Ultrasonic Vibration at Thermosonic Flip-Chip Bonding ... - IEEE Xplore

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IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 1, NO. 6, JUNE 2011

Ultrasonic Vibration at Thermosonic Flip-Chip Bonding Interface Fuliang Wang, Yun Chen, and Lei Han

Abstract— Thermosonic flip-chip (TSFC) bonding is a developing microelectronic packaging technology. To provide clear understanding of bonding process and bonding mechanism, the ultrasonic vibration at bonding interface was studied. The TSFC bonding was performed on a lab bonder, the ultrasonic vibration of tool and chip were measured by using a laser doppler vibrometer, and the effect of bonding force on chip vibration velocity harmonics was analyzed. Experimental results show that the bonding strength forming process can be divided into four stages. The abrupt “amplitude dropping” of chip vibration was observed, and could be considered as a sign of bonding strength formed. The bonding strength is formed after the bonding starts 6–8 ms. This moment can be indicated by a peak of fundamental and a ramp-up of third harmonics of chip vibration velocity. The results also show that relatively small bonding force was good for forming initial bonding strength. Then the gradually increased bonding force loading is thought to be more suitable for better bonding strength and reliability. Index Terms— Bonding process, laser doppler vibrometer, thermosonic flip-chip bonding, ultrasonic vibration, vibration measurement.

I. I NTRODUCTION

T

HERMOSONIC FLIP-CHIP (TSFC) bonding is a developing microelectronic packaging technology, which offers a snap, simpler assembly process in reducing the bond temperature, pressure and time. It also provides strong metallurgical joining, which is considered to be more reliable than conductive adhesive and comparable to solder interconnection, and has been used in light emitting diode and surface acoustic wave filter package applications [1], [2]. Experimental studies have been carried out to find key factors that affect the TSFC bonding performance. Timothy has developed a transverse TSFC bonder to assemble the chip with 64 gold bumps to gold pads. They found that a bonding process was affected by chip size and process parameters,

Manuscript received September 1, 2010; accepted March 15, 2011. Date of current version June 10, 2011. This work was supported in part by the Department of Science and Technology Program 973 under Contract 2009CB724203, the Natural Science Foundation of China under Contract 50705098 and Contract 50975292, and Program for Changjiang Scholars and Innovative Research Team in University under Grant IRT0549. Recommended for publication by Associate Editor C. Basaran upon evaluation of reviewers’ comments. The authors are with the School of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China. They are also with the State Key Laboratory of High Performance and Complex Manufacturing, Changsha 410083, China (e-mail: [email protected]; [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TCPMT.2011.2138703

such as bonding force, time, temperature, ultrasonic power and co-planarity [3], [4]. To overcome the planarity problem in transverse bonding system, Tan developed a longitudinal TSFC bonder and polymer was placed between tool and chip, the effects of polymer layer thickness and Yong’s modulus were studied by simulation and experiment [5], [6]. A model was deduced by Kang to study the tool configuration and their ultrasonic amplitude distribution [7], [8]. Bonding process was optimized by Luk, Yatsuda, Tomioka, and Cheah, respectively [1], [2], [9], [10], and the effect of bonding time, force, ultrasonic power, pad thickness and loading profiles on the shear strength of assembled flip-chip die were obtained. Although the results in these studies, are not fully consistent for their bonding system configuration, are different, but all studies show that the TSFC bonding is a complex process affected by many interacted factors. Currently, the industry heavily relies on experiments to establish a process window. This trialand-error process is time consuming and cost ineffective. It is important to fully understand the dynamic characteristics of the TSFC bonding process to obtain the optimum process parameters. Ultrasonic vibration and bonding force are two most important factors affecting bonding strength and yield. The ultrasonic vibration is generated by transducer, and propagated through tool and chip to bump/pad interface to form bonding strength. The ultrasonic propagation along that multilayer contracture is a complex dynamics process, which is also affected by the bonding force. To understand how the vibration affects the bonding strength formation, when the bonding starts to form, what is the interact effects among ultrasonic vibration, bonding force and bonding formation, it is needed to measure the ultrasonic vibration details at TSFC bonding interfaces. However, since the tiny flip-chip is only 0.3 mm height, and vibrated at the frequency of about 56 kHz during bonding, there is still no such experiment study. In this paper, the TSFC bonding system was established with a lab bonder. The ultrasonic vibration of tool and chip was monitored by a laser doppler vibration (LDV), the vibration propagation at the tool/chip interface, the bonding strength formation process and the effect of bonding force on bonding process were discussed. II. E XPERIMENT A. Configuration of TSFC Bonder and Bonding Process The TSFC bonding was performed with a lab TSFC bonder, as shown in Fig. 1. It consists of ultrasonic generator

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WANG et al.: ULTRASONIC VIBRATION AT TSFC BONDING INTERFACE

Bonding force

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PZT driver

Transducer

Horn

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Tool Trigger system

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Fig. 1.

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Transducer system of lab TSFC bonder. Fig. 3.

Schematic of laser doppler vibrometer measure system.

Bonding force

Vacuum Ultrasonic vibration Bonding tool

Flip-chip Pad

Gold bump Substrate Heat stage

Fig. 2. Details of TSFC bonding interfaces: tool/chip and bump/pad interface.

(USG), sandwich ultrasonic transducer (including piezoelectric (PZT) driver and horn, with the working frequency of about 56 kHz), bonding tool and heat stage. The whole bonding system was assembled on a spring beam, and the bonding force was controlled by a strain gage on the beam (not show in this figure). The ultrasonic vibration is generated by PZT driver under the excitation of USG, propagated through the horn, bonding tool, and applied at the bonding interface finally. The ultrasonic amplitude was mainly decided by drive voltage of USG, bonding force and states of bonding interface. The details of TSFC bonding interfaces are shown in Fig. 2. It consists of a bonding tool, silicon flip-chip, copper substrate and heat stage. In this paper, the 1 × 1 mm flip-chip containing eight peripheral Au bumps are employed. Bumps are prepared by using a gold wire bonder with 2 mil (50 µm) gold wire, resulted in approximately spherical bump of 80 µm and 27 µm in diameter and height, respectively. On the copper substrate, silver coated pads (10 µm in thickness) are fabricated facing up, corresponding to the chip bumps. There are two interfaces, tool/chip interface and bump/pad interface. As the applying of bonding force, friction force appears at the tool/chip interface when ultrasonic vibration acts on a tool, which drives the chip vibrated with the tool along a setting direction (parallel to the bump/pad interface), and the ultrasonic energy is propagated from tool to chip and to the bumps/pads interface. During this vibration propagation process, bumps and pads are welded together to form bonding

strength. As the constraint between bonding tool and chip plays an important role on the ultrasonic vibration propagation, the vacuum of the tool has been closed during bonding to avoid the constrain between them, but it is still used for pick and place the flip-chip. The TSFC bonding began with the substrate clamped on a heated stage with the temperature of 163 °C. The flipchip was sucked by tool and transferred to the substrate, with bumps aligned to the substrate pads. Tool was moved down, when chip contacted with substrate, the vacuum was closed to release the chip and bonding force was applied through tool simultaneously. When applied force reached the predetermined value, ultrasonic vibration was turned on to bond the bumps with pads. B. Ultrasonic Vibration Measurement To study the ultrasonic vibration at the bonding interface and describe the multilayer dynamics of them, a Polytec PSV400-M2 LDV measurement system was used to monitor the ultrasonic vibration of tool and flip-chip during TSFC bonding. The schematic of LDV measure system is shown in Fig. 3. The LDV consists of a LDV sensor head (PSV-400-M2) and a vibrometer controller. The head uses Helium–Neon laser and sends a monochromatic laser beam (wavelength is 633 nm) toward the target and collects the reflected radiation. According to the Doppler effect, the wavelength change of the reflected radiation is a function of the targeted object’s relative velocity. Thus, the velocity of the object is measured by the wavelength changing of reflected laser light. The vibrometer controller receives and decodes the output signals of head, and represents signals as the voltage in direct proportion to the target relative velocity. The LDV covers vibration frequency up to 1 MHz and vibration velocities up to 10 m/s. The measurement accuracy is several nanometers. While the typical vibration frequency of tool or chip is 56–60 kHz, and the vibration velocity is less than 1 m/s, so the vibration measurements of tool and chip by LDV are reliable. Additionally, a trigger device was developed to synchronize TSFC bonder and LDV system. The vibration measurement was started with the laser beam of LDV focused on and aligned normally to the lateral side of chip or tool tip with the guidance of CCD camera, as shown in Figs. 4 and 5. The LDV was set as pre-trigger mode with

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TABLE I TSFC B ONDING PARAMETERS I/O count Bonding force Substrate temperature Ultrasonic frequency Ultrasonic power Bonding time

Laser beam

Laser beam

8 bumps 30 g/bump 163 °C 56 kHz 2–3 W 22.5 ms

Bonding tool

Flip-chip Substrate

Fig. 5. Focus the laser beam on the transducer with CCD camera guidance.

Heat stage

III. R ESULTS AND D ISCUSSIONS A. Ultrasonic Vibration of Tool/Chip and Bonding Formation Process As confirmed by large number of repeats, a typical ultrasonic vibration at bonding interface consists of four stages, as shown in Fig. 6. At the phase lock stage (about 0.3–1.8 ms), the USG detects and locks the resonant frequency of transducer system (consists of transducer, tool, chip, and bonding interface), as the interfaces contact states are changed for each bonding device. At this stage, the transducer works in a forced

tool amplitude

Phase lock stage

a suitable trigger level of vibration signals, such as 0.01 m/s, to avoid trigger by noise. When bonding started, USG excited the transducer and trigged the LDV controller to record the vibration of chip and tool. The LDV has only one sensor head, so the vibration of tool and chip was measured separately. The chip vibration is hard to observe, for the laser point diameter is 0.2–0.3 mm, while the chip height is 0.3 mm. The laser beam has to be carefully aligned when measuring chip vibration, and repeated experiments in each parameter group (on the same condition, with new chip and substrate) were carried out to confirm the vibration of tool and chip. Moreover, the storage depth of LDV is limited by software. Therefore, two kinds of vibration measurements were carried out, recording 32 ms with the sample frequency of 5.12 MHz, or recording 128 ms with the sample frequency of 1.28 MHz. The TSFC bonding were carried out successfully with parameters set as in Table I, and the die-shear-force test resulted in an average strength of 20–30 g/bump (assembled parts without underfilling), which was acceptable for industry. With the above mentioned system and method, the vibration velocity of tool and chip are measured and the vibration amplitude is calculated, typically as shown in Fig. 6. The vibration looks like filled areas because each figure consists of 163 840 points (about 1916 ultrasonic cycles).

2 1.6 1.2 0.8 0.4 0 −0.4 −0.8 −1.2 −1.6 −2 1.6 1.2 0.8 0.4 0 −0.4 −0.8 −1.2 −1.6 −2 0

Bonding forming stage

Align the laser beam normal to the lateral side of tool and chip. Ultrasonic amplitude/µm

Fig. 4.

Bonding end stage

Bonding increase stage

chip amplitude 2.5

5

7.5

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12.5 15 17.5 20 Bonding time/ms

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25

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32

Fig. 6. Typical vibration amplitude of tool and chip during bonding process.

vibration mode. Therefore, the amplitude of tool and chip are small (< 0.2 µm), which is inadequate for bonding. After the resonant frequency is being locked, the transducer amplitude quickly increases to 1.4 µm in 5 ms. The amplitude of chip is always the same as that of the tool, which means the tool/chip interface has no relative movement and the ultrasonic vibration has been propagated to the bump/pad interface, which causes rubbing between bumps and pads, breaking surface oxides, exposing fresh surface and forming microwelds at the bump/pad interface [11]. Once the bonding forms, the chip is welded to substrate and will not move with tool, therefore, the chip amplitude abruptly decreases from 1.4 to 0.7 µm in 0.1 ms, as shown in Fig. 6. That was not often the case with a very strong amplitude dropping, but the frequency of its appearance was unbelievably high. It is still unclear whether it is associated with the surface cleaning and whether it is indispensable to a good flip-chip bonding or not. But one can make sure that the “amplitude dropping” represents a bonding strength formed between bumps and pads. This is the bonding forming stage (about 1.8–7.5 ms). According to the chip amplitude decrease velocity, it can be found that the bonding is formed suddenly (within 0.1 ms), instead of gradually, the earlier long vibration is the preparation for this sudden bonding.


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1.00 KX

KYKY-2800

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(b)

(c)

(d)

(e)

(f)

(g)

(h)

0°

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Fig. 7. Crack on the chip back caused by relative movement between tool and chip.

chip tool tip

1000 0 −1000 2.8

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2.82

2.83 2.84 Bonding time/ms

2.85

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1000 0 −1000 8.1

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8.12

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Fig. 8. (a) Vibration amplitude details of tool and chip at the bonding forming stage. (b) Vibration amplitude details of tool and chip at the bonding increase stage.

The continued ultrasonic vibration increases the bonding strength as the die shear strength increases with the bonding time. At this bonding increase stage (about 7.5–22 ms), only part of the ultrasonic vibration has been propagated to chip, the amplitude of chip is about 1.0 µm, which is adequate for bonding. The relative movement between tool and chip causes crack in the silicon chip, as shown in Fig. 7. At the bonding end stage (about 22–32 ms), the USG is switched off, the tool and chip are vibrated by inertia, and the vibration vanishes slowly as the damping consumes the kinetic energy gradually. B. Vibration Propagation from Tool to Chip The vibration amplitude details of tool and chip are shown in Fig. 8. At the bonding forming stage (before 7.5 ms), the vibration of tool and chip are sinusoidal, and are nearly the same, which shows that the tool/chip interface has no relative

Fig. 9. Vibration states of tool, chip, and bumps at an ultrasonic cycle (the bonding increase stage). (a) Origin position. (b) Sticking and moving forward simultaneously, gold bump is deformed by the chip displacement. (c) Sliding on each other. (d) Sticking and moving backward simultaneously. (e) Moving back to origin position. (f) Passing origin position, gold bump is deformed in opposite direction. (g) Sliding. (h) Sticking and moving simultaneously again.

movement, all ultrasonic vibrations have been propagated to the chip. After the bonding is formed (after 7.5 ms), the sinusoidal vibration of chip becomes cropped. Relative movement occurs at tool/chip interface, and a part of ultrasonic energy is consumed at tool/chip interface instead of propagating to bump/pad interface, which is unwilling for TSFC bonding for causing silicon damage, as shown in Fig. 7, and ultrasonic energy wasting. According to Fig. 8(b), the vibration of tool, chip, and bumps at an ultrasonic cycle can be illustrated as Fig. 9. The arrows indicate vibration directions of chip and tool, inclined lines indicate elastic and plastic deformation of bump. The substrate is clamped and will not move. The chip and tool: 1) at origin position; 2) sticking and moving forward simultaneously, gold bump is deformed by the chip displacement; 3) sliding on each other; 4) sticking and moving backward simultaneously; 5) moving back to origin position; 6) passing origin position, gold bump is deformed in opposite direction; 7) sliding; and 8) sticking and moving simultaneously again. By the alternation of sticking and sliding process, the


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Vibration Velocity v/mm/s

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(a)

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(b)

(b)

C. Ultrasonic Vibration Frequency Spectrum Decreasing the sample frequency of LDV to 1.28 MHz, the vibration can be recorded as long as 128 ms. Increasing the bonding time to 128 ms, and keep the other parameters as mentioned in Table I, the typical vibration velocity of tool and chip is obtained, as shown in Fig. 10. It shows the similar ultrasonic vibration propagation process as in Fig. 6. Chip velocity is always the same as that of the tool at the bonding forming stage (0–7 ms), which suggests interfacial rubbing mainly at the bump/pad interface. At the end of bonding forming stage (6–8 ms), the velocity of the chip slowed down abruptly, bonding strength at bump/pad interface is produced at this moment. This moment is several microseconds after ultrasonic start-up in all cases. The bonding increase stage has been extended, and bonding end stage is also about 10 ms. The frequency spectrum of vibration velocity is calculated with the 1-D fast Fourier transform. As shown in Fig. 11, the tool vibration is nearly a pure sinusoidal with the fundamental frequency of 56 kHz, while the chip vibration is composed of fundamental frequency, second, third, fourth, and fifth harmonics, and the third harmonic is greater than the second, the fifth harmonic is greater than the fourth. The fundamental, third and fifth harmonics of the chip vibration velocity is obtained and the root mean square (RMS) value of them is shown in Fig. 12. The fundamental (56 kHz) is the main component. It emerges as bonding starts, reaches peak at the end of bonding forming stage, and then decrease slowly to a lower and steady level until the end of bonding. The third and fifth harmonics emerge at the end of bonding forming stage, and keep in a steady level until the end of bonding. The peak of the fundamental and the emergence

Vibration Velocity (m/s)

ultrasonic energy is propagated to chip and consumed by the bump/pad interface. The cycled deformation of bump also caused dislocation multiplication and atom diffusion at the bonding interface, as mentioned in our earlier paper [12].

Fig. 11. (a) Frequency spectrum of tool vibration velocity. (b) Frequency spectrum of chip vibration velocity.

0.2 0.15

56 kHz 168 kHz

0.1

280 kHz

0.05 0

0

20

40

60 80 100 Bonding time (ms)

120

140

Fig. 12. RMS of the fundamental, third and fifth harmonic of the chip vibration velocity.


Fig. 10. (a) Typical vibration velocity of chip. (b) Typical vibration velocity of tool.

240 g 0.2 180 g

0.1

120 g 0

0

5

10 Bonding time (ms)

15

20

Fig. 13. Effect of bonding force on fundamental harmonic of chip vibration velocity.

of the third harmonics of chip velocity appear at the end of bonding forming stage, which can be used as a direct experimental criterion sensitive to an available strength in bump/pad interface. This moment is just several microseconds after ultrasonic start-up. D. Effect of Bonding Force on the Chip Vibration Harmonics To study the interact effects among ultrasonic vibration, bonding force and bonding formation, the chip vibrations were measured in three groups of experiments, where the bonding temperature, ultrasonic power and bonding time are 163 °C, 2.8 W, and 120 ms, respectively, while the bonding force is set as 120, 180, and 240 g, and the RMS of the fundamental and third harmonic of the chip vibration velocity is calculated.



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0.06 0.04 120 g 0.02 180 g 0

0

240 g

5

10 Bonding time (ms)

15

20

Fig. 14. Effect of bonding force on third harmonic of chip vibration velocity.

The effect of bonding force on fundamental harmonic of chip vibration is shown in Fig. 13 (only the first 18 ms are plotted in order to view the details). It shows that the fundamental harmonic increases with the increase in bonding force, and the change pattern are similar. The fundamental harmonic peak appears much earlier in small bonding force (120 g) condition than that in bigger force (240 g) condition, which means the bonding strength is easier to form in a small bonding force condition. The reasons for this maybe that small bonding force causes small friction force at bump/pad interface, which made the relative movement easier, erased the oxides layer and formed initial bonding strength more quickly. Small bonding force also causes small friction force at chip/tool interface, which is easier to be overcome by the initial formed bonding strength. The effect of bonding force on third harmonic of chip vibration is shown in Fig. 14. It shows that the third harmonic abruptly ramps up at the end of the bonding forming stage, the “ramp-up” comes earlier at small bonding force (120 g) and delays at big force (240 g) condition. This also means that small bonding force is essential for forming initial bonding strength. But small bonding force also results in small die shear strength for small contact bonding area. It was reported that the gradually increased bonding force loading can enhance the bonding strength and improve the bonding reliability [10]. A reason of it may be the initial small bonding force resulted earlier bonding formation and the latter large bonding force increases the contact bonding area. Therefore, the gradually increased bonding force loading is thought to be more suitable for better bonding strength and reliability. IV. C ONCLUSION The TSFC bonding was realized with a lab bonder, and the ultrasonic vibration of tool and chip during TSFC bonding process are measured with a LDV. With the chip and tool vibration signals, the ultrasonic vibration at the bonding interface and the bonding formation process was studied. 1) According to the ultrasonic vibration at bonding interface, the bonding process consists of four stages, phase lock stage, bonding forming stage, bonding increase stage, and bonding end stage. 2) The “amplitude dropping” is observed at the end of bonding forming stage, which represents a bonding strength formed at the bump/pad interface. According to the “amplitude dropping” of chip velocity, it is found that the bonding is formed suddenly (within 0.1 ms), instead of gradually. The bonding strength is formed

after the bonding starts 6–8 ms, and the earlier long vibration is the preparation for the sudden bonding. 3) At the bonding increase stage, the tool and chip works on a sticking/sliding state, only a part of ultrasonic energy is used for bump cycled deformation and bonding strength increasing. 4) The peak of fundamental and emergence of the third harmonics of chip velocity is a direct experimental criterion sensitive to an available strength in bump/pad interface. 5) The effect of bonding force on harmonics of chip vibration shows that small bonding force is good for forming initial bonding strength. The gradually increased bonding force loading is thought to be helpful for improving TSFC bonding strength and reliability. R EFERENCES [1] C. F. Luk, Y. C. Chan, and K. C. Hung, “Development of gold to gold interconnection flip chip bonding for chip on suspension assemblies,” Microelectron. Rel., vol. 42, no. 3, pp. 381–389, Mar. 2002. [2] T. Taizo, I. Tomohiro, and M. Ikuo, “Thermosonic flip-chip bonding for SAW filter,” Microelectron. Rel., vol. 44, no. 1, pp. 149–154, Jan. 2004. [3] T. S. McLaren and Y. C. Lee, “Modeling and evaluation criterion for thermocompression flip-chip bonding,” IEEE Trans. Adv. Packag., vol. 23, no. 4, pp. 652–660, Nov. 2000. [4] T. S. McLaren, S. Y. Kang, and W. Zhang, “Thermosonic bonding of an optical transceiver based on an 8 × 8 vertical cavity surface emitting laser array,” IEEE Trans. Comp., Packag., Manuf. Technol. Part B: Adv. Packag., vol. 20, no. 2, pp. 152–160, May 1997. [5] Q. Tan, W. Zhang, B. Schaible, L. J. Bond, T.-H. Ju, and Y.-C. Lee, “Thermosonic flip-chip bonding using longitudinal ultrasonic vibration,” IEEE Trans. Comp., Packag., Manuf. Technol. Part B: Adv. Packag., vol. 21, no. 1, pp. 53–58, Feb. 1998. [6] Q. Tan, B. Schaible, L. J. Bond, and Y.-C. Lee, “Thermosonic flip-chip bonding system with a self-planarization feature using polymer,” IEEE Trans. Adv. Packag., vol. 22, no. 3, pp. 468–475, Aug. 1999. [7] S.-Y. Kang, P. M. Williams, and Y.-C. Lee, “Modeling and experimental studies on thermosonic flip-chip bonding,” IEEE Trans. Comp., Packag., Manuf. Technol. Part B: Adv. Packag., vol. 18, no. 4, pp. 728–733, Nov. 1995. [8] S.-Y. Kang, P. M. Williams, T. S. McLaren, and Y.-C. Lee, “Studies of thermosonic bonding for flip-chip assembly,” Mater. Chem. Phys., vol. 42, no. 1, pp. 31–37, Oct. 1995. [9] H. Yatsuda, T. Horishima, T. Eimura, and T. Ooiwa, “Miniaturized SAW filters using a flip-chip technique,” IEEE Trans. Ultrason., Ferroelectr., Freq. Control, vol. 43, no. 1, pp. 125–130, Jan. 1996. [10] L. K. Cheah, Y. M. Tan, J. Wei, and C. K. Wong, “Gold to gold thermosonic flip-chip bonding,” Proc. SPIE High-Density Interconn. Syst. Packag., vol. 4428, pp. 165–170, Apr. 2001. [11] G. Harman, Wire Bonding in Microelectronics: Materials, Processes, Reliability, and Yield, 2nd ed. New York: McGraw-Hill, 1997. [12] F. Wang, L. Han, and J. Zhong, “Stress-induced atom diffusion at thermosonic flip chip bonding interface,” Sens. Actuat. A: Phys., vol. 149, no. 1, pp. 100–105, Jan. 2009.

Fuliang Wang received the B.S., M.S., and Ph.D. degrees in mechanical engineering from Central South University, Changsha, China, in 2001, 2003, and 2007, respectively. He is currently an Associate Professor at Central South University. His current research interests include microelectronics packaging processes, equipment, and reliability.

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Yun Chen received the B.S. degree in mechanical engineering from Central South University, Changsha, China, in 2009. He is currently pursuing the Ph.D. degree at Central South University. His current research interests include electronics packaging process and finite element simulation.

Lei Han received the B.S., M.S., and Ph.D. degrees from the University of Science and Technology of China, Hefei, China, in 1982, 1984, and 1989, respectively. He worked as a Research Associate at Oregon State University, Corvallis, Lehigh University, Bethlehem, PA, the State University of New York at Stony Brook, Stony Brook, and Case Western Reserve University, Cleveland, OH, from 1991 to 1995 and from 2000 to 2003. He is currently a Professor at Central South University, Changsha, China. His current research interests include experimental mechanics, smart structures, wavelet analysis, and electronics.