Lei Han

Interface mechanism of ultrasonic flip chip bonding

The authors demonstrate that 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. To minimize the intermetallic compound layer, the effectiveness of a different bonding approach is confirmed. 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 2.3:1.

Study on a cooling system based on thermoelectric cooler for thermal management of high-power LEDs

Abstract To improve the heat dissipation of high-power light-emitting diodes (LEDs), a cooling system with thermoelectric cooler (TEC) is investigated. In the experiment, the 6xa0×xa03xa0W LEDs in two rows are used to compose the light source module and the environment temperature is 17xa0°C. The temperatures of heat dissipation substrate of LEDs and cooling fins of a radiator are measured by K type thermocouples to evaluate the cooling performance. Results show that the temperature of the substrate of LEDs reaches 26xa0°C without TEC. However, it is only 9xa0°C when the best refrigeration condition appears. The temperature of the substrate of LEDs decreases by 17xa0°C since the heat produced by LEDs is absorbed rapidly by TEC and dissipated through the radiator, and the junction temperature of LEDs reaches only 45xa0°C which is much lower than the absolute maximum temperature of LEDs (120xa0°C). The experiment demonstrates that the cooling system with TEC has good performance.

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 Temperature Parameter in Au–Ag Wire Bonding

The effect of the temperature on bondability and bonding process for wire bonding are investigated. Bondability is characterized by shear bonding strength and bonding process is represented by input and output power of ultrasonic transducer. A laser Doppler vibrometer and Labview software were used to record the velocity, voltage and current of transducer at different temperature settings. A K-type thermocouple sensor was used to measure the bonding temperature. Experimental results show that unsuccessful bonding happens at low temperature, and over bonding appears if the temperature is too high. Only when the temperature is at appropriate settings, can a stable and satisfied bondability be attained. The reason for this experimental observation is analyzed. By using a high resolution transmission electron microscope, the atom diffusion depth of Au-Ag bonding interface was measured and the result is about 200 nm. By using joint time-frequency analysis, the instantaneous characteristics of bonding process were observed completely and clearly. It is found that input and output ultrasonic power vs. time-frequency in a bonding process, including resonance frequency, harmonic components and amplitude of ultrasonic energy, vary along with the change of temperature settings.

Temperature effect in thermosonic wire bonding

The temperature effect on bonding strength and ultrasonic transmission in a PZT transducer system was investigated. The results show that, the temperature change influences the material features of the bonding interface, such as elastic modulus, tensile strength of gold ball and Ag substrate, which results in different bonding strengths. Moreover, the temperature change also influences the impedance and dissipative ultrasonic energy in the PZT system. The current signal of PZT transducer was analyzed by join time-frequency analysis, which can reveal the current change in a bonding process more clearly and completely. The analysis shows that the bonding parameters influence mutually. These results can help build some criteria for parameter match and optimization in wire bonding processes.

Bondability window and power input for wire bonding

This paper presents a recent study by monitoring input power in wire bonding process on its performance. The instantaneous driving voltage and current to the PZT/transducer system were recorded and the input power histories for all tests were analyzed. A stable and satisfied bonding can be obtained at moderate ultrasonic power setting. A laser Doppler vibrometer was used to record the response of the structure. The initial power oscillating may represent the phase locking chaos, and the final attenuation may reflect the remains of kinetic energy in the structure. Strength of wire bonding should be attributed to the input power during the main loading segment.

Atomic diffusion properties in wire bonding

The lift-off characteristics at the interface of thermosonic bond were observed by using scanning electron microscope (JSM-6360LV). The vertical section of bonding point was produced by punching, grinding and ion-sputter thinning, and was tested by using transmission electron microscope (F30). The results show that the atomic diffusion at the bonded interface appears. The thickness of Au/Al interface characterized by atomic diffusion is about 500 nm under ultrasonic and thermal energy. The fracture morphology of lift-off interface is dimples. The tensile fracture appears by pull-test not in bonded interface but in basis material, and the bonded strength at interface is enhanced by diffused atom from the other side.

Experiment study of dynamic looping process for thermosonic wire bonding

Abstract Looping is a complex dynamic process affected by many interacted factors, and is becoming more and more important in the state-of-the-art thermosonic wire bonding. To provide an insight view of loop mechanism, the looping process of standard loop was experimentally studied with a high resolution and high speed video camera. The capillary trace and loop profile evolution process were obtained from looping video with a digital image process program. A phenomenological description was used to understand the looping forming mechanism. The effect of capillary trace on loop profile was investigated, and the kinks forming mechanism were discussed. The spring back and kink up were detaily described. Experiment results show that loop profile was affected by kinks number, position on gold wire and deformation. Kinks were formed by reverse motion of capillary. From the geometry point of view, kink is the wire segment with the local maximum curvature. From the mechanical point of view, kink is the partly plastic deformed wire segment with elastic deformed core inside. This study may be useful for loop design in industry and for loop dynamic research in academic.