NEW METHODS OF ULTRASONIC METAL WELDING Jiromaru TSUJINO, Tetsugi UEOKA, Ichiro WATANABE, Yusuke KIMURA, Takahiro MORI, Koichi HASEGAWA, Yuki FUJITA, Toshiyuki SHIRAKI and Mamoru MOTONAGA

Faculty of Engineering, Kanagawa University Yokohama 221, Japan

New methods of ultrasonic metal welding and characteristics of the welded specimens are studied. For welding of small specimens such as thin wire bonding, the bonding equipments designed using higher vibration frequency and complex vibration welding tips of 90, 120 and 190 kHz are significantly effective. For medium size welding specimens, complex vibration welding tips are also very effective, and one- dimensional complex vibration systems are developed to simplify the complex vibration systems. For welding of large specimens, ultrasonic butt welding methods joining thick metal specimens end to end are effective. Using the methods and large capacity vibration sources and power amplifiers of 5 to 50,100 kW, thick and large various metal plate specimens are successfully welded. Ultrasonic welding methods using two vibration systems are also very effective, but not mentioned here.

1. INTRODUCTION

Ultrasonic welding is featured that the same and different materials are possible to weld easily in short welding time, the characteristics of the weldment such as weld strength and fatigue strength are superior to the other welding methods, welding area is limited to very narrow area and being applied for joining various materials. New ultrasonic welding methods of metal materials are proposed by the author and their effectiveness is shown.

For welding of small specimens such as ultrasonic wire bonding of integrated circuits or electronic devlces, the bonding equipments using (1) higher vibration frequency3 than 40 or 60 kHz which is used in the conventional wire bonding systems, and using (2) complex vibration welding t ips14 vibrating in (a) elliptical to circular loci or (b) rectangular to square loci were proposed and have been shown that they are significantly effective. The bonding systems of 90, 120 and 190 kHz and complex vibration welding tips are designed. Using these equipments, required vibration velocity becomes smaller and required welding time shorter compare to conventional bonding systems.

For medium size welding specimens, (3) ultrasonic welding method using two vibration systems1 1-13) have been shown also effective for spot and multi-spot continuous welding same as for large specimens, and (4) complex vibration welding tips1-3) vibrating in (a) elliptical to circular loci or (b) rectangular to square loci

1051-0117/93/0000-0405 $4.00 0 1993 IEEE

were shown very effective same as bonding of thin wire and thick specimens. (5 ) One-dimensional complex vibration systems4.s) are developed to simplify these complex vibration systems and shown its effectiveness.

For welding of large and thick specimens, (6) welding method using two vibration systemsll-13) crossed at a right angle which drives the welding specimens from upper and lower sides simultaneously, and (7) ultrasonic butt welding methods-10) joining thick metal specimens end to end are effective. Using the methods and large capacity vibration sources and power amplifiers of 5 to 50, 100 kW, thick and wide plate specimens were successfully welded. Among these methods, Ultrasonic welding methods using two vibration systems (3)(6)11- 13) are not discussed here in detail.

2. ULTRASONIC WIRE BONDING USING COMPLEX AND HIGHER- FREQUENCY VIBRATION WELDING TIPS

Ultrasonic wire bonding using 60 kHz, 90 kHz and 120 kHz complex vibration welding tips which vibrate in linear to elliptical or circular and higher-frequency 190 kHz linear vibration welding tip are studied for improving welding characteristics of wire bonding system for various electronic devices. Aluminum wire specimens of 0.1 mm diameter are welded successfully using these welding equipments. The required vibration velocity of these complex vibration systems are about

120 kHz vibration Comp!ex transverse 120 kHz vibration system ( A ) vibration rod system ( B )

( 8.0 mm @ ) ( 1.2mm9) ( 8.0 mm @ )

Complex / vibration welding tip

Fig.1 Arrangement of a 120 kHz complex vibration

‘Welding specimen

ultrasonic wire bonding system.

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one-half to one-third and required vibration weld time is shorter than those for a conventional system of linear vibration locus. The required vibration velocity of a higher-frequency system is smaller than that of a lower-frequency system because of the large number of repetitions of vibration stress3).

2.1 Ultrasonic Wire Bonding Systems and welding specimens

2.1.1 Ultrasonic wire bonding systems of 60, 90, 120 and 190 kHz

The complex ultrasonic vibration welding system consists of a complex transverse vibration rod driven by two longitudinal vibration systems of the same frequency of 60 kHz, 90 kHz or 120 kHz which is crossed at a right angle and a welding frame with a static clamping pressure source.

Figure 1 shows a 120 kHz complex vibration bonding system with two longitudinal vibration driving systems crossed at a right angle. A free-free 3rd mode transverse complex vibration rod whose free edge is a welding tip is driven at a vibration loop position in two directions crossed at a right angles to each other. These vibration systems are driven simultaneously by independent controllers and amplifiers, and welding tip vibration locus shape and direction are controlled by regulating the vibration amplitude and phase difference of two driving systems. The vibration locus is changed from linear to elliptical or circular shape. The dimensions of vibration systems of 60 kHz, 90 kHz used are 15 mm in diameter and 1 .O in longitudinal wavelength. The dimensions off 120 kHz are 8 mm in diameter and 1.5 in longitudinal wavelength. Figure 2 shows a 190 kHz wire bonding system using linear vibration welding tip. The vibration systems of 190 kHz are 7 mm in diameter and 1.5 in longitudinal wavelength. The quality factor of these vibration systems is about 1000 under a no-load condition. Vibration amplitude is measured by ring-type magnetic vibration detectors installed at a loop position of the vibration systems. These complex vibration systems may consist in one-dimensional systems using complex vibration converters4.5).

2.1.2 Welding specimens Welding specimens used are 0.1-mm-diameter aluminum

wires (tensile strength=60 gf) and 1.0-mm-thick copper

190 kHz lonaitudinal vibration system ( 7.0 mm @ )

Linear vibration welding tip Fig.2 Arrangement of a 190 kHz linear vibration

1 Welding specimen

ultrasonic wire bonding system.

plates. The copper plates are cleaned and degreased by trichloroethylene and diluted hydrochloric acid. Weld strength measured is the maximum force required to break the specimen wire or welding surface in a direction perpendicular to the welding surface.

2.2 Welding characteristics of 60,90, 120 and 190 kHz linear and complex vibration bonding systems

2.2.1 Relationship between vibration velocity and welding strength of linear vibration welding tips of 60, 90 and 120 kHz

The relationship between linear velocity of 60 kHz. 90 kHz and 120 kHz welding tips and weld strength of 0.1- mm-diameter aluminum wire specimen is shown in Fig.3. The direction of the linear vibration welding tip is set parallel to the specimen wire length. Welding tip vibration velocity is altered to 1.3 m/s (peak-to-zero value), which is equal to vibration amplitudes of 3.45 pn for 60 kHz, 2.30 pn for 90 kHz and 6.9 pn for 120 kHz. Bold marks in the figures indicate that the specimens were not torn at the weldment and broken at wire specimen part under strength test.

The required vibration velocities for sufficient weld strength are about 1.1 4 s at the frequency of 60 kHz, 0.8 m/s at 90 kHz and 0.4 m/s at 120 kHz. The required velocity of 90 kHz is smaller than that of 60 kHz which is about 1.4 times that of 90 kHz. The required velocity of 120 kHz is about 1/3 of 60 kHz system. 2.2.2 Relationship between welding time and welding strength of circular vibration welding tips of 90 and 120 kHz

Figure 4 shows weld strength of aluminum wire specimens obtained from various welding times of 10 to 100 ms under the same vibration amplitude of 1.0 pn of 60 and 90 kHz circular vibration welding tips. The required welding time of the' 120 kHz complex vibration welding tip is about half that of the 90 kHz one3). Welding time required for the complex vibration systems is shorter than that of the linear system as shown in Fig.3.

110.1 mm diameter aluminum wire and I I

c 0, Y

5

22 0, C

U) c

60

40

0 s 20

- I f OArBrokenatwirepartl I

0 0.4 0.8 1.2 U '

Vibration velocity ( m/s ) Fig.3 Relationship between vibraion velocity and weld

strength of aluminum wire of 0.1 mm diameter welded by 60 kHz, 90 kHz and 120 kHz welding tips of linear vibration locus.

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2.2.3 Relationship between welding time and welding strength of linear vibration welding tips of 120 and I90 kHz

The relationship between linear velocity of 60 kHz and 90 kHz welding tips and weld strength of 0.1-mm- diameter aluminum wire specimen is shown in FigS. Clamping force used is 300 gf, which is the same as in the cases of 60 and 90 kHz. Required welding time of 190 kHz linear vibration welding system is smaller than 120 kHz and about 1/3 of 120 kHz system, although vibration velocity 0.42 m/s of 190 kHz is smaller than 0.45 4 s of 120 kHz.

2.2.4 Weld conditions of aluminum wire specimens

Relationship between weld strength and deformation of wire specimens welded by 60 kHz and 190 kHz welding tips of linear vibration locus is shown in Fig.6. Deformations of wire specimens joined by 60 kHz and 190 kHz with enough weld strength near to the wire specimen welding tips are about same, and also same as

80

- 60 5 c 40 ??

'D

c 0, v

0)

c v)

s 20

0 20 40 60 80 100 Welding time ( ms )

the deformations of the specimens welded by 90 kHz welding tips of linear and circular loci. Weld strength is maximum in the range where specimen width is 140 % to 180 % of wire specimen diameter. Excessive wire deformation damages specimens and decreases their weld strength. Weld conditions of the wire specimens of different deformations welded by 90 kHz welding tip are shown in Fig. 7. Figure 7 (b) shows sufficient deformation of specimen welded successfully~3.

A

c 0) v

'D s

80

60

40

20

0 100 150 200 250

Indentation width ( pm ) Fig.6 Relationship between indentation width and

weld strength of a 60 kHz and a 190 kHz vibration wire bonding system.

Weldment width Weldment width Weldment width Fig.4 Relationship between welding time and weld 120 % 150 O h 21 0 0%

Short Good Excessive strength of aluminum wire of 0.1 mm diameter weld condition weld condition weld condition welded by 90 kHz and 120 kHz welding tips of circular vibration locus. Fig.7 Welded conditions of aluminum wire specimens

of 0.1 mm diameter bv a 90 kHz wire bonding 80

- 60 5 2 40 ??

I!

c m v

c U)

s 20

0 Welding time ( ms )

Fig.5 Relationship between welding time and weld strength of aluminum wire specimens of 0.1 mm diameter welded by 120 kHz and 190 kHz welding tips of linear vibration locus.

- system.

3. ONE-DIMENSIONAL COMPLEX VIBRATION WELDING SYSTEM

Welding characteristics of metal plate specimens are significantly improved by ultrasonic lapped spot welding system using a complex vibration welding tip. The welding characteristics of the complex vibration

system have been shown very effective using (1) the same vibration frequency ( 27 kHz ), or (2) different frequency ( 20 kHz and 27 kHz ) complex vibration. Welding tip vibration locus shapes were controlled from linear to (1) elliptical, circular or to (2) rectangular, square by regulating vibration amplitudes and vibration phase difference of the driving vibration systems.

For simplifying the configurations of the complex vibration systems, one-dimensional complex vibration welding systems of 27 kHz are designedW.

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3.1 One-dimensional vibration converter

3.1.1 Configuration of one-dimensional longitudinal- transverse vibration converter

For simplifying the configuration of the complex vibration welding system, one-dimensional complex vibration welding systems of 27 kHz which vibrate in elliptical or circular locus are designed. Figure 8 shows an arrangement of a one-dimensional complex welding system which consists of a longitudinal vibration system using a 27 kHz bolt-clamped Langevin type PZT transducer and a stepped horn, a longitudinal to transverse vibration converter and a complex vibration rod which vibrates in longitudinal and transverse vibration modes simultaneously within near resonance frequency. A longitudinal-transverse vibration converter consists of two kind of metals of 17 mm diameter and 5 mm thickness which have different sound velocities. The converter is installed between the driving longitudinal vibration system and a complex vibration rod.

Welding tip vibration locus shape is controlled from linear to (1) elliptical, circular by regulating vibration frequency of the driving longitudinal systems.

3.1.2 Vibration characteristics of one- dimensional longitudinal-transverse vibration converter

Free admittance loops of a complex vibration system with a longitudinal to transverse converter are shown in Fig.9. It is shown that a longitudinal resonance mode at 26.7 kHz and a transverse resonance at 26.9 kHz are

mens

Fig.8 .Arrangement of a 27.kHz Ion 'tudina1.-transverse vibration converter usin a half!rrn Half ring used : Brass 5300 mls , H ~ ? S l S O m/s.

lu =26738.51 Hi! IY lmax L 0.038 ( s)

- 0.01 v) - al In= 26909.30 Hi!

0

4.01 Transverse vibration ho = 2691 5.52 Hi! IYlmax =0.0034(s)

I 0.02 t IU I 26745.78 Hi! I = 1493

Fig9 Free admittance loops of a longitudinal-transverse vibration converter.

measured. The amplitude of motional admittance of longitudinal vibration is larger than transverse one, but quality factors of these vibration modes are high as 2074 and 1493 and the both modes can be driven at large vibration amplitude simultaneously by compensating the power factor at the transverse mode.

Relationship between driving frequency, vibration amplitude and vibration phase of longitudinal and transverse vibration modes, and vibration locus shapes are shown in Fig.10. Vibration locus shape at welding tip part is almost circular where phase difference between these vibration modes is 90 degrees.

3.2 Welding characteristics of one-dimensional complex vibration system

Welding conditions of linear and complex vibration welding tips

Figure 11 shows relationship between welding time and weld strength of aluminum plate specimens of 0.5 mn thickness welded by welding tips of linear and circular loci under the same vibration amplitude of 2.0 pm. The obtained weld strengths by a welding tip of circular locus are larger than those joined by a conventional welding system whose welding tip vibration is linear.

26.65 26.70 26.75 26.80 26.85 26.90 Driving frequency ( kHz )

vibration locus shapes and phase difference between longitudinal and transverse vibrations.

Fig.10 Relationship between driving frequency,

30 10.5 mm and 0.5 mm thick pure aluminum plate specimens /

20

10

,. U ' ' I 0 2 4 6 8

Welding time ( s ) Relationship between welding time and weld strength using welding tip of linear and circular loci. Specimens : 0.5 mm thick pure aluminum plates.

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4. ULTRASONIC BUTT WELDING OF LARGE VARIOUS METAL PLATE SPECIMENS

Ultrasonic lapped spot welding is possible to weld up to 2 or 3 mm plate specimens. But, large and thick specimens are impossible to weld because the driving surface of the upper specimen can not endure the increasing driving force of the thick weldment and increasing shear displacement along the thickness of the specimen. Ultrasonic butt welding is adequate for welding thick specimens such as over 10 mm thickness.

4.1 Ultrasonic butt welding system

4.1.1 Ultrasonic butt welding system The ultrasonic butt welding system consists of one

powerful ultrasonic vibration source, an upper passive vibration system and a welding frame with hydraulic static pressure sources for clamping welding specimens between the upper and lower vibration system and for inducing static clamping pressure on a welding surface (Fig. 12). The ultrasonic vibration source consists of eight bolt-clamped Langevin-type PZT transducers of 60 mm diameter and a radial-to-longitudinal converter as a vibration power accumulator. Dimensions of the welding tip are 20 mm thickness and 52 mm width. The 15 kHz vibration source with 8 transducers used is 542 mmin diameter and 28.5 kg in weight. Vibration amplitude is measured by ring-type magnetic vibration detectors installed in various positions in vibration systems. The vibration source is driven by a 50 kW or 100 kW static induction thyristor power amplifier.

4.2 Welding characteristics of different metal plate specimens

4.2.1 Welding conditions of aluminum, copper and steel plate specimens

The relationship between vibration amplitude, input power and weld strength of 6-mm-thick aluminum, copper and steel plate specimens is shown in Fig.13. The maximum weld strength obtained is almost equal to aluminum specimen strength at a vibration amplitude of

Hvdraulic clarnDina vice

Welding plate specimen Lower of 30 mrn width welding tip

. Fig. 12 l5kHz ultrasonic butt welding system using a lower vibration source with eight BLT transducers.

about 18 pm and 15.5 to 17 pm. Thin burrs are produced on either side of the aluminum specimen. The required input power for joined aluminum, copper and steel plate specimens is about 5 kW/cm2 and 4 kW/cm2.

30 r h

E . -300 - 9 I Y VI

a, VI K a,

2 2 0 3

m - E,

u s

w - f 200

5 100

.-

Q I I

C

m 0 C 0

.II 1 0 - -0

. %

c

-

o L c

Specimen elongationI j position

- I 20 3 60 80 100 120

Weld strenqth ( MPa) Fig .14 Relationship between weld strength; measured temperature and elongation at tensile test of pure aluminum plate specimens.

x

U) U) - 80 a,

$ 4 0 t y , , (1 lAlurninum date I , #

116 Pm; 77 Mpal;/G-Welding suriace 0 1 0 8 6 4 2. 0 2 4 6 8 1 0

Specimen position from welding surface ( rnm ) Fig.15 Hardness distributions along weldment sections across welding surface of 6 mm thick (1) aluminum, (2) aluminum - copper and (3) aluminum - steel plate specimens.

I

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4.2.2 Temperature rise in welding specimens Figure 14 shows the temperature rise at a side surface of

aluminum Specimens measured by an infrared radiation thermometer. Maximum measured temperature is about 200 "C where the obtained weld strength is almost equal to the strength of aluminum. Hardness distributions along the sectional surface of the welded specimen are shown in Fig.15. Hardness decreases slightly at the welding surface by tempering effect due to temperature rise at the weldment. The temperature rise at welding surface is estimated about 480 "C at least from the measured tempering effect by an electric furnace.

4.2.3 Mechanical characteristics of welded specimens Figure 14 also shows relationship between elongation

length of welded specimens at tensile strength test and the weld strength of the same specimens. The elongation length increases as weld strength increases up to the elongation of base metal specimen. The mechanical characteristics of welded specimens are about same as . base metal specimens, and the hardness adjacent to the weldment decreases slightly by welding conditions.

4.2.4 Weld conditions of the same and different metals Figures 16 (1)(2)(3) show the cross sections of the

specimens. Burrs are produced on upper and lower welding parts of the aluminum-plate side mainly due to the higher hardness of copper or steel specimens.

Welding B U ~ Welding Weldlng .Bum\sudace - \surface Bum\surface

Alumlnum Alumlnum Copper Alumhum Steel ( 1 ) Aluminum

Fig.16 Weld conditions of 6 mm thick aluminum, copper, and steel plate specimens.

( 2 ) Aluminum and ( 3 ) Aluminum and plates copper plates steel plates

5. CONCLUSION

New ultrasonic welding methods of metal materials were proposed and their effectiveness was shown. 1) For ultrasonic bonding of small specimens such as thin wires, the bonding equipments using higher vibration frequency than 40 or 60 kHz which is used in the conventional wire bonding systems, and using complex vibration welding t ips vibrating in (a) elliptical to circular loci or (b) rectangular to square loci were proposed and it was shown they are very effective.

The bonding systems of 90, 120 and 190 kHz and complex vibration welding tips of the same vibration frequency were designed. Welding aluminum wire specimens of 0.1-mm-diameter were welded successfully by these higher-frequency complex vibration welding equipment. The deformation of the welded specimen

(specimen width at the weldment) for maximum weld strength was 140 to 180 % of wire diameter and about same in these frequencies and in the linear and complex vibration loci. The required vibration velocity becomes smaller and required welding time shorter compare to conventional bonding systems. Moreover, using these complex vibration methods, the weld strength of wire bonding becomes independent of the difference in the direction between welding tip vibration and wire length. 2) Formed ium size welding spec imeng, complex vibration welding t ips vibrating in (a) elliptical to circular loci or (b) rectangular to square loci were shown very effective same as the bonding of thin wire specimens. One-dimensional complex vibration systems were developed for simplifying the complex vibration systems, and may be applied for various purposes such as wire bonding hereafter and welding of large specimens in future. 3) Using -- ' -and large capacity vibration sources and power amplifiers of 5 to 50,100 kW, aluminum plate specimens of 6 mm thickness and 400 m width were successfully welded by shifting welding driving position with the input power of 3.5 . kW/cm2. The same and different metal specimens of aluminum, copper, and steel plate specimens of 6 nm thickness were joined successfully with the weld strength almost equal to the specimen. Temperature rise in an aluminum welding surface was estimated about 480 OC. The elongation lengths of the welded specimens at tensile test which had weld strength near to the specimen were almost equal to that of the base metal.

REFERENCES

Ultraso nic wire bondi 'ng 1) J. Tsujino, H. Furuya and Y. Murayama: Proc. IEEE 1989 Ultrasonics Symp. (IEEE, New York, 1990) pp. 1103-1 106. 2) J. Tsuiino. Y. Muravama and H. Furuva: Proc. 10th SVIIID. Ultrasonics Electronics,- Tokyo, 1989, Jpk J. Appl. Phyc 2'9

3) J. Tsujino, T. Mori and K. Hasegawa: Jpn. J. Appl. Phys. 32 (1993) pp.2435-2440, Part 1, No. 5B, May 1993

(1990) Suppl. 29-1, pp. 173-175.

.. nal complex vibration s v m

4) J. Tsujino, Y. Kimura, and M. Motonaga: Proc. National meeting of Acoustical Society of Japan, 1993.3., pp.797-798. (in Japanese) 5) J. Tsujino, T. Sbiraki, Y. Ishii, S . Kimura and T. Ueoka: Proc. National meeting of Acoustical Society of Japan, 1993.10.. pp.913-914. (in Japanese) Ultrasonic butt weldinp 6) J. Tsujino, T. Ueoka and S. Tsuboi: Proc. IEEE 1990 Ultrasonics Symp. (IEEE, New York,1991) p.371-374. 7) J.Tsujino, T.Ueoka, Y.Suzaki, K.Uchida. 1.Watanabe and A.Andoh: h c . 11th Symp. Ultrasonic Electronics, Kyoto, 1990, Jpn. J.Appl. Phys. 30 (1991) Suppl. 30-1, p. 212-215. 8) J. Tsujino, T. Ueoka and I. Watanabe: Proc. 14th Int. Conf. on Acoustics (1992) C6-4. 9) J. Tsujino, T. Ueoka, I. Watanabe, M. Ogawa. M. Hirasawa and Y. Fujita: Jpn. J. Appl. Phys. 32 (1993) pp. 2441-2446, Part 1, No. 5B. May 1993 10) J. Tsujino, T. Ueoka and I. Watanabe, M. Ogawa, M. Hirasawa and Y. Fujita: Proc. IEEE 1992 Ultrasonics Symp.(IEEE, New York,1993) p.859-866.

11) J. Tsujino, T. Ueoka and K. Kenmotsu: Proc. IEEE 1985 Ultrasonics Symp.(IEEE, New York.1986) p.557-562. 12) J. Tsujino: Proc. Ultrasonics Int. 1989 (Buttenvorths, Guildford.1990) p.346-353. 13) J. Tsujino, T. Ueoka, Y. Fujita, K. Maru and Y. Onishi : Proc. National meeting of Acoustical Society of Japan, 1993.10., pp.915-916. (in Japanese)

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