Development of Cu-Sn Alloy Plating with Superior Excellent
Sliding Characteristics and Corrosion ResistanceKazuo Yoshida*, Shuichi Kitagawa*, Kengo Mitose*, and Kyota Susai*

As a matter of consideration of plating materials for automobile terminals, wemade a sample in which a Cu-Sn diffusion layer was exposed to the surface.
We evaluated its anti-fretting characteristics, its corrosion resistance, its coefficient of friction, itssolder wettability. As a result, when compared to the conventional plating materials with Sn on
its surfaces, the material with the Cu-Sn diffusion layer (Cu-Sn alloy plating) showed muchsuperior anti-fretting characteristics. Also, it met or exceeded its capabilities of coefficient offriction and corrosion resistance has sufficient performance for practical use. Its solder
wettability was a little inferior to the conventional ones, though.

ABSTRACT1.

INTRODUCTION

In automobile terminals, electric currents are carried byconnecting male and female terminals. In general,
Sn-plating is on the surface of the terminals for corrosion
prevention. Also, when female terminals nip male terminals, contact pressure of the contact points inside thefemale terminals is stable. The higher the contact pressure is, the more stable the current flow in male andfemale terminals is. However, in this case, the terminal
insertion force which is generated when fitting connectors
becomes stronger.
In recent years, connectors for automobiles are becoming more and more performing. Along with that, the number of terminals and their insertion force are increasing.
The increasing workload of connector-fitting operators inautomakers is becoming a problem. Therefore, the insertion force per terminal requires to be weaker. One of thepossible countermeasures is designing low contact- pressure terminals.
However, it is known that when Sn-plating contact
points slide at low contact pressure in a minute distanceof about 20-30 μm continuously, the contact resistancerises rapidly. This phenomenon is called fretting 1). The
main factors affecting it are contact pressure, sliding distances, contact shapes, coating oil, kinds of plating andplating thickness 2), 3).
We researched this phenomenon focusing on the thicknesses of Sn layers from the material research point ofview. It was made clear that thinning Sn layers (0.25 μm)
had a certain effect on reducing the amount ofSn-abrasion powder, which is the source of oxidative sed-iment, and restraining the rise of resistance during fretting 4).
In accordance with the result, we made a sample inwhich the Cu-Sn layer was deliberately exposed to thesurface by thinning the Sn layer. We researched its antifretting characteristics, its corrosion resistance, its coefficient of friction and its solder wettability.

2.

MATERIAL

In this research, a 0.25 mm thick Cu-Ni-Si alloy strip wasselected as a substrate. It is frequently used as terminals
for automobiles because it has a high strength, a highconductivity and a low stress-relaxation characteristic.
Table 1 shows the chemical composition of the alloy.
Table 1Chemical composition of the Cu-Ni-Si alloy substrate(mass%).

NiSiZnSnMg

Cu

2.0~2.8

0.45~0.6

0.4~0.55

0.1~0.25

0.05~0.2Bal.

Using other kinds of copper alloys makes no differencein this research. The alloy strip was electroplated with Ni,
Cu and nonglossy-Sn in this order. Then, it was reflowprocessed. The thickness of each layer was adjusted bychanging the current density. Figure 1 shows the section
images of the samples. Table 2 shows the surface area
rate of the plating samples and the thickness of eachlayer.

* Metal Research Center. R&D Div.

Furukawa Review, No. 40

20118Development of Cu-Sn Alloy Plating with Superior Excellent Sliding Characteristics and Corrosion Resistance

ab- 1
Sn
Cu-Sn

Sn

Approximately
1.0 μm

Cu-Sn

0.25 mmNi
Copper alloy
substrate(FAS 680)

CuNi
Copperalloy
Substrate
substrate
Cu-Ni-Si Alloy(FAS 680)

No.1
Figure 1

Table 2b- 23.

cCu-SnCu
Ni
Copper alloy
substrate(FAS 680)

No.2

TEST METHOD

3.1 Fretting test
Figure 2 shows the schematic view of the fretting tester.
Dataacquisition
AmplifierOscilloscope

No.3

Laser/Eddy current
displacement
sensorRider
Flat

Temp, humid
control

Stepping motor

Section images of the plating samples.

Substrate
Substrate
Substrate
Copper Alloy
Copper Alloy
Copper Alloy680)area rate (FAS680)average thickness(FAS 680)of
The (FAS
surfaceand thethe plating.

Surface area rate (%)

Current source

Thickness (µm)

No.

Sn

Cu-Sn

SnCu-Sn

CuNi1100

00.40

0.43

00.50

24060

0.12

0.40

0.20

0.50

30100

00.23

0.02

0.50

No.1 in figure 1 shows the section image of the sample
in which the most surface layer was formed by a pure Snlayer. No.2 shows the one in which the Cu-Sn diffusion
layer is partly exposed to the surface. No.3 shows the one
in which the Cu-Sn diffusion layer is exposed all over tothe surface. The term “surface area rate” in Table 2 shows
the rate of the plated layers which are exposed to themost surface. The surface area rate was defined as thefollowing:First, the sections of the plated layers were exposed by
FIB (Focused Ion Beam) process, then it was analyzed by
AES (Atomic Emission Spectroscopy). Using AES, eachlayer in the plated sections was clearly sorted into thepure Sn layer, the Cu-Sn diffusion layer, the pure Cu layer,
the Ni layer and the substrate by color mapping per element.
In No.2, the plated layers of the pure Sn layer and theCu-Sn diffusion layer fluctuated in thickness. To be more
precise, the Cu-Sn diffusion layer was uneven and thepure Sn layer was scattered.
Therefore, in this report, the surface area rate wasdefined as the exposed rate of each layer to the most surface. It was evaluated by measuring the lengths of thepure Sn layers (a and b-1 in Figure 1) and the Cu-Sn diffusion layers (b-2 and c in Figure 1) which are exposed tothe most surface of the observation area (11.7 μm width)and dividing it by the overall lengths.
The average thicknesses were evaluated by measuring
and averaging out randomly selected 5 points in the pure
Sn layer, the Cu-Sn diffusion layer and the Ni layer.

Figure 2

Schematic view of the fretting experiment tester.

A hemisphere shaped bulge of 1.0 mm radius was processed in the test specimen to simulate the contact point
in a female terminal with a male terminal (dimple). It iscalled “the rider” (see Figure 2). Also, the strip-formed
test specimen was cut flat to simulate the male tab of amale terminal. It is called “the flat” (see Figure 2). The test
specimens of the rider and the flat were washed by ultrasonic in acetone solution for five minutes to eliminate
contaminants like oil. Then, they were used as test specimens of the fretting test. In this report, the plating materialof the rider side was standardized to No.1 in which the Snlayer was formed all over the surface. And on the flat side,
the plating materials of No.1-3 were used. The contactload was set 3 N and the sliding distance was set 20 μm
in the test.
3.2 Coefficient of friction measurement
The coefficients of friction were measured by a Bowden
tester on the following conditions. [n=3]Measurement condition: 300 g load; 0.5 R bulge reflowSn plating probe
Sliding condition: one sliding one way; 10 mm sliding
distance, 100 mm/min sliding speed
3.3

Measurement of contact resistance before andafter environmental tests
3.3.1 Conditions of environmental tests
Conditions of four types of environmental tests are the following:• High temperature exposure test
The test specimens were heated in a temperature-controlled bath on the following condition.
Heating condition: 160°C×120h• High temperature and humidity test
A high temperature and humidity test was conducted ina pressure cooker on the following condition.
High temperature and humidity condition: (105°C,100%RH)×16h• Mixed pollution gas test
The specimens were exposed to mixed pollution gas ina mixed pollution gas tester on the following conditions.
Exposure condition: (30°C, 70%RH)×48h
Density of gas: H2S 100 ppb, NO2 200 ppb, Cl2 20 ppb• Salt spray test

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Development of Cu-Sn Alloy Plating with Superior Excellent Sliding Characteristics and Corrosion Resistance

3.4 Solder wettability
As measurement test specimens, samples of No.1-3 without heating and after heating (155°C×16h) were used.
Their solder wetting time was measured by a solder
checker. [n=2]Kind of solder: Sn-3Ag-0.5Cu; 245°C
Flux: RMA type (Solbond RM615)
Steeping time: 10 mm/sec steeping speed, steeping
time: 10 sec, steeping depth: 9.9 mm

4.

generated by abrasion. On the other hand, plating materials which don’t form pure Sn on the most surfaces (suchas No.3) or form little Sn (such as No.2) are not prone togenerate tin oxide, the cause of the increase in resistance. Therefore, their anti-fretting characteristics aresuperior 5).
4.2 Measurement result of coefficient of friction
Figure 4 shows the measurement results of coefficient offriction.

4.1 Result of fretting test
Figure 3 shows the results of the fretting test. The initial
contact resistance was nearly equal to 0.8 mΩ in No.1,1.0 mΩ in No.2, 0.5 mΩ in No.3.

Increase in contact resistance (mΩ)

100
80
60

No.1
No.2
No.3

4020

1

10100

0.20

1000

No.1

Figure 4No.2

No.3

The coefficient of friction in the samples.

The results of No.1 and 2 are close. No.3 showed thelowest coefficient of friction. One likely reason is that thescraped amount in No.3 is small due to the hard Cu-Sn
diffusion layer on the surface. Another likely reason isrelated to the contact with the Sn contact shoe. The contact between Sn and Cu-Sn is likely to be less prone toadhesion than the one between Sn and Sn.
4.3

Measurement results of contact resistance
before and after environmental tests
Figure 5 shows the measurement results of the increase incontact resistance (average value of n=10) after the environmental tests. The initial contact resistance was nearly
equal to 1.4 mΩ in No.1, 1.2 mΩ in No.2, 1.8 mΩ in No.3.

10000

CyclesProfile of contact resistance on fretting test.

In the plating material of No.1 in which the pure Sn
remained on the surface, the resistance increased up toabout 100 mΩ after 100 times of sliding, then, it
decreased to less or equal to 10 mΩ. The contact resistance stayed low after a certain period of time. When sliding exceed 1000 times, the resistance increased again.
Next, the resistance of the plating materials of No.2 and3, in which formed the Cu-Sn alloy layers on the surfaces,
stayed less or equal to 10 mΩ until sliding exceeded 1000times, then, it increased as in No.1.
This result shows that plating materials, which form
pure Sn on the most surfaces (such as No.1), are prone
to the rapid increase of resistance because tin oxide is10
Increase in contact resistance (mΩ)

Figure 3

0.25

0.15

RESULT AND REVIEW0n=30.30
Coefficient of friction (µk)

The test specimens were exposed to salt water in a salt
spray tester (compliant to JIS Z 2371) on the followingcondition.
Exposure condition: 35°C×24h
Density of salt water: 5%NaCl
3.3.2 Measuring method of contact resistance
The voltage during a flow of a low current was measured
by a contact resistance measuring machine. [n=10]
Measurement condition: 100 g load; 5 R bulge Ag
probe
Conducting condition: 10 mA currency

8n=10No.1
No.2
No.3

64
20-2

Figure 5High
temperature
test

High
temperatureand humidity
test

Mixed
pollution
gas test

Salt spray test

Contact resistance after the environmental tests.

Furukawa Review, No. 40

201110Development of Cu-Sn Alloy Plating with Superior Excellent Sliding Characteristics and Corrosion ResistanceIf contact pressure exceeds 10 mΩ after the environmental tests, it means that the three samples have somedefects as electric contacts. However, the increase in contact resistance didn’t exceed 10 mΩ in all of them. Also,
no wide gap was observed among them. Therefore, theplating material in which the Cu-Sn diffusion layer is
exposed to the most surface has sufficient corrosion
resistance for practical use.
4.4 Evaluation result of solder wettability
Figure 6 shows the evaluation results of solder wettability.
109Zero cross time (sec.)

8No.1
No.2
No.3

>
=10 sec.

n=25.

CONCLUSION

Compared with the conventional plating materials with Sn
on the surfaces, the material with a Cu-Sn alloy plating
showed superior anti-fretting characteristics. Also, it
showed the equivalent or better capabilities of friction ofcoefficient and the sufficient corrosion resistance for practical use. Its solder wettability was a little inferior to theconventional ones, though.
Therefore, the materials with the Cu-Sn diffusion layer
are promising as plating materials for automobile terminals.
This article involves the contents 5) reported in the 50th
Japan Copper and Brass Research Association.

76REFERENCES

51) P. G. Slade, “Electrical Contacts: Principles and Applications”,
Marcel Dekker. Inc., New York (1999), 299-332
2) Motohiko Suzuki, Toshihisa Hara: Abstracts of the Japan Copperand Brass Research Association, (2005), 19-203) Tetsuya Ito, Masato Matsushima, Kensaku Takata, Yasuhiro
Hattori: SEI TECHNICAL REVIEW, July 2007 No.171, 75-794) Kengo Mitose, Kazuo Yoshida, Kyota Susai: Furukawa ElectricReview, No.121 (2008), 28-335) Kazuo Yoshida, Syuichi Kitagawa, Kengo Mitose, Kyota Susai:“Journal of the JRICu”, 50 (2011), 170-174
6) Hisaaki Takao, Nobuyuki Yamamoto, Hideo Hasegawa: R&D
Review of Toyota CRDL, 31 No.4 (1996.12), 61-69

432
10Not heat-treatedFigure 6

Heat treated (155°C×16h)

Wetting time of Sn-3Ag-0.5Cu solder.

As test specimens, non-heat-treated one and heat-treated (155°C×16h) one were used. The purpose of using
the latter one is to simulate an oxidized state of the surface to some extent. The results are: In No.1, the zerocrossing time of both non-heat-treated and heat-treatedones were approximately two seconds. However, in theheat-treated one of No.2, it was 10 seconds or longer,
and in the heat-treated one of No.3, it was a little less than
10 seconds. It means solder wettability deteriorated inNo.2 and 3. The likely reason is that stable Cu-Sn composite oxide 6), which is difficult to eliminate with flux, wasformed on the surface of the Cu-Sn diffusion layers.

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