interface microcharacteristics of the specimens of flip-chip bonding were inspected by using a transmission electron microscope. 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. The acceleration of ultrasonic vibration was about several ten thousand times as acceleration of gravity, which activates dislocations inside the metal crystalline lattice which act as the fast diffusion channels. Dislocation diffusion is more prominent than the crystal diffusion when the temperature is low. Differing from thermal melting mechanism of the reflow bonding, the ultrasonic bonding is much faster than the reflow solder bonding.

Atom diffusion mechanism of thermo-sonic flip chip bonding interface

The TSFC (thermosonic flip chip) bonding was realized in a self-structured TSFC bonder. The atom inter-diffusion on Au-Ag bonding interface was characterized by TEM, and high-resolution TEM pictures reveales that the dislocation density in the bump increases after the acting of ultrasonic. And the interlaced dislocation slip lines were observed in the SEM pictures of bumps surface, which indicates the dislocations motion in the interior of bumps. A FEM model was used to simulate the stress on the bonding interface. It is noticed that the ultrasonic vibration causes high stress in the contact interface of bump and pad, which increases the dislocation density and provides short-circuit diffusion channel for Au and Ag atom inter-diffusion. Finally, a preliminary discussion about the atom diffusion, based on the atom diffusion theory, is also presented. Studies show that the stress is a significant component of atom diffusion driving force. The Gibbs free energy, chemical potential and acting force of Au and Ag atom on bonding interface are increased by the stress gradient. With the driving force caused by stress, the probability for atom to overcome the energy barrier increased, and the diffusion speed increased. And the atom diffusion depth on bonding interface is about 200-500 nm in several hundred milliseconds, which forms good bonding strength.

Heavy Aluminum Wire Wedge Bonding Strength Prediction Using a Transducer Driven Current Signal and an Artificial Neural Network

Heavy aluminum wire wedge bonding is commonly used in the packages of power electronic devices. In this paper, a heavy aluminum wire wedge bonding strength prediction system was developed. In this system, the driving current of the transducer system was recorded by a data acquisition system, decomposed, and analyzed using a wavelet model. The fundamental frequency component was extracted, and seven characteristics (five in the time domain and two in the frequency domain) were obtained from the experimental observations. An artificial neural network using back-propagation as training (using 1440 bonding samples) was used; 2200 bonding test samples were then used to verify the performance of the bonding strength prediction system. Experimental results show that this method can be used to accurately predict the shear strength of heavy aluminum wire wedge bonding.

The analysis of heat pipe cooling in high power LED lighting system

In order to study the problems of high power LED lighting system, a cooling device based on the combination of heat pipe and fan was designed. In the experiment, the LEDs array of 12 W composed of 4 LEDs is used and the room temperature is 23°C. The thermocouple temperature gauge is used to collect and record the change of the operating temperature of the LED lighting system in 30 minutes. Results show that the temperature of the substrate of LEDs reaches 40°C within 5 minutes when cooling by fins. In the condition of cooling by heat pipe, the cooling effect is little better than by fins. However, the operating temperature is only 25°C when cooling by the combination of the heat pipe and fan. The experiment demonstrates that the cooling in the LED lighting system is very important and heat pipe has excellent cooling capacity only when combination with the fan.

System design for anti-interference of smoke in metal liquid level detecting

In order to eliminate the smoke noise in metal liquid level laser detecting for cast machine, a new system is integrated with semiconductor laser, optical lens, Charge-Coupled Device (CCD), high speed analog signal processor, Field Programmable Gate Array (FPGA). It is based on the theory of triangulation method and the idea of real time surface compensation. The location of the scattering spot will change with the change of metal liquid level, so the level can be calculated by the relationship between the location of the scattering spot and the metal liquid level. However, smoke will influence the location of spot. This system can improve the received light intensity by adjusting the light integration time, and as a result eliminate the interference of smoke, improve the displacement measuring accuracy in smoke environment.