dynamics of ultrasonic transducer system for thermosonic flip chip bonding

IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGIES, VOL. 32, NO. 2, JUNE 2009 261

Dynamics of Ultrasonic Transducer System forThermosonic Flip Chip Bonding

Zhili Long, Yunxin Wu, Lei Han, and Jue Zhong

Abstract—Thermosonic flip chip bonding is used in certain finepitch IC packaging for its unique features. By using this bondingprocess in this paper, 1 mm 1 mm chip with 8 gold bumps hasbeen bonded onto a silver-coated substrate. Dynamical propertiesof transducer system, which is the key component for providingthe ultrasonic energy, have been investigated using finite elementmodel (FEM) and measurement using impedance analyzer andlaser doppler vibrometer (LDV). The simulation results show thatthe ultrasonic transducer vibrates by coupling with all excitedmodes, 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 isseverely disturbed by undesirable non-axial modes such as flexuralmodes. There are some other unwanted parasitic modes close todominant mode. Measured velocities of the transducer horn showthat the system vibrates under several modes simultaneously. Theimpedance measurements reveal additional frequencies overlap-ping the working frequency. All non-axial modes of the ultrasonictransducer disturb the bonding process and degrade the bondingquality. A subtle control is needed to obtain unique axial modeand stable vibration for high bonding quality.

Index Terms—Dynamical properties, finite element method(FEM) analysis, thermosonic flip chip bonding, ultrasonic trans-ducer.

I. INTRODUCTION

W IRE bonding and flip chip bonding are the mainmicroelectronic packaging ways to make fine-pitch

connections between the chip and the substrate. Althoughwire bonding holds over 80% in packaging market, it is ex-tremely challenging because of the demand of ultracompactand miniaturized electronic devices in the future [1], [2]. Flipchip bonding, which includes C4 (controlled collapse chip con-nection) soldering, thermosonic flip chip, thermocompressionbonding and adhesive bonding, offers an attractive solutionfor high density IOs and miniaturized devices [1]. Comparedto other equivalent processes, thermosonic flip chip bonding

Manuscript received May 02, 2006; revised November 30, 2006. Current ver-sion published July 22, 2009. This work was supported in part by the NationalScience Foundation of China under Contracts 50390064 and 50605064, in partby the Ph.D. Program Foundation of Ministry of Education of China underContract 20060533068, in part by the China Department of Science and Tech-nology Program 973 under Contract 2009CB724203, and in part by the HunanTechnology Project under Contract 2007FJ3098. This work was recommendedfor publication by Associate Editor K. Ramakrishna upon evaluation of the re-viewers’ comments.

The authors are with the Department of Mechatronics, Central South Uni-versity, 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 onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TCAPT.2009.2017380

offers several unique advantages such as cost-effectiveness,low temperature process and lead-free material. Nowadays,thermosonic flip chip bonding is used in such applicationsas MEMS, LEDs, optoelectronic modules, SAW filter, etc.[3]–[5].

Thermosonic flip chip bonding is a process where gold ballsand substrate are joined together by ultrasonic energy and heatunder certain pressure. Several methods of thermsonic flipchip bonding have been developed. An experimental platformby assembling ultrasonic generator, transducer, and bondingtool has been built to investigate this bonding process [3], [4].This platform has been successfully demonstrated bonding for64-I/O GaAs-silicon optoelectronic modules with the bondingstrength reaching up to 23 gf/bump. A novel longitudinalsystem for thermosonic flip chip, where the chip is underlongitudinal “hammering,” has also been developed [6], [7].This bonding process can overcome the planarity problem anduses a simplified bonding tool configuration. In thermosonicflip chip bonding, ultrasonic energy softens to the metal. Thetransducer plays an important role in turning electrical energyinto ultrasonic energy and then transmitting energy to bondinginterface. Its dynamics and the ultrasonic energy propagationhave been the subject of many studies. Vibrational mode of thetransducer that includes the effect of ultrasonic energy has beenused to select tool length and mass [3], [4]. In order to controlultrasonic energy efficiently, a layer of polymer has been usedto smooth the contact between the bonding tool and the chip[7]. A simple model treating mount barrel as unilateral contactfor longitudinal flip chip bonding is developed [8]. Studies onthe dynamical behavior of thermosonic flip chip transducerare scarce [9]–[12]. Extrapolation of the dynamics of wirebonding transducer to flip chip is inappropriate as the twosystems are different. With increase of I/Os for thermosonic flipchip, dynamics of transducer system, such as vibration modesand resonance frequencies, can influence directly ultrasonicenergy delivered to the bonding interface. In order to obtainfavorable bonding quality, it is necessary to control accuratelythe vibration of the flip chip transducer system.

In the present study, a thermosonic flip chip has been devel-oped. Dynamics of ultrasonic transducer have been investigatedby FEM and experiments. The results can be applied to trans-ducer design, and to understand the mechanism of thermosonicflip chip bonding.

II. EXPERIMENTAL PLATFORM

The process and equipment for thermosonic flip chip bondingare similar to the conventional wire bond system. In this study,

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262 IEEE TRANSACTIONS ON COMPONENTS AND PACKAGING TECHNOLOGIES, VOL. 32, NO. 2, JUNE 2009

Fig. 1. Ultrasonic transducer for thermosonic flip chip bonding.

an existing aluminum wire bonder has successfully been mod-ified for flip chip bonding process with following parameters:

ultrasonic power, normal pressure in the rangeand bonding time . As shown in Fig. 1, the

ultrasonic transducer consists of a PLL (Phase Locked Loop) ul-trasonic generator, piezoelectric rings, back and front slabs, anultrasonic horn and a bonding tool. The ultrasonic vibration en-ergy generated by piezoelectric rings is transmitted to the chipthrough the horn and the bonding tool. Metallurgical bond isthen formed at the interface. As the most important actuator inflip chip bonder, the dynamical properties of transducer deter-mine the bonding quality between chip and substrate.

On this system, a 1 mm 1 mm mm chip with 8 gold bumps[Fig. 2(b)] is successfully bonded to Ag substrate [Fig. 2(c)].Shear strength of the bonding interfaces has been measured byusing DAGE 4000 Tester and the results are shown in Fig. 3.The graph shows that the bonding strength reaches as high as30 gf/bump, which is much higher than the standard strengthcriterion of 5 gf/bump [4]. Table I lists corresponding bondingparameters.

III. FINITE ELEMENT MODEL

By ignoring small features, a finite element model (FEM) ofthe transducer system has been built. A spring and damper ele-ment is used to simulate frictional force between bonding tooland the chip. The spring constant is calculated with the squareroot of sum of the error (SSE) method [4], as

(1)

where and are the calculated and measured frequencies,respectively. The spring constant can be determined by min-imizing the SSE. Fixed boundary is applied to the mounting lo-cation of the ultrasonic transducer. The finite element model in-cludes 19,716 3-D structural solid elements and 38,905 nodes.The Block Lanczos method is used to compute the vibrationalmodes and frequencies.

FEM results show that there are 54 natural frequencies andvibrational modes within 100 kHz range. Axial, flexural, tor-sional, and coupling modes of vibration are observed. Whendriven by electrical signal with appropriate frequency, those vi-brational modes would be excited. (See Fig. 4.)

A. Dominant Working Vibration Mode

The axial vibration modes of transducer are the most desir-able in flip chip bonding because they cause the tip of bonding

Fig. 2. (a) Bonding model, (b) chip and (c) substrate.

Fig. 3. Bonding strength.

TABLE IBONDING PARAMETERS

tool move in a direction parallel to the chip, which can success-fully bond the chip to substrate. Therefore, these axial modes aremostly worth investigating. Results from FEM calculation showthat there are four modes of axial vibration at 24.149, 44.291,59.383, and 81.157 kHz, respectively. In thermosonic flip chip,all these axial modes can be regarded as working modes duringbonding process. Furthermore, the third axial vibration mode at

LONG et al.: DYNAMICS OF ULTRASONIC TRANSDUCER SYSTEM FOR THERMOSONIC FLIP CHIP BONDING 263

Fig. 4. 3D finite element model of ultrasonic transducer.

Fig. 5. (a) Vibration mode and (b) its corresponding displacement for the dom-inant working mode.

59.383 kHz is in a good agreement with the designed frequencyband of 58.0–61.0 kHz. It is the dominant working mode in prac-tical flip chip bonding process.

Fig. 5 shows the dominant working mode of ultrasonic trans-ducer and its relative displacement distributions. For axial ( )

TABLE IIICOMPARISONS OF AXIAL RESONANT FREQUENCIES

BETWEEN FEM AND IMPEDANCE MEASUREMENT

direction, the transducer has 3 nodes designated as node A,B, C. The length of the whole transducer is about 1.5 wave-lengths, where the piezoelectric driver and the taper horn cor-respond to 0.5 and 1 wavelength, respectively. Node B, whereaxial amplitude is zero, can be used for mounting transducerfor least loss of ultrasonic energy. Axial amplitude of trans-ducer at horn tip is amplified to provide largest possible en-ergy to the chip and bonding interface. In addition, it is foundthat the relative amplitudes of and direction are non-zero,which suggesting the presence of other excited modes besidesthe axial mode. Although amplitudes of these non-axial com-ponents are small, they will adversely effect the desirable axialvibration and degrade bonding quality. FEM results also sug-gest that when locked at about 60 kHz signal by PLL ultrasonicgenerator, the transducer system vibrates at a complex state andcouples the axial with non-axial components. The non-axial un-desirable components should be reduced or controlled.

B. Vibration Modes Near the Dominant One

Several undesirable parasitic vibrational modes close to dom-inant mode are present. As seen in Fig. 6, the parasitic modes in-clude flexural (56.422 kHz) and torsional (57.331 kHz) modes.Because these unwanted mode shapes are very close to the dom-inant mode and may be inside generator PLL frequency range,some of these modes will be excited. Once these parasiticalmodes are excited, they cause undesirable motion of the bondingtool, which can deteriorate the bonding strength. Moreover, it


Fig. 6. Undesirable vibration modes near the dominant one: (a) flexural mode(�� ) and (b) torsional mode (�� ).

TABLE IIUNDESIRABLE VIBRATION MODES AND THEIR BAD EFFECT

may destroy the bonding interface and finally result in unsuc-cessful bonding. Table II lists the bad effects of these unwantedvibrations.

IV. EXPERIMENT

A. Frequency Properties

Frequency and impedance properties of ultrasonic transducerhave been measured in the range 20 to 100 kHz 5 Hz steps usingan Agilent 4294A impedance analyzer (see Fig. 7). The resultsshow that the resonant frequencies of transducer system are verycomplicated. The sharpest resonant frequency is 60.289 kHzand is the dominant frequency locked by the PLL ultrasonic gen-erator in practical bonding. In addition, other three axial reso-nant frequencies can be found. As shown in Table III, there isa good agreement between FEM simulation and experimentalresults. The error of simulation is due to the uncertainty in ma-terial parameters and structure sizes and the simplifications inthe FEM model.

Moreover, it is found that there are a number of weaker reso-nant frequencies close to the dominant frequency. They are the

Fig. 7. Impedance characteristics of transducer system.

Fig. 8. Laser Doppler vibrometer measurement platform.

Fig. 9. Measured marked points.

frequencies of undesirable non-axial modes which act as para-sitics on the dominant mode.

B. Velocity Measurements of Transducer Horn

Velocity measurement on the transducer horn has been car-ried out using a laser doppler vibrometer. The measurementplatform is shown in Fig. 8.

At normal ultrasonic power 0.2 to 2.5 W, velocities of markedpoints “C,” “F,” and “E” of the horn tip, “A” and “K” of tool tipare detected (refer to Fig. 9), where and : axial velocities( direction); : vertical velocity ( direction); and :horizontal lateral velocities ( direction).

The ratios of , , andare referred to the effect of non-axial on axial vibration.


Fig. 10. Vibration velocities in horn tip: (a) velocities of marked points and (b)ratios of non-axial to axial vibration.

The measurement results are shown in Figs. 10 and 11. Thefollowing observations can be made:

a) Velocities of axial vibration ( and ) are dominantamong all modes. This ensures a successful bonding;

b) Velocities of non-axial components ( , and ) arenot zero. It indicates that the undesirable non-axial modeshave been excited during bonding;

c) The vertical vibrational velocity ( ) among all non-axialmodes influences significantly the axial vibration and

can reach as high as 70%;d) The LDV measurements show that during bonding

process, ultrasonic transducer vibrates in a the couplingmode, where the axial mode is dominant, but disturbed byother non-axial vibrations such as vertical flexure mode.

C. Effect of Non-Axial Vibration on Bonding Quality

The effect of non-axial vibration on bonding quality is inves-tigated by tuning the length of bonding tool. As the length ofbonding tool increases, the effect of non-axial on axial vibra-tion turns up with the effective ratio exceeding 25%. It is foundthat there is a favorable window of the length ( ) ofbonding tool for best bonding. See Fig. 12.

The experimental measurement and FEM analysis demon-strate that there are multiple vibrational modes of the ultrasonictransducer. The dominant axial component is adversely im-pacted by the non-axial modes, which in turn lowers thebonding quality. Therefore, it is important to control the

Fig. 11. Vibration velocities in tool tip: (a) velocities of marked points and (b)ratio of non-axial to axial vibration.

Fig. 12. Effect of non-axial vibration on bonding quality.

non-axial vibration modes. FEM analysis and experimentalmeasurements offer the following remedial methods.

a) Choosing the piezoelectric ring with higher and loweris an effective way to enhance axial vibration and re-

strain the non-axial ones, as the piezoelectric ring is thevibration source of transducer system;

b) It is important to choose an appropriate material for ul-trasonic propagation, and improve the design of the hornstructure with optimal length and mounting method fortransducer system;

c) Since the frequencies of the non-axial components arevery close to the frequency of the axial vibration, it is re-quired to enhance the precision of the phase locked loop


in the ultrasonic generator, which can lock accurately withthe frequency of the axial vibration.

V. MOVEMENT PROPERTIES OF THE CHIP

In thermosonic flip chip bonding, the movement properties ofthe chips are very critical because they finally cause the forma-tion of the bond. Vibration of the chip and bonding tool tip hasbeen detected by laser vibrometer. Displacement of the chip andbonding tool is calculated by root mean square (RMS) methodand the results are shown in Fig. 13.

A. Synchronized Phase

During , the chip and bonding tool vibrate synchro-nously in the initial bonding phase. It implies: (a) there is norelative movement between bonding tool and the chip, and (b)the friction force between bonding tool and chip under a certainpressure is higher than the one between gold bump and sub-strate, resulting in relative movement between gold bump andsubstrate. From (a) and (b), it can be deduced that the surfaceoxides and other forms of contamination were broken and re-moved to expose clean surfaces and provide a necessary envi-ronment for atom diffusion.

B. Separated Point

At about 5 ms, the displacement of the chip is lower thanthe displacement of tool tip. It means that bonding strength beformed initially, and the bonding interface is in a slip-stick state[17].

C. Asynchronous Phase

After 5 ms, the displacement of the chip decreases continu-ously and the relative displacement between tool and chip in-creases gradually. It suggests that atoms in both the gold andthe Ag diffuse into each other and bonding strength develop pro-gressively.

VI. SUMMARY AND CONCLUSION

In this study, the process of thermosonic flip chip was de-veloped successfully at a conventional aluminum wire bonder,and the bonding strength reached as high as 30 gf/bump. Thefinite element method and LDV measurement were used to an-alyze the dynamics of ultrasonic transducer. The FEM analysisshows that transducer system works under its third axial vibra-tion mode including 1.5 wavelengths and 3 nodes. In the axialmode, the vibration amplitude is amplified and the ultrasonicenergy is concentrated to the bonding interface. However, thisaxial mode is disturbed by other non-axial components such asflexural vibration. There are some other undesirable vibrationsparasitizing closely to the axial mode. From experiment, it isfound that the bonding quality is degraded when the ratio ofnon-axial vibration increase, especially over 25%. The measure-ment from impedance analyzer and LDV are in good agreementwith the results of FEM analysis. The movement details of thechip are observed and can be divided into two phases. Some sug-gestions for controlling the pure axial vibration of the transducerare put forward. The research results can be helpful to control

Fig. 13. Movement of the (a) chip and (b) bonding tool tip. (c) Movement de-tail, (d) RMS displacement.

the bonding process, design transducer system, and understandthe bonding mechanism of thermosonic flip chip.


ACKNOWLEDGMENT

The authors would like to thank the reviewers for their valu-able comments on the paper.

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[15] W. T. Thomson, Theory of Vibration with Application. EnglewoodCliffs, NJ: Prentice-Hall, 1981.

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Zhili Long received the B.S., M.S., and Ph.D. de-grees from the Central South University, Changsha,China, in 2000, 2002, and 2007, respectively, all inmechatronics engineering.

His research interests are ultrasonic packaging andthe design of piezoelectric actuator.

Yunxin Wu received the B.S. and M.S. degrees inmechanical engineering from Central South Univer-sity, Changsha, China and the Ph.D. degree from theFaculte Polytechnique de Mons, Belgium, in 1999.

He is currently a Professor at Central South Uni-versity. His main research interests are dynamicalmodel and control of mechanical engineering.

Lei Han received the B.S., M.S., and Ph.D. degreesfrom the University of Science and Technology ofChina, in 1982, 1984, and 1989, respectively.

During 1991 to 1995 and 2000 to 2003, heworked as a Research Associate at Oregon StateUniversity, Lehigh University, the State Universityof New York at Stony Brook, and Case WesternReserve University. He is currently a Professor atCentral South University, Changsha, China. Hisresearch interests include experimental mechanics,smart structures, wavelet analysis and electronics

packaging reliability.

Jue Zhong graduated from Beijing University of Science and Technology,China, in 1958.

She is currently a Professor at Central South University, Changsha, China.Her research interests include industrial and microelectronics packagingequipment.

Prof. Zhong is a Fellow of the Chinese Academy of Engineering.

dynamics of ultrasonic transducer system for thermosonic flip chip bonding

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