Pb-Free Plating for Electronic Components
by Morimasa Tanimoto *, Hitoshi Tanaka *,Satoshi Suzuki * and Akira Matsuda *The authors have developed Pb-free tin alloy plating materials. Preliminary
screen tests showed that several alloy candidates had some shortcomings. SnAg plating had high costs, Sn-Bi plating had shortcomings in both thermal stability and workability, and Sn-Zn plating in wettability and jointing capability.
In the Sn-Bi plating, thermal stability and workability have been improved by adopting a doublelayer plating, i.e., an Sn-Bi surface layer/Sn undercoat layer. The double-layer plating reduces
the total amount of Bi and hence reduces pollution of the soldering material. Also reflow-process
improves the environmental resistivity of the plating.

ABSTRACT1.

INTRODUCTION

Recently, a lot of environmental issues such as fluorocarbon, ethane, dioxin, acid rain, carbon dioxide and lead
have been discussed. Various regulations are being proposed and some are now being executed. Among them,lead (Pb) regulation is discussed a lot and there are plans
for strong controls. Lead has long been suggested to beharmful to the human nervous system. It is known thatlead causes cancer, retardation in infants, genital functional disorder and high blood pressure, and as a result there
are increasing regulations against Pb-ion elution fromlead-containing products.
Regulations to stop or limit the use of lead in electronic
components are also being requested. The uses of lead inthese components include Sn-Pb alloys for soldering. The
substitution materials of the Sn-Pb alloy have been widely
researched by many manufacturers. They are tin-based
multi-element alloys including Ag, Bi, Cu, In or Zn, etc.
Another application that uses Pb is solder plating as a surface treatment method. Therefore, in order to protect
human beings against its harmful effects, it is necessary
not to use lead in these applications (Pb-free).
As a Pb-free plating process, Cu-alloy leadframes forsemiconductors with Pd-plating have been developed andare now being used.1) Because of its high cost and cost
fluctuations due to the use of a noble metal, the Pd-plating
is available only for small, limited applications, and not for
general use as an alternative to Pb-plating.
Pb-free plating materials under development are tinbased alloys, such as Sn-Ag, Sn-Bi and Sn-Zn. Eachmaterial has both good qualities and bad qualities. For* 1st Research Dept., Metals Research Center, R & D Div.

example, the bath of Sn-Ag plating is unstable and thecost is high. Sn-Bi plating is less ductile and its workability
is not very good. Sn-Zn plating does not have good wettability in practical use.
This paper reports on the development of the Pb-freeplating that improves the shortcomings of tin-alloy platingby thoroughly investigating its properties.

2.

PRELIMINARY SCREENING OF PbFREE PLATING

2.1 Introduction
We selected three tin alloys for screening tests from various tin alloys available: Sn-Ag, Sn-Zn and Sn-Zn, basedon the viewpoints such that the alloy is usable for electroplating of electronic components, its plating bath is basically established, and the alloy has a low melting temperature. In order to confirm their electroplating properties, we
examined their diffusion rates, wettability, solder joint
strength and whisker generation against Cu substrates byheating them.
2.2 Test Samples
Substrates of test samples were tough pitch copper coils(C1100) of 0.4 mm thickness. We prepared three tin alloy
samples with the following compositions: Sn-3.5% Ag, Sn5% Bi and Sn-9% Zn. Reference samples of Sn-10%Pb,
Sn and Pd electroplating were also prepared.
Electroplating was carried out after electrolytic cleansing
and pickling. The thickness of each Sn-alloy plating was
10 µm, and the thickness of the Pd-plating was 0.5 µm. A
Ni-plating of 1.0 µm thickness was done as a pre-coat for
the Pd-plating.

912.3.2 Wettability
The wettability was measured using the Menisco-graphmethod. The immersion speed was 10 mm/sec, the depthof immersion was 10 mm, and the immersion time was 10
sec. Two solder baths used were: eutectic solder (Sn-36%Pb) of 230°C and Pb-free solder (Sn-7.5% Bi-2.0% Ag0.5% Cu) of 250°C. The measurement was carried out at
two levels: just after plating and after 5000 hrs at room
temperature.
2.3.3 Solder joint strength
A copper plated wire of 0.6-mm diameter was jointed tothe specimens in a hemisphere-like-shape. The sample
was tested with a pull strength tester. The joint strength
measurements were carried out at two levels: just after
jointing and after heat treatment at 150°C for 1000 hrs. As
jointing solders, a eutectic solder (Sn-36% Pb) and a Pbfree solder (Sn-7.5% Bi-2.0% Ag-0.5% Cu) were used.
Elements analysis was carried out for some fractured surfaces with fluorescent X-ray and EPMA.
2.3.4 Whisker test
Samples were kept at 50°C for 3000 hrs in a constanttemperature oven, and during this period, the appearances of whiskers were checked under an optical microscope(x20) after 100, 200, 500, 1000 and 3000 hrs,
respectively.
2.4 Test Results
2.4.1 Growth speed of intermetallic compound
Figure 1 shows the relationship between the eutectic point
and the growth speed of intermetallic compound (IMC).
The order of the thickness reduction rate before and after
heat treatment at 155°C for 16 hrs was as follows: Sn-5%

Bi > Sn-10% Pb > Sn-3.5% Ag = Sn-9% Zn > Sn. Thisorder depends on the eutectic point of the alloy rather
than on the melting points of layer compositions.
2.4.2 Wettability
Figure 2 shows the results of wettability tests. The figure
shows that only the Sn-Zn plating after 5000 hrs deteriorated in its wettability due to surface oxidation. The othersamples were all good. There was no difference in the
wettability by the plating thickness between the eutectic
solder of 230°C and the Pb-free solder of 250°C.
2.4.3 Solder joint strength 2)
Figure 3 shows the results of joint strength tests. The joint
strength with the eutectic solder showed approximately
the same values except for the Sn-Zn plating both before
and after heating in the deterioration test. The joint
strength with the Pb-free solder was a little lower than the
one with the eutectic solder.
The joint strength between the eutectic solder and SnZn plating is far weaker than the strength of conventional
Sn-Pb plating. The elements analysis of the boundaryinterface showed that Zn was highly condensed at theboundary between Sn and Cu. Therefore, a lot of voids
were generated which substantially reduced the joint
strength.
The joint strength of Pb-free-solder and Sn-Pb plating
after 1000 hrs at 150°C fell sharply. However, the combination of the eutectic solder and the Sn-Bi plating, or theeutectic solder and the Sn-Ag plating did not show degradation in joint strength. Therefore, adopting the Pb-freesolder requires careful consideration of the types of solder
and plating combinations.

over5(a) Sn-36%Pb

Soldering time (sec)

2.3 Evaluation Items
2.3.1 Growth speed of intermetallic compound
The growth speed was measured by the reduction in solder weight after a heat treatment at 155°C for 16 hrs. The
thickness of the solder was measured by the constant current anodic dissolution method with an anode current density of 20 mA/cm2 in a Kokour R-50 bath.

1.5

Normal state4
5000-hr duration
3
2
1
0
Sn-Ag

Sn-Bi

Sn-Bi

Sn-Zn

Sn

Sn-Pb

Pd

5
1

(b) Sn-7.5%Bi-2.0%Ag-0.5%Cu

Sn-Ag

Soldering time (sec)

Growth speed (µm)

Sn-Pb

Sn-Zn
Sn
0.5

0
100

150

200

92

5000-hr duration
3
2
1

250

Eutectic point (°C)

Figure 1

Normal state

4

Relationship between eutectic point and intermetallic
compound growth speed

0
Sn-Ag

Figure 2

Sn-Bi

Sn-Zn

Sn

Sn-Pb

Pd

Wettability of Sn-Pb solder (a) and Pb-free solder (b)Furukawa Review, No. 19. 2000

without an undercoat, is essential. Thus we studied the
optimization of the layer composition of Sn-Bi plating to
improve its thermal stability and workability.

30(a) Sn-36%Pb

Joint strength (MPa)

25
20
15

3.

10

Normal state
After 150°Cx1000 hrs
heating5

3.1 Introduction
We examined two methods to find an optimum layer composition. One is the optimization of the Sn-Bi composition.
A lower concentration of Bi improves the workability andthermal stability of the layer. The other is a double-layer
plating with Sn-undercoat plating. This process canimprove thermal stability due to the Sn undercoat layer, or
improve workability by a reduction in the Bi concentrationof the total layer due to the double plating.
Here, we examined 6 levels of double-layer thickness
including a single layer, and 4 levels of Sn-Bi compositions, totaling 24 samples to evaluate Sn-Bi plating properties.

0
Sn-Ag

Sn-Bi

Sn-Zn

Sn

Sn-Pb

Pd

20
Normal state

Joint strength (MPa)(b) Sn-7.5%Bi-2.0%Ag-0.5%Cu

After 150°Cx1000 hrs
heating

15

10

5

0
Sn-Ag

Figure 3

Sn-Bi

Sn-Zn

Sn

Sn-Pb

Pd

3.2 Test Samples
Tough pitch copper strips of 0.4 mm (C1100) were used
as a substrate. The plating processes are; electrolytic
cleansing, pickling, undercoat Sn plating, Sn-Bi plating,neutralization and drying, respectively. The thickness of
the Sn-Bi plating is 5 levels of 0.2, 0.5, 1, 2 and 5 µm, andthe balance is Sn plating, totaling 10 µm. An Sn-Bi singlelayer plating was prepared as a reference. Four types ofplating baths with different metal concentrations were prepared for the Sn-Bi layer composition.

Solder joint strength of Sn-Pb solder (a) and Pb-freesolder (b)2.4.4 Whisker test
The generation of whiskers was found within 100 hrs in
the Sn plating. Rugged projections on the surface were
found after 500 hrs in the Sn-Zn plating, and whisker generation after 1000 hrs was also found. On the contrary, no
whisker was found in the Sn-Ag, Sn-Bi or Sn-Pb plating
after 3000 hrs.

3.3 Evaluation Items
3.3.1 Layer thickness and composition
The thickness was measured by the constant current
anodic dissolution method. The composition was measured with a fluorescent X-ray micro thickness tester.

2.5 Summary of Screening (Table 1)
Among the three combinations, Sn-Ag, Sn-Bi and Sn-Zn
plating, two types (Sn-Ag, and Sn-Bi plating) turned out tobe promising. From a practical viewpoint, the Sn-Ag plating is expensive due to not only the expensive material
cost, but also the need for a large quantity of a chelating
agent in the plating bath to compensate for the large gap
in the precipitation potential between Sn and Ag. Thus,
waste treatment costs increase.
Sn-Bi plating requires improvement in thermal stability
and workability. In order to improve the thermal stability,
consideration of the layer composition, such as with or
Table 1
Type of

Growth speed

platingof IMC

Sn-Ag

○

Sn-Bi

×

Sn-Zn
Sn

IMPROVEMENT OF Sn-Bi PLATINGPROPERTIES

3.3.2 Growth speed of intermetallic compound
The diffusion speed was evaluated by the reduction in solder thickness before and after heat treatment for 72 hrs.
The heating temperature was at two levels, 130°C which
is lower than the eutectic point of Sn-Bi (139°C), and170°C which is higher than the eutectic point.

Screening test results of Pb-free plating
Joint

Anti-

strengthwhisker

○

○

○

○

○

×

○

△

Sn-Pb

△

Pd/Ni

◎

Wettability

Workability

Cost

○

○

×

○

×

△

×

△

○

△

○

×

○

◎

◎

○

○

○

◎

○

△

◎

△

×

Evaluation : (Good) ◎ > ○ > △ > ×(No good)93

3.3.3 Bendability
The outer portion which was 90 degrees V-bent in 0 mmR
was observed under a scanning electron microscope(x300). Evaluations were made of cracking and substrate
exposure.
3.3.4 Wettability
A load test for wettability was carried out by the Meniscograph method. Soldering conditions were the same as in
the clause 2.3.2. The flux used was Rosin 25%/IPA.
Measurements were implemented before and after heat
treatment at 170°C for 72 hrs.
Table 2Composition of Sn-Bi single-layer plating

Reduction in solder thickness (µm)

Bi concentration (wt%)

No.1

No.2

No.3

No.4

1.7

4.3

8.6

14.3

7
(a) 170°Cx72 hr
6
5
4
3

3.4.2 Growth speed of intermetallic compound
Figure 4 shows the results of the measurement of growth
speed of intermetallic compound. Under the thermal treatment of 170°C for 72 hrs, beyond the eutectic point of SnBi (139°C), the diffusion rates of the intermetallic compound were the same as those of Sn-plating within 2% in
the total Bi concentration and increased with Bi concentration beyond that point (2%). In the case of a material,
which is subjected to thermal hysteresis beyond theeutectic point, the total Bi concentration governs the diffusion rate; and the concentration should, therefore, be lessthan 2%, as the diffusion rates between the single anddouble-layer plating are the same.
Under the thermal treatment of 130°C for 72 hrs, lower
than the eutectic point, while the diffusion rate in single
plating increases with Bi concentration, that in the case of
double-layer plating remains at the same level as for the
Sn-plating. Consequently, thermal property improvementin the double-layer plating has been confirmed.

Sn-Bi/Sn

2

Sn-Bi

1

Sn

0
0

5

10

15

Total Bi concentration (Bi wt%)
Reduction in solder thickness (µm)

3.4 Test Results
3.4.1 Solder composition
Table 2 shows the composition of the Sn-Bi single-layer
plating. As for the average composition of the double-layer
plating, it was confirmed that the ratio was almost proportional to the deposited layer thickness.

4(b) 130°Cx72 hr
3

2
Sn-Bi/Sn
Sn-Bi

1

Sn
0
0

5

10

15

3.4.3 Bendability
We observed the bent portion of the specimens to evaluate the bendability of the specimens in four levels. Photo 1
shows the evaluation standards, and Figure 5 shows theevaluation results. They clearly revealed the difference
between the single-layer and double-layer plating. In thecase of the single-layer plating, there are large cracks(level 1) at the lower Bi concentration of 1.7%, and in thecase of the double-layer plating, there are no cracks (level3) at the combination of Sn-8.6% Bi 2 µm surface layer/Sn
8 µm undercoat.
The lower Bi concentration of 1.7% still yielded cracks inthe single-layer plating because this layer is less ductile
than the Sn-layer. While in the double-layer plating, the Sn
undercoat prevented substrate exposure, even when thesurface layer of Sn-Bi was sheared.

Total Bi concentration (Bi wt%)

Figure 4

Growth speed of intermetallic compound at 170°Cx72
hr (a) and 130°Cx72 hr (b)Sn-Bi/Sn

Evaluation level

3

Sn-Bi
Sn

2

1
50 µm

Level 3
No cracks

Photo 1

94

Level 2
Small cracks

Level 1
Large cracks

Bendability rating samples

0

5

10

15

Total Bi concentration (Bi wt%)

Figure 5

Bendability

Furukawa Review, No. 19. 2000

3.4.4 Wettability
Figure 6 shows the results of the wettability test. At room
temperature and after the heat treatment under 170°C for
72 hrs, the higher Bi concentration in the Sn-Bi surface
layer showed better wettability. The layer of Sn-14.3% Bi
10 µm, however, showed substantially reduced wettability
after the heat treatment of 170°C for 72 hrs because ofincreased diffusion of the layer into the substrate. Goodwettability is obtained by raising the Bi concentration of
the surface layer to the extent that diffusion into the substrate does not occur.
3.5

Conclusion for Characteristics Improvement of
Sn-Bi Plating
In the Sn-Bi single-layer plating, a higher Bi concentration
improves wettability but remarkably reduces workability
and thermal stability; a lower Bi concentration reduces
wettability and does not substantially improve workability.
The double-layer plating is essential for good workability. The lower Bi concentration in total improves thermal
stability, too. Improvement in wettability is effectively
achieved with a higher Bi concentration in the surface
layer; a higher Bi concentration in the Sn-Bi surface layer
is particularly effective. Also environmental resistivity isimproved by the Bi condensation from the reflow treatment. Therefore, this treatment is very practical in use.

4.

STUDY FOR COMMERCIALIZATIONVIABILITY

4.1 Introduction
Based on the above-mentioned results, we tried to produce and evaluate the Pb-free plating samples in a production line. The samples were double-layer-plated wires
with Sn-Bi surface layer/Sn undercoat that generally used
for lead wires in electronic components.
4.2 Test Samples
A copper plated wire of 0.57 mmφ was used as a substrate material. The composition of the layer is an Sn-7%
Bi 2 µm surface layer/Sn 8 µm undercoat. The plating was
carried out in a pilot line: first, electrolytic cleansing, followed by pickling, Sn plating, Sn-Bi plating, neutralization,
hot water rinsing and drying. We also prepared reflowed
samples.
We used Sn-Pb wires, Sn plated wires, and Sn-Bi single-layer plated wires as references for the same evaluation items.
4.3 Evaluation
4.3.1 Growth speed of intermetallic compound
Refer to the clause 3.3.2.
4.3.2 Whisker test
We observed whisker generation in samples kept at 50°C
for 6 months. The test samples were made to enhancewhisker generation by welding aluminum wire; this gives
rise to whiskers due to surplus jointing stress.
4.3.3 Oxidation
We evaluated the level of oxidation by the electricity for
reduction. The constant current anodic reduction method
was implemented using a 0.1 mA/cm2 cathode current
density in a 0.1 N KCl solution. There were three samples:
just after plated, after a heat treatment of 170°C for 72
hrs, and after a pressure cooker test of 105°C, 100% RH
for 16 hrs.

40.0%

Soldering load (mN)3(a) Normal state

1.7%
4.3%

2

8.6%
14.3%

1
0
-1
-2
0.2

0.5

1

2

5

10

Plating thickness of surface layer of Sn-Bi (µm)-1

Soldering load (mN)

-2
-3
0.0%

-4

1.7%
-5

4.3%
8.6%

-6

(b) 170°Cx72 hr

14.3%
-7
0.2

0.5

1

2

5

10

4.3.4 Wettability
We evaluated the samples with the Menisco-graphmethod. The test was implemented at 2 mm/sec immersion rate, 2 mm immersion depth and 10 sec immersion
time using a 230°C eutectic solder bath (Sn-36% Pb).
There were three tested samples: just after plated, after aheat treatment of 170°C for 72 hrs, and after a pressure
cooker test of 105°C, 100% RH for 16 hrs.
4.3.5 Workability
We observed the appearance of twisted samples after
170°C for 72 hrs heat treatment. The 40 mm long specimens were twisted 20 times forward and 5 times backward, followed by scanning electron microscopic observation.

Plating thickness of surface layer of Sn-Bi (µm)

Figure 6

Wettability

95

Table 3
Surface

Growth speed

Evaluation test results of Sn-Bi/Sn plated wire
Electricity for reduction (mC/cm2)

of IMC

Normal

(µm)

state

Reflow

2.2

1.7

Plating

2.2

1.6

treatment

Solder wetting hours (sec)Pressure

Normalcooker test

state

8.4

12.6

0.9

2.6

1.6

12.0

16.2

1.1

3.5

2.3

HeatingHeating

Pressure
cooker test

Plating composition of samples: Sn-7% Bi 2 µm / Sn 10 µm

4.5 Summary of Commercial Applications
We have developed a satisfactory alternative for the SnPb plating. This method is hard to yield whiskers giving
good thermal stability and workability. Furthermore, we
found that the reflow treatment improved the anti-oxidation
property and wettability.
Sn-Bi surface layer/
Sn undercoat plating

Photo 2

Sn plating

0.1mm

Observation of whisker

4.4 Test Results (Table 3)
4.4.1 Growth speed of intermetallic compoundWe found the growth speed of intermetallic compound of
the Sn-Bi/Sn undercoat plating into the wire to be 2.2 µm
in specific depth, the same with the preliminary test, anddid not find any difference between non reflowed andreflowed samples.
4.4.2 Whisker test
Photo 2 shows the appearance of the sample after six
months, revealing no sign of whisker generation in the SnBi surface layer/Sn undercoat plated wire, while a lot of
whiskers were observed in the reference samples of the
Sn plated wire. Therefore, Sn-Bi surface plating isapproved for suppressing whisker generation.
4.4.3 Oxidation
Improvements against oxidization of both heat-treated andpressure cooker tested samples were achieved throughthe reflow process. The reflow treatment is useful to prevent oxidation of a layer surface.
4.4.4 Wettability
The effect of reflow treatment on wettability was observed
and is shown in Table 3. The improvement in wettability
resulted due to the improvement in anti-oxidization fromthe reflow treatment.

5.

CONCLUSION

In this report, a Pb-free plating was introduced: i.e., an SnBi surface layer/Sn undercoat double-layer plating proved
to be very useful. This method requires no noble metals.
In particular, it is useful today when Pb-containing joints
are still being used in the electronic component market.
The double-layer plating allows the total amount of Bi to
be reduced compared to the single-layer plating, resulting
in the reduction of pollution in the solder.
Selection between the Pd plating or Sn-Bi plating
depends on the actual application and the results expected. The Pd plating is likely to be preferable for higher temperature use, shorter lead-time and high-reliability connections, and the Sn-Bi/Sn undercoat plating is useful for lowcost applications. The Sn-Bi/Sn undercoat plating as a
substitute for the Sn-Pb plating offers, in addition to lower
material costs, a benefit that conventional equipment canbe used with only a small remodeling cost.

REFERENCES

1) Mori et al.; Furukawa Electric Review, 100, (1997), 111. (inJapanese)
2) Tanaka et al.; Proceedings of 38th Copper Products Technology
Conference, (1998), 118. (in Japanese)3) Tanimoto et al.; Proceedings of 13th Electronics Mounting
Technology Conference, (1999), 3. (in Japanese)
Manuscript received on July 15, 1999.

4.4.5 Workability
The double-layer plating showed no stripping; on the contrary, large cracks were generated, and layer stripping
occurred in the Sn single-layer plating.

96

Furukawa Review, No. 19. 2000