a study of the effect of indium filler metal on the … koyama et al.: a study of the effect of...

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Koyama et al.: A Study of the Effect of Indium Filler Metal (1/6)

1. IntroductionIn recent years, the applicability of low-temperature

bonding in industrial processes has been explored in an

effort to miniaturize electronic equipment. However, low-

temperature bonding has been difficult to achieve because

of the presence of oxide films at the joint boundaries,

which inhibits the proper bonding of electrode materials

such as Cu and Sn. Although these oxide films can be

removed effectively via ultrasonic welding,[1–6] surface

activated bonding,[7–12] and the metal salt generation

bonding method,[13] it is still difficult to fill the gaps

between the bonding surfaces using plastic deformation

alone during the low-temperature bonding process.

The liquid-phase diffusion bonding method using a filler

metal has been used for large materials with the purpose

of filling the gap between the bonding surfaces and

increasing the bond strength.[14–20] However, there have

been few reports on the application of the method to elec-

tronic packaging. The intention of this study was the appli-

cation of the liquid phase diffusion bonding method to

minute junctions such as electrodes. Liquid phase diffu-

sion bonding has the advantage that the bonding tempera-

ture can be decreased, and the melting point of the joint

increases after solidification, creating a joint that is harder

to melt. This occurs because the filler metal diffuses into

the base metal and causes isothermal solidification of the

base metal. In addition, attention is necessary to melt at

the eutectic temperature or higher again when the bond-

ing process is finished without completing the isothermal

solidification. In addition, the low bonding temperature

also enables the creation of a high-strength joint without

causing the growth of an intermetallic compound layer. To

achieve low-temperature bonding, In is used as the filler

metal for the joint because when In is mixed with other

metals, various kinds of reactions occur. For example, the

eutectic reaction occurs and lowers the melting point of

the materials, which becomes 427 K when it is mixed with

Cu and 393 K when it is mixed with Sn. After liquid phase

[Technical Paper]

A Study of the Effect of Indium Filler Metal on the Bonding

Strength of Copper and TinShinji Koyama, Seng Keat Ting, Yukinari Aoki, and Ikuo Shohji

Faculty of Science and Technology, Division of Mechanical Science and Technology, Gunma University, 1-5-1 Tenjin-cho, Kiryu City, Gunma

376-8515, Japan

(Received August 21, 2013; accepted November 18, 2013)

Abstract

In was deposited onto the joint interface of a Cu/Sn joint to examine the effect of the filler metal on the tensile strength

of the joint and on the reduction of the bonding temperature. Observations were carried out using Scanning Electron

Microscopy (SEM) to observe the interfacial microstructures and fracture surfaces. After the In filler was deposited onto

the joint surface, liquid-phase diffusion bonding was carried out. Compared to a specimen that did not have the In filler,

a specimen with the In filler could be joined with an approximately 30% lower degree of deformation and at a temperature

that was approximately 30 K lower. At the same time, the achieved tensile strength was comparable to that of the base

metal on which the In was deposited. Two possible reasons were considered for the increase in the tensile strength. First,

when heated to 393 K, In and Sn react with each other and turn into a liquid phase. This layer of liquid then covers the

entire bonding surface and increases the area of contact between the bonding surfaces, which in turn causes the increase

in the bonding strength. Second, because In is easier to oxidize than Cu and Sn, the metallic In becomes In oxide, which

is formed by the In taking oxygen from the native oxide films of the Cu and Sn. Subsequently, the intimate contact that

is achieved between the metallic Cu and metallic Sn increases the bonding strength.

Keywords: Copper, Tin, In, Bonding Strength, Filler Metal, Liquid Phase Diffusion Bonding, Intermetallic Com-

pound

Copyright © The Japan Institute of Electronics Packaging

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

diffusion bonding, observations of the interfacial micro-

structures and fracture surfaces of the joint are carried out

using SEM.

2. Experimental DetailsThe Sn specimen used in this study was a 15 mm × 15

mm × 5 mm block with 99.9% purity. As shown in Fig. 1,

this block was joined to a Cu block (15 mm × 15 mm × 10

mm) to make it easy to perform tensile tests. To remove

the smear metal and the work straining layer, the bonding

surface of the Sn was first polished with emery paper fol-

lowed by electrolytic polishing using a solution composed

of 5 vol% perchloric acid, 10 vol% ethyl glycol monobutyl

ether, and 85 vol% ethyl alcohol. The surface roughness Ra

of the bonding surface was 0.33 µm, and the In was depos-

ited onto the bonding surface as a filler metal to a surface

thickness of 300–700 nm using a vacuum deposition appa-

ratus (JEOL JEE-4X, Japan). The Cu specimen used in this

study was a 15 mm × 15 mm × 10 mm block with 99.9%

purity. To match the roughness of the commonly available

Cu electrode pad (0.07 µm), the bonding surface of the Cu

was polished with emery paper (#4000).

The liquid phase diffusion bonding was carried out in a

vacuum chamber at temperatures ranging from 413 to 463

K (higher than the eutectic temperature of the Sn–In alloy

used in this study), and the heating rate was fixed at 0.35

K/s. The bonding pressure and bonding time were fixed at

7 MPa and 1.8 ks, respectively, during the bonding pro-

cess, so that the degree of deformation obtained is approx-

imately 50%. The bonding pressure was removed as soon

as the bonding process was completed.

After the liquid-phase diffusion bonding process, the

specimen was separated into three different pieces to be

used for the tensile test and the metallographic observa-

tion of the bonded interface. The specimen used for the

tensile test had a cross-sectional area of 3 mm × 3 mm, and

the tensile test was carried out in the vertical direction of

the bonded interface. The tensile test was performed at

room temperature using a displacement speed of 0.017

mm/s. In addition, the slow displacement speed was cho-

sen to examine whether a fracture occurred with the

breaking extension.

3.1 Optimization of In filler metal thicknessTo examine the effect of the In filler metal thickness on

the tensile strength of a joint, tensile tests were performed

after liquid phase diffusion bonding was carried out using

various filler thicknesses ranging from 0.3 µm to 0.7 µm.

The temperature during the bonding process was fixed at

443 K. The relationship between the filler metal thickness

and tensile strength is shown in Fig. 2, while Fig. 3 shows

SEM images of the fracture surfaces and the result of the

EDX analysis.

In Fig. 2, it is observed that the tensile strengths of all

the specimens reached the tensile strength of Sn (approxi-

mately 12 MPa) regardless of the filler metal thickness

used. Among all the filler metal thicknesses used, the

specimen that had 0.5 µm of In deposited showed the larg-

est breaking extension and broke at the Sn base metal,

away from the joint interface. However, when the filler

metal thickness was 0.3 µm, the specimen broke at the

bond interface; in addition, when the filler thickness was

Fig. 1 Illustration of specimens used in this study.

Fig. 2 Relationship between tensile strength, filler thick-ness, and breaking extension for specimens bonded at 443 K and 7 MPa.

Fig. 3 SEM micrographs and EDX analysis of fractured sur-faces of specimens with filler metal (0.3 µm and 0.7 µm) bonded at 443 K and 7 MPa.

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0.7 µm, the specimen broke at the bond interface after the

reduction of the area occurred. To clarify the reason

behind each fracture mode, SEM observations were car-

ried out on specimens that had 0.3 µm and 0.7 µm of In

deposited. These SEM observations showed that when the

filler metal thickness was 0.3 µm, an adherent substance

was found on the fracture surface of the Cu. At the same

time, the polishing marks on the Cu surface were also

found to imprint on the Sn surface. Moreover, small traces

of Cu were detected when an EDX analysis was carried out

on the fracture surface of the Sn. On the other hand, when

the filler metal thickness was 0.7 µm, the polishing marks

found on both the Cu and Sn surfaces became smaller and

harder to detect. In addition, traces of Cu were also found

to cover the entire fracture surface of the Sn when an EDX

analysis was carried out on the specimen. From these

observation results, it is understood that when the thick-

ness of the filler metal was 0.3 µm, the amount of liquid

phase present in the joint interface was insufficient

because of the relatively thin layer of filler metal. In con-

trast, when the thickness of the filler metal was 0.7 µm, the

In could not fully diffuse into the Cu and Sn within the

bonding time used in this experiment because of the rela-

tively thick layer of filler metal. Because In is much softer

than Cu and Sn, it is easily inferred that a strong junction

was not provided. Hence, it is believed that the most suit-

able filler thickness used in this study was 0.5 µm.

3.2 Effect of In filler metal on tensile strengthFigure 4 shows the relationship between the tempera-

ture and the tensile strength of a joint. The chart in Fig. 4

shows the temperature (K), joint efficiency (%), and

degree of deformation (D (%)), as represented by the hori-

zontal axis and two vertical axes in the figure. The joint

efficiency was derived by taking the ratio of the tensile

strength to the strength of the Sn base metal. Hence, for a

joint that broke in the Sn, the joint efficiency was 100%.

The degree of deformation (D (%)) can be calculated using

the following equation, D (%) = (1 - height of Sn block

after bonding / height of Sn block before bonding) × 100.

Here, the height of the Sn block is in the direction where

the bonding pressure is applied and includes the thickness

of the filler metal deposited on its surface. In addition,

analysis results for specimens without In deposition are

also shown in Fig. 4 to illustrate the effect of the filler

metal. In Fig. 4, it is observed that the tensile strength

increases with an increase in the bonding temperature

with or without the use of filler metal. However, although

the tensile strength increases with an increase in the bond-

ing temperature, even without using filler metal, the ten-

sile strength of a joint with filler metal is able to match the

base-metal strength at a temperature at least 30 K lower

than a joint without filler metal. An example of this can be

illustrated by the temperature needed to achieve a bond

that breaks at the Sn base metal. Without using filler

metal, the temperature required to obtain such a joint is

473 K. However, when filler metal is used, the temperature

required is 30 K lower, 443 K. Therefore, it is believed that

a high-strength joint can be achieved at a lower tempera-

ture and with less deformation when filler metal is used.

Figure 5 shows SEM micrographs of the fracture sur-

face of a joint without In deposition. For comparison,

observations were carried out on two sets of specimens

bonded at 433 K and 443 K. For the specimens bonded at

433 K, the fracture surface was smooth, and the fracture

mode of the specimens was brittle. In addition, no traces of

Cu were found on the fracture surface of the Sn. Based on

the results of the TEM observations carried out in a previ-

ous study,[21] this can be explained by the presence of

Fig. 4 Effect of filler metal on tensile strength and bonding temperature for specimens bonded at fixed bonding pressure and bonding time (7 MPa and 1.8 ks).

Fig. 5 SEM micrographs of fractured surfaces of joints after tensile tests (without filler metal).

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oxide films between the bonding surfaces, which pre-

vented the diffusion of the materials and resulted in a joint

with low tensile strength. For the specimens that were

bonded at 443 K, adhesions of Sn were found on the frac-

ture surface of the Cu and vice versa. It is thought that the

mutual diffusion of materials between the bonding sur-

faces occurred as a result of the removal of the oxide films

from the bonding surfaces of the Cu and Sn, and that this

mutual diffusion of materials caused the increase in the

tensile strength of the joint. It is supposed that if a thick

intermetallic compound (IMC) layer was formed, the ten-

sile strength decreased because the joint was broken in

this IMC layer by brittle fracture. However, in the area

where a large amount of Cu was detected on the fracture

surface of the Sn, it is not known whether the compound

was in a stoichiometric proportion such as Cu3Sn and

Cu6Sn5.

Figure 6 shows SEM micrographs of the fracture sur-

face of a joint with In deposition. As with the specimens

with filler metal, two sets of specimens bonded at 413 K

and 433 K were prepared for comparison. For the speci-

mens that were bonded at 413 K, adhesions of Sn were

found on the fractured surface of the Cu. At the same time,

adhesions of Cu were detected on the fractured surface of

the Sn. When the bonding temperature was increased to

433 K, the surface roughness of the fractured surface was

high. In addition, the area where Cu was detected

increased. This is believed to have been caused by the

reducing atmosphere created by the introduction of the In

filler. In other words, because the In filler took oxygen

from the native oxide films of the Cu and Sn, metallic Cu

and metallic Sn adhered and were able to successively

start a reaction. A detailed explanation of this phenomenon

will be given in the next section.

3.3 Observation of bonded interfaceFigure 7 shows SEM images of a joint interface. As

shown in Fig. 7(a), a non-adhered area measuring 1 µm in

width and an intermetallic compound layer approximately

2 µm in thickness (indicated by the dotted line) were

detected when the temperature of a specimen without In

deposition was 443 K. When a semi-qualitative analysis

using EDX was carried out on the intermetallic compound

layer, its composition was found to mostly consist of

Cu6Sn5. Therefore, the accretions observed on the fracture

surface were understood to be composed of Cu6Sn5. Under

this condition, because the joint fractured at the interme-

tallic compound layer, the tensile strength of the joint

decreased. On the other hand, as shown in Fig. 7(b), no

surface defect or intermetallic compound layer was found

on the bonding surface when the temperature of a speci-

men with In deposition was 433 K. The following two rea-

sons can be given for the increase in the tensile strength.

First, when heated to 393 K, the In and Sn react with each

other and are converted to a liquid phase. This liquid layer

then covers the entire bonding surface and increases the

contact surface area between the bonding surfaces. This,

in turn, caused the joint to increase in strength. Second, as

shown in Fig. 8, because the Gibbs free energy of In2O3

⊿G (kJ/mol) in the range of 413–473 K is lower than the

those of SnO2 and of Cu2O, a reducing atmosphere is cre-

ated when In is added between the Cu and Sn.[22] This

reducing atmosphere causes the oxide layers on the bond-

ing surfaces to lose oxygen to In to form In2O3, while

simultaneously causing the SnO2 and Cu2O to revert back

into Sn and Cu. This reduces the amount of oxide film

present between the bonding surfaces and causes the ten-

sile strength of the joint to increase as a result of the

increase in the diffusion between materials. In our previ-

Fig. 6 SEM micrographs of fractured surfaces of joints after tensile tests (with filler metal).

Fig. 7 SEM micrographs showing bonded interfaces of spec-imens: (a) without filler metal at 443 K and (b) with filler metal at 433 K.

97


ous study, when Bi filler was applied at the bond interface

of Sn/Sn, it was observed that the native oxide films of Sn

were cohered to reduce the surface area of the liquid

phase formed by a eutectic reaction of Bi and Sn.[23]

Therefore, in this study, it is supposed that aggregated

In2O3 oxide particles did not influence the tensile strength

of the joint.

4. Conclusions The following conclusions were drawn from this study:

(1) When In filler is used, it decreases the bonding tem-

perature by at least 30 K, while simultaneously pro-

ducing a bonding strength comparable to the

strength of the base metal, Sn.

(2) The use of In filler reduces the amount of plastic

deformation needed to increase the contact surface

area between the bonding surfaces, and greatly con-

tributes to securing a tight contact surface in the

early stage of the bonding process.

(3) When In filler is not used, a higher temperature is

needed to achieve a tensile strength comparable to

that of base metal Sn because a larger plastic defor-

mation is needed to achieve close contact between

the bonding surfaces. However, the higher bonding

temperature also causes the growth of a brittle

intermetallic compound between the bonding sur-

faces, which lowers the tensile strength of the joint.

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Shinji KoyamaSeng Keat TingYukinari AokiIkuo Shohji

a study of the effect of indium filler metal on the … koyama et al.: a study of the effect of...

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