38

Transactions of The Japan Institute of Electronics Packaging Vol. 6, No. 1, 2013

1. IntroductionThe development of packaging technologies such as

system-in-a-package (SiP) has greatly contributed to mak-

ing electronic devices smaller and more multifunctional.

Smaller, high-density packages require micro-interconnec-

tions between the die and substrate. Wire bonding and

flip-chip bonding are the main techniques used to make

these interconnections. Flip-chip bonding is a process in

which bumps are fabricated on a die and interconnected to

the package substrate, with the active die face down. Flip-

chip bonding has many advantages, such as high preci-

sion, high density, short interconnections, and small para-

sitic elements. However, the reliability of the

interconnections is crucial for flip-chip bonding.[1] Low-k

materials are used for insulating layers for large-scale inte-

gration, in order to reduce the signal delay; however,

because low-k materials are mechanically fragile, the bond-

ing force must be reduced. Ultrasonic vibration is com-

monly applied to the bonding head to decrease the force.

It has been reported that the structure and power of the

ultrasonic vibration horn affects the bonding state.[2]

However, failure analysis of flip-chip bonding is not easy,

because it is difficult to observe the connected area under

the face-down condition. Arai et al.[3] have reported on a

method to evaluate the bonding status using a die-pull tes-

ter, which can be used to evaluate whether the bonding

conditions are suitable for the production environment.[4]

It is known that the directions of the leads on the substrate

also affect the bonding state,[5] i.e., the bonding behavior

of longitudinal and lateral leads is different under the hori-

zontal vibration condition.

In this paper, the bonding states are investigated using

test element group (TEG) dies with gold stud-bumps.

Ultrasonic bonding is applied to substrates with longitudi-

nal and lateral leads, and the die-pull mode is investigated

systematically. The die is physically removed from the sub-

strate, and the failure mode is analyzed. The bonding

behavior with a vibrational head rotated by 45 degrees

with respect to the die configuration is compared with that

of a conventional head, and the process margin of flip-chip

bonding for both heads is discussed along with the failure

modes.

2. Experimental ProcedureThe conventional flip-chip bonding equipment has a

fixed vibration direction with respect to the die configura-

tion, as shown in Fig. 1(a). The flip-chip bonder we devel-

oped has an ultrasonic vibration head that can be rotated

in advance and operated with constant weight and fre-

quency, as shown in Fig. 1(b).

[Technical Paper]

Effect of Direction of Ultrasonic Vibration on Flip-Chip BondingMutsumi Masumoto*, Yoshiyuki Arai*,**, and Hajime Tomokage*

*Department of Electronics Engineering and Computer Science, Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan

**Research and Development Div., Toray Engineering Co., Ltd., 1-1-45 Oe, Otsu, Shiga 520-2141, Japan

(Received May 14, 2013; accepted October 29, 2013)

Abstract

Flip-chip bonding has several advantages, such as high precision, high density, short interconnections, and small parasitic

elements. However, creating reliable interconnections between chips and substrates is the key issue in flip-chip bonding.

In this work, bonding states are investigated using test element group (TEG) dies with gold stud bumps. Ultrasonic

bonding is applied to a substrate with longitudinal and lateral leads, and the die-pull mode is investigated systematically.

For conventional flip-chip bonding equipment, the die-pull test shows different bonding states for longitudinal and lateral

leads. However, we have developed a flip-chip bonder with a rotational vibration head, where the direction of the angle

of the vibration with respect to the die configuration can be changed. For a head rotated to 45 degrees, uniform bonding

is established on both the longitudinal and the lateral leads. A wide process margin for flip-chip bonding is obtained, with

a high yield.

Keywords: Flip-chip bonding, Die-pull test, Bump shear strength, Ultrasonic vibration

Copyright © The Japan Institute of Electronics Packaging

39

Masumoto et al.: Effect of Direction of Ultrasonic Vibration on Flip-Chip Bonding (2/5)

The test element group (TEG) dies used in this mea-

surement were 8.22 × 8.22 mm with 500 gold stud-bumps

per die. The pad size and pad pitch were 45 μm and 50 μm,

respectively. The substrates to be connected were FR-4

type with copper leads plated with nickel and gold. The

thickness of the nickel and gold were 0.08 ± 0.04 μm and

0.50 ± 0.25 μm, respectively. The lateral and longitudinal

lead patterns were placed equally on the substrates. The

conditions for flip-chip bonding were as follows: trigger

force 12 N (or 2.5 g per bump), bonding force 34.5 N,

bonding time 0.7 s, and amplitude of ultrasonic vibration

1.3 μm.

The reflow test was performed at 260°C, three times.

The direct-current resistance of a daisy-chain connection

between a die and a substrate was measured, and any sam-

ple which increased in resistance by more than 20% after a

reflow test was classified as not good (NG).

The die-pull test was performed using the mode count-

ing system.[3] The die was physically removed from the

substrate in the vertical direction. The pull speed was 1

mm/s, and the maximum force was 300 N. Then the pull-

off images were observed, and classified into four modes.

The different failure modes are shown schematically in

Fig. 2, along with the pull-off marks observed on the pad

and substrate sides for each mode. Mode A indicates

weaker bonding between the chip pad and stud-bump com-

pared with that between the stud-bump and lead. Mode B

is fracture of the bump, and implies strong bonding, while

mode C indicates incomplete bonding between bump and

lead. Finally, mode D is fracture between the lead and sub-

strate material. This also implies strong bonding, but

sometimes mode D also occurs because of copper delami-

nation problems when making the substrates.

The shear test was performed with a conventional shear

tester. To measure the shear strength of the bonds

between bumps and leads, the aluminum pads on a TEG

Fig. 1 Ultrasonic bonding head and die configuration: (a) conventional flip-chip equip-ment and (b) equipment with head rotated with respect to the die.

(a) (b)

Ultrasonic vibration

Fig. 2 Die-pull mode and typical photographs at pad side and substrate side: (a) mode A, (b) mode B, (c) mode C and (d) mode D.

(a) (b) (c) (d)

Pad-side

Substrate-side

(a)

TEG

(b) (c) (d)

Substrate

40

Transactions of The Japan Institute of Electronics Packaging Vol. 6, No. 1, 2013

die were chemically etched off, as shown in Fig. 3. The

etching was performed by immersing the die into a KOH

23% solution at 50°C for 40 min. We ascertained that this

chemical treatment did not affect the shear strength of the

stud-bump.

3. Results and DiscussionThe typical failures observed after the reflow tests are

shown in Fig. 4. From the resistance measurements, an

individual connection was determined as “NG”, and then

cross-sectioning was performed. The cracks usually

occurred between the die pad and bump, and between the

bump and lead. In order to obtain the distribution of bond-

failure modes on a wafer, the die-pull test was performed

with conventional flip-chip equipment. Figure 5 shows the

distribution of the bond-failure modes obtained for five

dies. The results for the longitudinal lead and the lateral

lead are shown in Fig. 5(a) and (b), respectively. Mode B

is dominant for the longitudinal lead (the proportions of

modes A, B, C and D were 2%, 96%, 0% and 2%, respec-

tively). On the other hand, the lateral lead connection usu-

ally failed in either mode A or B (the proportions of modes

A, B, C and D were 46 %, 53%, 0% and 1%, respectively), as

shown in Fig. 5(b). Mode B indicates strong bonding

between a bump and a lead. However, for the lateral lead

under this condition, there is a significant proportion of

mode A failures, meaning that excess vibrational energy

was applied to the pad during the lateral-direction

bonding.[3] Mode A is supposed to correspond to the

presence of a crack between the pad and bump after the

reflow test.

Figure 6 shows the distribution of bump shear strengths

obtained from the shear tests on the dies bonded using

conventional flip-chip bonding equipment. The mean value

and standard deviation are tabulated in Table 1. The mean

shear strength and variation in shear strength are smaller

for the longitudinal leads than for the lateral leads. The

small variation in shear strength for the longitudinal lead is

consistent with the dominance of die-pull Mode B in Fig.

5(a). For the lateral leads, on the other hand, almost half of

failures correspond to Mode A, which accounts for the

larger value of the mean bump strength, and also the

larger variation in bump strength. Although Mode A cor-

responds to a crack between a pad and a bump, the result

of Fig. 6 was obtained with the bumps etched off from the

substrate. That might be the reason why the shear

strength for the lateral leads was larger than for the longi-

tudinal ones.

We consider that the ultrasonic vibration first induces a

low bonding force; the bonding strength then reaches its

maximum value, followed by fracture, as the vibration con-

tinues. Figure 7 shows the flip-chip bonding process as a

function of bonding time. When the bonding time is short,

lead open failures occur, because of the low bonding force.

Fig. 3 Process for carrying out a bump shear test: (a) after bonding, (b) alu-minum pad etched off chemically and (c) shear test.

Substrate

TEG chip

BumpLead

)c()b()a(

Substrate

TEG chip

BumpLead

)c()b()a(

Fig. 4 Typical defects observed after reflow test: (a) crack between a pad and a bump, (b) crack between a bump and a lead.

)b()a(

20 mµ

Table 1 Shear test for conventional and 45-degree-rotated bonders.

Conventional 45 degree rotated

Longitudinal Lateral Longitudinal Lateral

Sample number

36 36 36 36

Mean value (g)

6.86 10.4 8.24 8.48

Standard deviation (g)

1.17 2.12 0.89 0.96

(a) (b)

41

Masumoto et al.: Effect of Direction of Ultrasonic Vibration on Flip-Chip Bonding (4/5)

On the other hand, excess bonding energy causes cracks

when the bonding time is too long. For the lateral leads,

the ultrasonic energy can be absorbed more easily than for

the longitudinal leads. Therefore, the bonding states

obtained for the longitudinal and lateral leads can be

explained as shown in Fig. 7. The bonding energy, which

is the product of vibration power and bonding time, is dif-

ferent between the longitudinal and the lateral leads,

which can cause a time-lag in forming firm bonds, result-

ing in a reduced process margin for effective ultrasonic

bonding.

To change the bonding energy applied by the ultrasonic

vibration, the bonding head was rotated with respect to the

die configuration. Figure 8 shows the results for die-pull

tests and Fig. 9 shows the bump shear strength distribu-

tion for a head rotated 45 degrees. The mean value and

Fig. 8 Proportions of mode A, B, C and D failures for 45-degree-rotaed flip-chip bonding: (a) longitudinal lead, (b) lateral lead.

Fig. 6 Distribution of bump shear strengths.

10

20

30

40

50

60

0 5 10 15 20 25

Inci

denc

era

tio (%

)

Bump shear strength (g)

Lateral

Longitudinal

Fig. 7 Diagram showing bonding states and times for longi-tudinal and lateral leads bonded using a conventional flip-chip bonder. Note the small process margin.

Lead open

Lead open

CrackGood

CrackGood

Longitudinal lead

Lateral lead

Bonding time (s)

Process margin

0.5 1.0 1.5 1.05

Fig. 5 Proportions of mode A, B, C and D failures for conventional flip-chip bonding: (a) longitudinal lead, (b) lateral lead.

Mode A Mode B Mode C Mode DMode A Mode B Mode C Mode D

Die pull mode

Inci

denc

e ra

tio(%

)

Inci

denc

e ra

tio(%

)

Die pull mode

(a) (b)

Mode A Mode B Mode C Mode DMode A Mode B Mode C Mode D

Die pull mode

Inci

denc

e ra

tio(%

)

Inci

denc

e ra

tio(%

)

Die pull mode

(a) (b)

Die pull mode Die pull mode

(a) (b)

Inci

denc

e ra

tio(%

)

Inci

denc

e ra

tio(%

)

100

80

60

40

20

0

100

80

60

40

20

0

Die pull mode Die pull mode

(a) (b)

Inci

denc

e ra

tio(%

)

Inci

denc

e ra

tio(%

)

100

80

60

40

20

0

100

80

60

40

20

0

(a) (b)

42

Transactions of The Japan Institute of Electronics Packaging Vol. 6, No. 1, 2013

standard deviation are tabulated in Table 1. The curves for

the longitudinal and lateral leads have the same mean

strength and variation, and the overall variation is small

compared with that obtained using conventional flip-chip

equipment. SEM images of the longitudinal and lateral

leads after the die-pull test are shown in Fig. 10. Mode B

pull-off patterns, with similar structures, were formed on

both the leads.

Figure 11 shows a schematic of the bonding status ver-

sus bonding time for a 45-degree head. The ultrasonic

energy was delivered to both longitudinal and lateral leads,

resulting in a much wider process margin for effective

bonding.

Using the 45-degree-rotated head improved the yield for

flip-chip bonding. For example, in one product, the yield

on 3,000 dies increased from 85.1% to 99.4%. Low-k Black

DiamondTM material was used for the insulating layer.

After open and function tests, dies bonded using the

45-degree-rotated head also showed high reliability in tests

such as preprocessing humidity resistance level 3, heat

cycle test 55/125°C, high temperature and humidity pres-

ervation test 110°C/85%, and high temperature self-test at

150°C.

We are still doing research on other factors such as flip

chip bonding temperature, and gold stud bump shape and

will publish the results in a next paper.

4. ConclusionUltrasonic flip-chip bonding of TEG dies with gold stud-

bumps was applied to substrates with longitudinal and lat-

eral leads, and the die-pull mode was investigated system-

atically. When conventional flip-chip bonding equipment

was used, the die-pull test showed different bonding states

for the longitudinal and lateral leads. A flip-chip bonder

with a vibration head that could be rotated with respect to

the die configuration was developed. For a 45-degree-

rotated head, uniform bonding was established on both

longitudinal and lateral leads. A wide process margin for

flip-chip bonding was obtained with a high yield.

References[1] Y. Jin, Z. Wang, and J. Chen, “Introduction to micro-

system packaging technology,” Science Press, pp.

73–83, 2011.

[2] A. Yamauchi, S. Kuwauchi, S. Sato, and S. Nakai,

“Enhancements in ultrasonic flip chip bonding to flex-

ible printed circuit substrate (in Japanese),” MES

2003 Symposium Proceedings, pp. 200–203, 2003.

[3] Y. Arai, W. Jimyung, S. Aoki, K. Imai, and Y.

Miyamoto, “Die pull tester for flip-chip bonding,”

ICEP2011 Proceedings, pp. 688–692, 2011.

[4] M. Masumoto, N. Nakanishi, and A. Okazaki,

Japanese Patent JP2010-258302A, 2011.

[5] Y. Arai, Y. Miyamoto, S. Aoki, and K. Shimatani,

“Mode counting system for die pull test,” ICEP2012

Proceedings, pp. 373–377, 2012.

Mutsumi MasumotoYoshiyuki AraiHajime Tomokage

Fig. 9 Distribution of bump shear strengths for bonds made with the 45-degree-rotated bonder.

10

20

30

40

50

60

0 5 10 15 20 25

Bump shear strength (g)

Inci

denc

era

tio (%

)

Lateral

Longitudinal

Fig. 10 Mode B pull-off patterns for the longitudinal (a) and lateral (b) leads.

Fig. 11 Diagram showing bonding states and times for longi-tudinal and lateral leads bonded using a 45-degree-rotated bonder. Note the increased process margin.

Lead open

Lead open

CrackGood

CrackGood

Longitudinal lead

Lateral lead

Bonding time(s)

Process margin

0.5 1.0 1.5 0.95 1.2

(a) (b)(a) (b)