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

1. IntroductionRecent electronic devices and cables generally use a

connector as an electrical connection component. From

the viewpoint of spring properties and conductivity, most

connector terminals use copper or a copper alloy as the

substrate material on which Sn, Au, or Ag is electrodepos-

ited, depending on the contact reliability and solderability

requirements of the application. Connector terminals that

require particularly high reliability often use a hard Au

film on a Ni electroplated substrate. However, the high

cost of Au has accelerated the trend to limit the Au plated

area and thin the deposit in order to reduce Au consump-

tion. Although the applied area of the Au film has been

reduced and thinned, for connectors that are inserted and

pulled a number of times or are required to have high cor-

rosion resistance, efforts to reduce Au consumption while

maintaining reliability have been insufficient. The reliabil-

ity of connectors is ensured by electrodepositing a thick

Au film.[1] Therefore the effect that the film surface mor-

phology of the connectors has on friction and wear proper-

ties, contact resistance, soldering strength, and corrosion

resistance as an approach to reducing precious metal con-

sumption was investigated.

2. Experimental2.1 Sample preparation

As the base material, a 0.5 mm thick oxygen-free copper

plate was used. The plate was cut into 30 mm × 40 mm

pieces and pretreated according to the previously reported

method.[2] Following pretreatment, smooth surface sam-

ples were electroplated in a sulfamic-acid-type Ni bath to

give a 2 µm thick Ni film, and then electroplated in a com-

mercially available hard Au bath to give a 0.05 µm thick Au

film. The Au film thickness was measured with an X-ray

fluorescence analyser (SFT-9500: SII, 0.1-mm-diameter col-

limator). As a post treatment, samples were treated with a

commercially available water-soluble sealing agent (here-

after referred to as “smooth surface plating”). Micro-hol-

lowed surface samples were prepared using the method

described except that additives were included in the Ni

plating bath to induce rough surfaced deposition (hereaf-

ter referred to as “micro-hollowed surface plating”).

The surface morphology of each sample was observed

with a FE-SEM system (S-4800: Hitachi), the surface

roughness and specific surface area were measured using

a laser microscope (VK-9700: Keyence), and cross-sec-

tional observation was performed with a FIB system (FB-

[Technical Paper]

Surface Morphology and Characteristics of Electroplated

Au/Ni Films for Connector Contact MaterialsYoshiyuki Nishimura*,**, Mariko Maebara**, Katsuhiko Tashiro***,

Hideo Honma***, and Tsugito Yamashita*,***

*Graduate School, Kanto Gakuin University, 1-50-1, Mutsuura-Higashi, Kanazawa-ku, Yokohama-shi, Kanagawa 236-8501, Japan

**OM Sangyo Co., Ltd., 3-18-48, Noda, Kita-ku, Okayama-shi, Okayama 700-0971, Japan

***Kanto Gakuin University Materials and Surface Engineering Research Institute, 1-1-1, Hukuura, Kanazawa-ku, Yokohama-shi,

Kanagawa 236-0004, Japan

(Received July 7, 2013; accepted October 22, 2013)

Abstract

The surfaces of the connector terminals used for electrical connection of components are electroplated with Sn, Au, or

Ag in order to improve reliability. For electronic devices that require particularly high reliability, hard gold is used. The

characteristics of gold deposits of various surface morphologies were examined. Gold films with increased surface rough-

ness exhibited superior friction and wear properties without a large increase in contact electrical resistance after friction

testing. Increasing the surface roughness also resulted in higher solder adhesion strength and improved corrosion

resistance to sulfur dioxide gas.

Keywords: Connecter, Hard Gold Plating, Surface Morphology, Friction and Wear Property, Contact Resistance

Copyright © The Japan Institute of Electronics Packaging

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Nishimura et al.: Surface Morphology and Characteristics of Electroplated Au/Ni Films (2/6)

2100: Hitachi).

2.2 Evaluation of friction and wear propertiesThe friction and wear properties of each sample were

evaluated using a reciprocating slide friction tester (SSWT:

Shinko Engineering). A 9.8-mm-diameter brass ball with a

0.4-µm thick electroplated hard Au layer on a 2-µm thick

Ni substrate was repeatedly slid on each sample for 3,600

cycles with a sliding length of 4 mm and load of 0.5 N

applied on the ball. During this time, the friction coeffi-

cient was measured. For friction coefficient measure-

ments, data was collected after every 20 cycles from the

beginning of the test until its termination and then aver-

aged. Following the test, the film surface of each sample

was observed with the FE-SEM system and analyzed with

an EDX system (EX-350: Horiba) at an acceleration voltage

of 20 kV. In addition, a micro ohmmeter (CRS-113-AU:

Yamasaki-Seiki) was used to measure changes in contact

resistance that occurred due to the friction and wear test.

2.3 Evaluation of soldering strengthThe soldering strength was measured in accordance

with JIS H 8504. A 0.5 mm thick oxygen-free copper plate

was used as the L-type metal fitting. Test samples were

prepared by press-molding the substrates into the shape

specified to provide a soldering area of 5 mm by 5 mm

prior to electroplating. A Pb-free solder paste (TCS-254-

5042SF 12-1: Tarutin Kester) was applied to an 8 mm by

0.2 mm area on one of the samples and then heated for 30

seconds at 260°C to solder the sample to the L type metal

fitting. Soldered samples were measured using a tensile

testing machine (3382: Instron).

2.4 Evaluation of corrosion resistanceCorrosion resistance was evaluated by a sulfur dioxide

gas test and a neutral salt spray test. The sulfur dioxide

gas test was carried out in accordance with JIS H 8502 for

96 h. A constant flow gas corrosion test cabinet (GPL-91-C:

Yamasaki-Seiki) was used for the sulfur dioxide gas test,

with sulfur dioxide gas concentration set at 10 ppm at 40°C

and relative humidity at 80%RH. Samples without the seal-

ing treatment were also tested for comparison. For the

neutral salt spray test, the test was carried out in accor-

dance with JIS Z 2371 for 96 h using a salt spray test instru-

ment (CAP-90: Suga Test Instruments). In both tests, the

corrosion resistance was judged using the rating number

method. For the samples tested with sulfur dioxide gas,

the surfaces were observed with an optical microscope

(VHX-900: Keyence) and the FE-SEM system, and surface

analysis was performed with the EDX system.

3. Results and Discussion3.1 Surface and cross-sectional morphology of samples

Figure 1 shows the surface morphology, cross-sectional

images, and surface roughness parameters of the smooth

surface plating and the micro-hollowed surface plating as

observed with the FE-SEM system, FIB system, and laser

microscope. Unlike the smooth surface plating, the plating

surface and cross-sectional images of the micro-hollowed

surface exhibited non-uniform circular hollows in the sur-

face. However, the Au film could not be observed with the

FIB system because the film was too thin. Thus, both the

Fig. 1 Surface morphology and surface roughness of electroplated Au/Ni film from various Ni plating bath.

Smooth Micro-hollowed

SEMimages

Cross sectionimages

Roughness(μm)

Ra 032.0260.0

Rz 550.3916.0

Specific surface area 115.1400.1

10μm

Ni

Cu Cu

Ni

2.5μm2.5μm

C deposited layerC deposited layer

10μm

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

specific surface area and surface roughness in terms of

both the Ra and Rz of the micro-hollowed surface plating

were confirmed to be greater than those of the smooth

surface plating.

3.2 Friction and wear propertiesFigure 2 shows the results of the friction and wear test

on each sample. At the beginning, the friction coefficient

of the smooth surface plating was stable at about 0.4, but

after 2,000 cycles of sliding the friction started to increase

leading to seizure before reaching 3,000 cycles at which

point the test was terminated. The average friction coeffi-

cient was 0.436. On the other hand, for the micro-hollowed

surface plating, the friction coefficient increased sharply at

the beginning of the test and started decreasing gradually

from about 100 cycles. After about 500 cycles, the friction

coefficient stabilized at about 0.3 and was maintained until

termination of the test. Figure 3 shows the SEM and EDX

surface analysis of the tested sections after the friction and

wear test. C and Cu could not be detected, but for the

smooth surface plating, a large amount of Ni was detected

at the center of the tested section compared to the micro-

hollowed surface. On the other hand, a larger amount of

Fig. 2 The relationship between the friction coefficient and slide frequency of electro-plated Au/Ni film from various Ni plating bath.

Smooth Micro-hollowed

Average Friction

Coefficient203.0634.0

0.00.51.01.52.0

0 1,000 2,000 3,0000.00.51.01.52.0

0 1,000 2,000 3,000cycle

μ

cycle

μSeizure

Fig. 3 Observation results of wear tracks by FE-SEM/EDX image of various electro-plated Au/Ni film after sliding test.

Smooth surface Micro-hollowed surfacePlate Ball Plate Ball(smooth)

SEMimages

C

O

Ni

Cu

Au

100μm

Sliding direction

100μm 100μm 100μm

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Nishimura et al.: Surface Morphology and Characteristics of Electroplated Au/Ni Films (4/6)

Au was detected in test region of the micro-hollowed sur-

face plating compared to the smooth surface plating. The

ball used on the smooth plating in the test exhibited a

higher degree of Au film wear and a larger area of Ni expo-

sure at the tested section despite fewer cycles, clearly illus-

trating the difference in wear. The increased Au wear on

the sample and Ni exposure of the ball suggests partial

shearing during the test.[4] Detection of O along the slid-

ing path at the tested section of the ball used on the

smooth plating surface may have arisen due to removal of

Au exposing Ni and the heat of friction causing surface

oxidation.[3] This result provides an explanation for the

seizure that occurred during the test. Contact resistance

values shown in Table 1 were measured before and after

the friction and wear test. The results show that the initial

contact resistance value of the micro-hollowed surface

plating was lower than that of the smooth surface plating.

Post-test, both samples increased in contact resistance, the

smooth surface plating more so than the micro-hollowed

surface plating despite fewer sliding cycles. These results

show that the plated film surface morphology has a signifi-

cant influence on the friction and wear properties and that

friction and wear characteristics could be improved using

micro-hollowed surface plating. This variation may result

from a wider real contact area and a lower shear force pro-

vided by the multiple real contact points of the micro-hol-

lowed surface plating, which has many hollows.

3.3 Soldering strengthTable 2 shows the results of the solder bonding strength

per unit area measurement. When both the substrate and

the L-type metal fitting were smooth surface plated, the

solder bonding strength was 0.5 N/mm2, which increased

to 0.8 N/mm2 when the L type fitting was micro-hollow

surface plated and the substrate was smooth surface

plated. It further increased to 1.1 N/mm2 when both sur-

faces were micro-hollow surface plated. More than twice

the strength was obtained when both surfaces were micro-

hollow surface plated compared to the samples where

both surfaces were smooth surface plated. These results

indicate that changing the surface morphology influences

the soldering strength. The increase in soldering strength

is most likely due to the wider specific surface area and

consequently wider actual joint area that the micro-hollow

surface plating has in contrast with the smooth surface

plating.

3.4 Corrosion resistanceTable 3 shows the results of the neutral salt spray test

and the sulfur dioxide gas tests. Neither sample showed

significant corrosion in the neutral salt spray. On the other

hand, the sulfur dioxide gas test resulted in visible spot-

like corrosion of the smooth surface plating, less pro-

nounced on the sample where sealing treatment was per-

formed. In comparison, both of the micro-hollowed surface

plating samples showed minimal corrosion, which was

reduced even further by the sealing treatment. Figure 4

shows optical microscope images of the surface of each

sample that underwent the sulfur dioxide gas test, and Fig.

5 shows the FE-SEM/EDX observation results. Both O

and S were detected in the corrosion that formed, suggest-

ing that SO2 oxidized Ni to produce Ni and sulfoxide ion

containing compounds.[5] Corrosion on the micro-hol-

lowed surface plating samples was faint and evenly distrib-

uted as opposed to the smooth surface plating samples on

which corrosion was concentrated at certain points and

included Cu. The presence of Cu in the corrosion on the

Table 1 Results of contact resistance of electrodeposited Au/Ni film from various Ni plating bath which before and after sliding test.

Smooth (mΩ) Micro-hollowed (mΩ)

Before After Before After

7.2 8.05 6.4 6.65

Table 2 Results of soldering joint test of electrodeposited Au/Ni film from various Ni plating bath.

Substrate Smooth SmoothMicro-

hollowed

L type metal fitting

SmoothMicro-

hollowedMicro-

hollowed

Tensile strength (N/mm2)

0.5 0.8 1.1

Table 3 Results of neutral salt spray test and sulphur dioxide corrosion test of electrodeposited Au/Ni film from various Ni plating bath.

Rating number

SmoothMicro-

hollowed

Neutral salt spray test

Sealing treated

10 10 　

Sulphur dioxide corrosion test

Sealing untreated

4 9.0

Sealing treated

6 9.3

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

smooth surface plating indicates deep localized corrosion.

These results suggest that the micro-hollowed surface

plating infinitely dispersed galvanic corrosion,[6] which

improved the corrosion resistance similar to the mecha-

nism by which micro-porous chromium plating does so.[7]

4. ConclusionThe correlations between surface morphology and the

durability, soldering, and corrosion characteristics of Ni/

Au plating for connector terminals were examined. The

formation of a micro-hollow composed Ni surface was

achieved by the use of additives to the Ni plating bath used

prior to hard Au plating. The micro-hollowed surface was

Fig. 4 Surface morphology of electroplated Au/Ni film from various Ni plating bath after sulphur dioxide corrosion test.

Sealing untreated Sealing treated

Smooth

Micro-hollowed

250μm

Fig. 5 Observation results of corrosion tracks by FE-SEM/EDX image of various elec-troplated Au/Ni film after sulphur dioxide corrosion test.

Smooth Micro-hollowed

Treated

SEMimages

C

O

S

Ni

Cu

Au

Undetected

60μm 60μm 60μm 60μm

Sealing Untreated Treated Untreated

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Nishimura et al.: Surface Morphology and Characteristics of Electroplated Au/Ni Films (6/6)

found to considerably improve friction and wear proper-

ties, soldering strength, and corrosion resistance, all

important characteristics required of connectors. In all

three cases, the mechanism for improvement appeared to

be related to the surface microstructure. When Au electro-

plating has been used as a surface treatment, increasing

the Au film thickness was the only previous means for

improving the reliability. This technique of controlling sur-

face morphology may provide an alternative means of

ensuring reliability such that Au consumption could also

be minimized. In future work, the optimization of surface

morphology for connector terminals will be studied.

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pp. 559–567, 2011.

[2] Y. Nishimura, K. Baba, Y. Saka, M. Hattori, H. Honma,

and T. Yamashita, “Friction and Wear Resistance of

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26–30, 2013.

[3] E. Takeuchi, Tribology for Mechanical Engineers,

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[4] H. Hashimoto, Learning Tribology from Basics,

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[5] Y. Tadokoro, N. Takezawa, S. Ito, M. Sato, T.

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[6] M. Kawasaki, “Quality of Electroplating,” Boshoku-

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[7] T. Koga, “Improvement of Corrosion Resistance of

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Yoshiyuki NishimuraMariko MaebaraKatsuhiko TashiroHideo HonmaTsugito Yamashita