Fuliang Wang

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.

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.

Dispensing of high concentration Ag nano-particles ink for ultra-low resistivity paper-based writing electronics.

Paper-based writing electronics has received a lot of interest recently due to its potential applications in flexible electronics. To obtain ultra-low resistivity paper-based writing electronics, we developed a kind of ink with high concentration of Ag Nano-particles (up to 80 wt%), as well as a related dispensing writing system consisting an air compressor machine and a dispenser. Additionally, we also demonstrated the writability and practical application of our proposed ink and writing system. Based on the study on the effect of sintering time and pressure, we found the optimal sintering time and pressure to obtain high quality Ag NPs wires. The electrical conductivity of nano-silver paper-based electronics has been tested using the calculated resistivity. After hot-pressure sintering at 120 °C, 25 MPa pressure for 20 minutes, the resistivity of silver NPs conductive tracks was 3.92 × 10−8 (Ωm), only 2.45 times of bulk silver. The mechanical flexibility of nano-silver paper-based electronics also has been tested. After 1000 bending cycles, the resistivity slightly increased from the initial 4.01 × 10−8 to 5.08 × 10−8 (Ωm). With this proposed ink preparation and writing system, a kind of paper-based writing electronics with ultra-low resistivity and good mechanical flexibility was achieved.

Effect of Capillary Trace on Dynamic Loop Profile Evolution in Thermosonic Wire Bonding

Thermosonic wire bonding remains the most commonly used interconnection technology in microelectronic packaging, and looping is an important aspect in modern wire bonders. To identify the loop formation mechanism, the effect of capillary trace on the standard wire looping process was studied. Dynamic looping processes with different capillary trace parameters and reverse motions of 4, 8, and 16 mil were recorded by a high-speed camera. The capillary trace and wire profile evolution were obtained from the looping videos using a digital image processing program, and the relationship between capillary trace and loop profiles was analyzed. A finite-element model was established to study the strain distribution on wire during looping. Experimental and simulation results show that the wire profile of the standard loop is mainly affected by capillary position and is not sensitive to capillary velocity. The upward capillary trace mainly affects the loop configuration, including the number, position, and deformation of kinks and the loop length. The downward capillary trace affects the stress states, loop height, kink deformation, and loop profiles. A kink is the wire with the largest local curvature, and it is a plastically deformed wire segment with little elastic core. The kink has two functions: 1) shaping the loop and 2) isolating the pulling force on the first bond and neck caused by capillary movement. This paper can be of great help in loop profile optimization in the industry and in academic research of loop dynamics.

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.

Experimental and Modeling Study of Breakup Behavior in Silicone Jet Dispensing for Light-Emitting Diode Packaging

Silicone jet dispensing has grown increasingly common in light-emitting diode (LED) packaging owing to its high speed and low cost. However, differences in breakup behavior result in different silicone jetting volumes that create challenges in maintaining dispensing volume consistency during silicone jet dispensing process; this severely affects the performance of LED packaging. To reveal the breakup behavior during the jet dispensing process, the experimental and modeling studies were conducted. First, the ultrafast breakup process was experimentally observed using a high-speed camera, which shows that this process only requires approximately 50 ms. Then, an analytical model was developed and verified through an experiment. Using this analytical model, the effects of initial disturbance amplitude, initial disturbance wavelength, and thread length on the thread breakup were studied. The simulation demonstrated that it is necessary to control the disturbance amplitude under 0.005 for volume consistency in jet dispensing; thus, a moving work piece rather than a moving nozzle is recommended in jet dispensing. A disturbance wavelength between 0.3 and 0.6 mm is beneficial for both thread breakup and high-volume consistency. Thread length has little effect on the breakup position but significant effects on the volume consistency and breakup time. A low substrate increases the volume and the consistency of thread breakup. This paper will provide a useful guide for dispenser designs of modern LED packages.

Modeling and Experimental Study of the Kink Formation Process in Wire Bonding

Looping is a key technology for the modern wire bonder. A kink is a critical structure in a wire loop. In order to understand the kink formation mechanism, a 2-D dynamic finite element model is developed using ANSYS/LS-DYNA, in which the air tension force, friction between capillary and wire, and real capillary trace are considered. The simulated kink formation process was verified by an experiment. With this model, the strain distribution on a gold wire was calculated, and the effects of wire material properties and capillary trace parameters on the kink number, position, and loop profiles were studied. Simulation results show that a minute average plastic strain of 0.14 is needed to form a distinct kink in a wire. Similarly, an elastic core with an average plastic strain of less than 0.08 at the center of a kink provides stiffness and sag/sway resistance for loops. A kink is a wire segment with plastic deformation outside and an elastic core inside, and the number of kinks and their positions are mainly affected by the capillary trace. In contrast, wire material properties only slightly influence the kink properties. This paper may provide helpful insights into loop design for modern microelectronics packages.

Ultrasound aided smooth dispensing for high viscoelastic epoxy in microelectronic packaging.

Epoxy dispensing is one of the most critical processes in microelectronic packaging. However, due its high viscoelasticity, dispensing of epoxy is extremely difficult, and a lower viscoelasticity epoxy is desired to improve the process. In this paper, a novel method is proposed to achieve a lowered viscoelastic epoxy by using ultrasound. The viscoelasticity and molecular structures of the epoxies were compared and analyzed before and after experimentation. Different factors of the ultrasonic process, including power, processing time and ultrasonic energy, were studied in this study. It is found that elasticity is more sensitive to ultrasonic processing while viscosity is little affected. Further, large power and long processing time can minimize the viscoelasticity to ideal values. Due to the reduced loss modulus and storage modulus after ultrasonic processing, smooth dispensing is demonstrated for the processed epoxy. The subsequently color temperature experiments show that ultrasonic processing will not affect LEDs lighting. It is clear that the ultrasonic processing will have good potential to aide smooth dispensing for high viscoelastic epoxy in electronic industry.

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.