87

Kato et al.: High-Temperature-Resistant Interconnections (1/6)

1. IntroductionTo reduce the size and increase the efficiency of power

converters mounted in hybrid vehicles or electric vehicles,

research on the development of methods to increase the

output power density using technology such as silicon car-

bide (SiC) devices is in progress. High temperature SiC

devices require the materials for packaging capable of

working at higher temperature than those for Si devices.

While studies have been conducted on heat-resistant pack-

aging materials that can replace Al bonding wire or solder

materials having low melting points, various problems in

alternative methods have been pointed out.[1, 2]

Our approach described in this paper is to form the con-

ductive connection with Ni at relatively lower temperature.

[3] For the connection of chip electrodes Ni micro-plating

bonding (NMPB)[4] was used and die attach connection

was carried out by sintering nickel nanoparticles at a low

temperature.[5] An example of the concept for these inter-

connections is shown in Fig. 1.

2. Experimental Procedure2.1 Bonding by nickel micro-plating

We used nickel electroplating to form nickel micro-plat-

ing bonds. The plating conditions were as follows: Ni plat-

ing bath of Watts solutions, bath temperature of 50°C,

electrical current density of 5 A/dm2, and substrate and

lead material of mainly copper. The growth rate of copper

plating on a flat plate was ranging from 0.75 μm/min to 0.9

μm/min.

To simulate bonding between the chip electrodes and

the substrate leads of a power device, we brought a copper

wire (diameter: 172 μm) into contact with a copper plate

and plated the wire and the plate as shown in Fig. 2 (a). To

simulate die bonding, we brought the gold surface of a sili-

con chip with vapor-deposited gold into contact with a lead

frame cut into strips, and plated the chip and the lead

frame as shown in Fig. 2 (b)

[Technical Paper]

High-Temperature-Resistant Interconnections Formed by Using

Nickel Micro-plating and Ni Nano-particles for Power DevicesNoriyuki Kato, Suguru Hashimoto, Tomonori Iizuka, and Kohei Tatsumi

Graduate School of Information, Production and Systems, Waseda University, 2-7 Hibikino, Wakamatsu-ku, Kitakyushu, Fukuoka 808-0135, Japan

(Received August 6, 2013; accepted November 14, 2013)

Abstract

The improvement of interconnection technology is becoming a top priority for the operation of SiC devices at high tem-

peratures. We proposed a new interconnection method using nickel electroplating to form bonds between chip electrodes

and substrate leads. We also newly proposed low-temperature nickel nanoparticle sintering to form die bonding connec-

tions. SiC devices assembled with these new connection methods operated successfully in a high-temperature environ-

ment of about 300°C. We confirmed that these methods had adequate potential as an advanced heat resistant package in

comparison with conventional interconnections.

Keywords: High Temperature Resistant Packaging, Power Device, Micro Electro-plating Bonding, SiC Device,

Nano-Nickel

Fig. 1 Model of bonded SiC diode specimen.

Nano-Ni

Cu-wireNi-pla�ng

Ni-pla�ng

A -electrode

A -electrode

Cu plate

A -pla�ng

Cu plate Fig. 2 (a) Copper wire bonded to plate (b) Silicon chip bonded to lead frame.

2.7

lead frame

Si-chip

Au-electrode5.0

5.0

Cu-wire Cu plate

)b( )a(

Copyright © The Japan Institute of Electronics Packaging

88

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

To evaluate the high-heat-resistance reliability of the

specimens after micro-plating bonding, we heated them at

various temperatures between 100 and 500°C in an argon

atmosphere for 60 min and then measured the shear

strength of the bonds with a shear tester (Rhesca model

PTR-01). The specimens were heated to 500°C for acceler-

ated aging testing to evaluate the interface reliability and

the change in resistivity.

To observe the cross section of the bonded portions we

used a scanning electron microscope (SEM Hitachi model

S-3400N.).

2.2 Low-temperature nickel nanoparticle sinteringWe used a 5 wt.% nickel nanoparticle solution as the

bonding material. The nickel nanoparticles had a diameter

of 20 nm and were dispersed in a solution of isopropyl

alcohol (IPA). Since IPA is highly volatile, it evaporated

after application, leaving only the nickel nanoparticles.

We used an electrostatic atomizer (made by Apic

Yamada)[6] to apply the solution. As shown in Fig. 3(a),

this device enabled a selective and uniform application of

the solution on the specimen surface by means of electro-

static force.

For the evaluation of bonding characteristics with Ni

nanoparticles we prepared Si dummy chips with size (A)

2.7 mm × 2.7 mm and (B) 8.0 mm × 8.0 mm. After applying

the solution to chip (A), electrodes were placed on chip

(B) with their surfaces facing each other and were heated

and pressurized in atmosphere to form a bond as shown in

Fig. 3(b). The bonding conditions were as follows: the

device stage and the pressurizing head were heated to

300°C, and a constant load of 147 N was applied by the

pressurizing head. After heating the bonded specimens in

an argon atmosphere for 60 min (between room tempera-

ture and 500°C), we measured their shear strength.

2.3 Chips used for bonding strength evaluationThe chips used for bonding were silicon dummy chips

with aluminum film electrodes, and SiC-SBD(Shottky bar-

rier diode) chips (1,200 V, 15 A, 2.7 × 2.7 mm, made by

SiCED Electronics Development GmbH) having an alumi-

num electrode film on the top surface and a silver film on

the back surface. They had a thickness of 365 μm. To

study bonding on these aluminum electrode surfaces, we

used chips having an electroless nickel plating film formed

on an aluminum electrode surface, and chips having vapor-

deposited nickel and gold on aluminum electrode films.

We evaluated nickel nanoparticle bonding strength using

bonding between pairs of silicon dummy chips (A) and

(B). We applied the nickel nanoparticle solution to the

electrode surface and studied on the bonding characteris-

tics between both electrodes.

3. Results and Discussion3.1 Nickel micro-plating bonding (NMPB) and evaluation

3.1.1 Plating bonding studyTo simulate bonding between the chip electrodes and

the substrate leads of a power device, the copper wire was

into contact with a copper plate and plated the wire and the

plate, as shown in Fig. 2(a). Figure 4 shows the cross sec-

tion of a copper wire (diameter: 172 μm) bonded to the Fig. 3a Schematic diagram for electrostatic atomization.

Fig. 3b Bonding steps of dummy chips with using Ni nano-particle atomization.

Chip(A) Stage

Chip(A)

Chip (B)

147N

Nano Ni Heating

89

Kato et al.: High-Temperature-Resistant Interconnections (3/6)

copper plate by Ni micro-plating.

To simulate die bonding, we plated the chip and the lead

frame as shown in Fig. 2 (b). Figure 5 shows the observed

cross section.

Both cross sections indicate no problematic voids or

other defects generated by the nickel plating.

3.1.2 Shear testingTo evaluate whether specimens with plated bonds had

high heat resistance, we measured their shear strength

after heating. We measured specimens in which a copper

wire (with a length of 1 mm and diameter of 172 μm) was

brought into contact with a copper plate and plated for 15

min or 30 min. The specimens were heated in an argon

atmosphere and then shear-tested. We carried out shear

testing nine times at each heating temperature and found

the average shear strength of the specimens. As shown in

Fig. 6, specimens with plating times of both 15 and 30 min

exhibited high shear strength after being heated to 500°C.

The shear strength of both specimen types increased with

an increase in the heating temperature. This finding may

have been the result of increasing copper-nickel alloying

caused by diffusion, resulting in greater adhesion and

strength at the interface. From the results described in

3.1.4, it appeared that no second phase was formed at the

interface between copper and nickel layers, indicating that

no deterioration caused by the formation of brittle phases

or intermetallics with voiding occurred in the high-temper-

ature environment.

3.1.3 Copper-nickel alloy resistance measure-mentSince the electrical resistance of a copper-nickel alloy

layer formed in a high-temperature environment may

increase, we measured the change in resistance caused by

alloying. We used the four-terminal method to measure the

resistance of copper wire specimens (having a length of 30

mm and diameter of 172 μm) plated for 30 min and then

heated in an argon atmosphere. After plating, the wire

diameter increased to 200 μm, and the distance between

the terminals used for the wire resistance measurement

was 12.5 mm. Figure 7 shows the results. Diffusion and

alloying proceeded with an increase in the temperature,

causing an increase in resistance.[7] The increase in resis-

tance even for a heating temperature of 500°C (for 1 h)

was a value of around 10%. The diffusion condition of

500°C for 1 h is equivalent to that of 350°C for 4,580 h,

assuming that the activation energy for Ni diffusion in Cu

is 225 kJ/mol.[8] This indicates that problematic defects

are not considered to be generated in the interconnection

during practical use up to 350°C.

Fig. 4 Plating cross section of copper wire bonded to copper plate.

Cu plate

Cu-wire

Ni-pla�ng5μm

Fig. 5 Plating cross section of silicon chip with vapor-depos-ited gold bonded to lead frame.

Ni-pla�ng lead frame

Si-chip

50μm

Fig. 6 Heating temperature-shear strength characteristic for plated bonding specimens of copper wire and copper plate made using different plating times.

Fig. 7 Heating temperature-resistance and heating-tempera-ture-resistivity characteristics for specimens of nickel-plated wire on copper.

90

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

3.1.4 Analysis of copper-nickel diffusion stateTo verify the state of copper-nickel diffusion in speci-

mens, after heating, we used a SEM to observe the cross

sections of a copper plate covered by Ni plating. We mea-

sured concentration profiles of each element in the vicinity

of interfaces using an energy-dispersive X-ray spectros-

copy (EDX) line analysis. Figure 8 shows the results. The

positions of EDX line analysis are indicated within each

SEM image and the concentration profiles are displayed in

the graphs.

A comparison of the graphs of unheated specimens and

specimens heated at 300°C in Fig. 8 (a) and (b) reveals

almost no change in the concentration distribution. On the

other hand the concentration distribution of the specimen

heated at 500°C in Fig. 8 (c) shows the formation of an

alloy layer of approximately 5 μm. There were no second

phase observed, as predicted from a phase diagram of

Cu-Ni system. Further, although Kirkendall voids have

formed in the copper plate, the results described in Sec-

tions 3.1.2 and 3.1.3 indicate that their effect is not serious.

3.2 Nickel nanoparticle bonding and evaluationAs described in Section 3.1.1, plating-based die bonding

can be carried out by using a strip or lattice substrate

structure, but it is difficult to use for bonding flat plate

surfaces together. It has been shown that flat plates can be

bonded using low-temperature nickel nanoparticle bond-

ing[5, 9]; hence, we studied the application of this method

to die bonding. We brought the electrode surfaces of sili-

con dummy chips that are (A) 2.7 × 2.7 mm and (B) 8.0 ×

8.0 mm in size into facing position, bonded them using

nickel nanoparticles, and then shear-tested the specimens

after heating. We performed shear testing five times at

each temperature and plotted the average shear strengths.

Figure 9 shows that a slight drop in the shear strength was

Fig. 8 SEM images and concentration distribution graphs of cross section of nickel-plated copper plate specimens.

(a) Heating condition: no heating (as bonded).

(b) Heating condition: heated for 60 min at 300°C.

(c) Heating condition: heated for 60 min at 500°C.

91

Kato et al.: High-Temperature-Resistant Interconnections (5/6)

found when the specimens were heated at 500°C, but

since the value still remained high (over 10 N).

3.3 High-temperature circuit operation testing and evaluation using SiC diode chips

To evaluate whether the bonding technologies that we

have discussed are practical for use in power device con-

nections, we carried out high-temperature circuit opera-

tion testing on SiC-SBD chips (1,200 V, 15 A, made by

SiCED). As shown in Fig. 10, the interconnections were

created for the specimen by using nickel micro-plating to

bond connections between chip electrodes and leads of a

silver-plated copper substrate joined and bonded by a cop-

per wire (using a plating time of 30 min). Nickel nano-par-

ticles were used for die bonding, with a surface-processed

aluminum electrode bonded to a silver-plated copper sub-

strate.

After bonding the specimens, we performed the electri-

cal operation tests while heating each specimen on a hot

plate at a different constant temperature (ranging between

20 and 325°C). We then measured the diode voltage (VD)

and current (ID), varying the power supply voltage

(between 0 and 15 V) using the circuit illustrated in Fig.

10. Figure 11 shows the results for different temperatures.

In a high-temperature environment of 325°C, we obtained

the diode V-I characteristics and did not observe the

change in the resistivity caused by the bond deterioration

during measurements. With using Ni interconnections

newly introduced in this paper the high temperature oper-

ation of the SiC diode was first confirmed.

4. Conclusions1. Our findings demonstrated that nickel micro-plating

bonding(NMPB) could be used for bonding chips

to substrate electrodes and for die bonding to chip

substrates in power devices. We demonstrated that

low-temperature bonding ensured bond reliability

in high-temperature environments of over 300°C.

We observed no bond deterioration during acceler-

ated diffusion tests of bonds formed by copper-

nickel plating.

2. We used nickel micro-plating bonding and low-tem-

perature nickel nanoparticle sintering to form

bonds between SiC power device chips and sub-

strate electrodes and to form die bonding connec-

tions, respectively. We tested device operation in a

high-temperature environment of about 300°C.

3. Our findings demonstrated that micro-plating-based

chip bonding technology could be considered to

have adequate potential as a practical, simple

mounting technology that is highly heat-resistant

and ensures high reliability and low cost.

References[1] H. A. Mantooth, M. M. Mojarradi, and R. W. Johnson,

“Emerging Capabilities in Electronics Technologies

for ExtremeEnvironments. Part I - High Temperature

Electronics,” IEEE Power Electronics Society News-

letter, issue 1, 2006.

[2] L. Coppola, D. Huff, F. Wang, R. Burgos, and D.

Boroyevich, “Survey on High Temperature Packaging

Materials for SiC-Based Power Electronics,” Proc.

PESC, Orlando, FL, pp. 2234–2240, 2007.

[3] S. Terashima, Y. Yamamoto, T. Uno, and K. Tatsumi,

“Significant reduction of wire sweep using Ni plating

to realize ultra fine pitch wire bonding,” Proceedings

of the 52nd Electronic Components and Technology

Fig. 9 Heating temperature-shear strength characteristic of specimens bonded using nickel nanoparticles.

Fig. 10 Circuit used for measuring diode V-I characteristics.

Variable DC power source

ID

VD

2 Ω

SiC diode 0 15V

Fig. 11 V-I characteristics of bonded SiC diodes at different heating temperatures.

0

1

2

3

4

5

6

7

8

0 200 400 600 800 1000

I D[m

A]

VD[mV]

92

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

Conference, 52, pp. 891–896, 2002.

[4] K. Tatsumi and T. Ando, “Plating micro bonding used

for Tape Carrier Package,” Proc. NIST/IEEE VLSI

PACKAGING WORKSHOP, YORKTOWN HEIGHTS,

N.Y. 1993, 10.12.

[5] K. Tatsumi et al., “Electronic component bonding

material, composition for bonding, bonding method,

and electronic component,” PCT/JP2012/065242.

[6] http://www.apicyamada.co.jp/pdf/seiden_hunmu.pdf

(accessed Aug 2013).

[7] C. Y. Ho, M. W. Ackerman, K. Y. Wu, T. N. Havill, R.

H. Bogaard, R. A. Matula, S. G. Oh, and H. M. James,

“Electrical resistivity of ten selected binary alloy sys-

tems,” J. Phys. Chem. Ref. Data, Vol. 12, No. 2, pp.

183–322, 1983.

[8] Metal data book, edited by Japan Institute of Metals,

Maruzen, 2010.

[9] S. Hashimoto, T. C. Lun, K. Tatsumi, A. Nogami, and

Y. Sawa, “Study on bonding by using Ni nano particles

for high temperature packaging,” Proceedings of

autumn meeting of the Japan Institute of Metals and

Materials 2013, p. 192.

Noriyuki KatoSuguru HashimotoTomonori IizukaKohei Tatsumi