Jue Zhong

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.

Effect of ultrasonic power on wedge bonding strength and interface microstructure

During the aluminum wire wedge bonding, the ultrasonic power and bonding strength were obtained. Based on those data, the relationship between ultrasonic power and bonding strength was studied. The results show that: 1) ultrasonic power is affected by ultrasonic power ratio and other uncontrolled factors such as asymmetric substrate quality, unstable restriction on the interface between wedge tool and aluminum wire; 2) when ultrasonic power is less than 1.0 W, increasing ultrasonic power leads to increasing bonding strength and decreasing failure bonding; on the contrary, when ultrasonic power is greater than 1.6 W, increasing power leads to decreasing bonding strength and increasing failure bonding; 3) only when ultrasonic power is between 1.0 W and 1.6 W, can stable and high yield bonding be reached. Finally, the microstructure of bonding interface was observed, and a ring-shaped bond pattern is founded in the center and friction scrape besides the ring area.

Features of Machine Variables in Thermosonic Flip Chip

An assembly bed on thermosonic flip chip bonding was set up, two different structures of tool tips were designed, and a series of experiments on flip chip and bonding machine variables were carried out. Lift-off characteristics of thermosonic flip chip were investigated by using Scanning Electron Microscope (JSM-6360LV), and vibration features of tool tips driven by high frequency were tested by using PSV-400-M2 Laser Doppler Vibrometer. Results show that, for chip-press model, slippage and rotation phenomena between tool tip and chip have been solved by using tool with greater area tip pattern during flip-chip bonding process, and welding failures appeared in chip-collet model have been controlled. Greater area pattern on tool tip is better than small area pattern. The power of ‘n’ bumps on flip chip bonding is far smaller than that of n×(the power of single wire bonding). The power is directly proportion to vibration displacement driven by the power, high-power decrease positioning precision of flip chip bonding or result in slippage and rotation phenomena. The proper machine variables ranges for thermosonic flip chip had been obtained.

Dynamics of Ultrasonic Transducer System for Thermosonic Flip Chip Bonding

Thermosonic flip chip bonding is used in certain fine pitch IC packaging for its unique features. By using this bonding process in this paper, 1 mm times 1 mm chip with 8 gold bumps has been bonded onto a silver-coated substrate. Dynamical properties of transducer system, which is the key component for providing the ultrasonic energy, have been investigated using finite element model (FEM) and measurement using impedance analyzer and laser Doppler vibrometer (LDV). The simulation results show that the ultrasonic transducer vibrates by coupling with all excited modes, therefore resulting in complicated motions during bonding. The third axial mode, which includes 1.5 wavelengths and 3 nodes, is the dominant working vibration. However, this axial mode is severely disturbed by undesirable non-axial modes such as flexural modes. There are some other unwanted parasitic modes close to dominant mode. Measured velocities of the transducer horn show that the system vibrates under several modes simultaneously. The impedance measurements reveal additional frequencies overlapping the working frequency. All non-axial modes of the ultrasonic transducer disturb the bonding process and degrade the bonding quality. A subtle control is needed to obtain unique axial mode and stable vibration for high bonding quality.

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.

Ultrasonic Applied in Super Precision polishing of Magnetic Recording Heads

In the traditionary nanogrinding way, a sub-nanometer smooth surface can be achieved, but the PTR can be only controlled under ten nanometers. To find a new way to reduce the PTR. ultrasonic was applied in the polishing experiment. At first, the experiment with two kinds of samples. AlTiC bulk and NiFe bulk were carried out in free abrasive lapping way with the float-piece polisher. It was discovered that the material removal rate and the roughness of surface is improved in different extent of different material, the material removal rate (MRR) of the AlTiC bulk was increased from 10 nm/m to 16 nm/m. while the MRR of the NiFe bulk was increased from 23 nm/m to 30 nm/m. Encouraged by the result of the experiment, the specimen of 70 mm*1.25 mm rowbar with 60 GMR magnetic head was polished in the same condition, as a result, the PTR was reduced from 65 nm to 45 nm. Furthermore, the ultrasonic was introduced into nanogrinding of recording head. In this way. the PTR was controlled at lnm or so and smoother surface was achieved. Two kinds of mechanism about the role of the ultrasonic were prompt to explain in what way that the material removal rate is improved and why the improved ratio is different in different materials in the polishing.