18. - 20. 5. 2011, Brno, Czech Republic, EU
INFLUENCE OF Ce ON GROWTH OF IMC DURING AGING AT THE INTERFACE
OF LEAD - FREE SOLDER AND Cu SUBSTRATE
Katarína POCISKOVÁ DIMOVÁ , Lýdia RÍZEKOVÁ TRNKOVÁ ,
Milan OŽVOLD, Milan TURŇA
Slovak University of Technology, Faculty of Materials Science and Technology,
Paulínska 16, 917 24 Trnava, Slovak Republic, [email protected], [email protected],
Abstrakt
The paper deals with the rare earths, which are added to lead-free solders in order to improve the properties
such as wettability, structural stability and the related growth and formation of intermetallic compounds (IMC)
at the Cu substrate/solder interface. SnCu0.7 eutectic solders with a small addition of Ce (0.1, 0.2 and 0.5 wt
%) were used for the experiment. To compare the impact of cerium, default solder SnCu0.7 was also
analysed. Soldered samples were annealed at the temperature of 150°C for 50, 200, 500 and 1000 hours.
Microstructure changes in volume of the solder and at the interface were observed using optical microscopy.
Influence of Ce on the growth of the IMC was mainly studied. Increasing the Ce content in solders resulted in
the refinement of the microstructure and the change of the IMC morphology at the interface.
Keywords: lead-free solder, SnCu0.7 alloys, Rare Earth (RE), Cerium, intermetallic compound (IMC)
1.
INTRODUCTION
It is well known that conventional Pb-containing solders are harmful to health and environment. In recent
decades, the introduction of legislation (e.g. Waste from electrical and electronic equipment - WEEE and
restriction of the use of certain hazardous substances - RoHS) accelerated the research into lead-free
solders. Under the pressure of legislation and trade competitions, large quantities of work have been carried
out to find the substitutes for SnPb solders.
The lead-free solders in electronic industry need to meet the following requirements: low melting point, good
wettability and low cost [1].
Rare earth (RE) elements have been regarded as the addition of metals, which means that a minute amount
of RE elements may greatly enhance the properties of solder alloys. RE elements are the surface-active
elements, which play an important role in metallurgy of materials, such as refinement of microstructure,
alloying and purification of materials.
In recent studies [2], the experimental result showed that the wetting property was improved by 0.25 - 0.5
wt.% addition of (La,Ce) into the SnAg3.5 solder alloy, or 0.03 wt.% of Ce into the SnAgCu based solder.
However, addition of an excessive amount of the RE elements reduced the beneficial influence [2]. Dudek et
al. found that the addition of La (up to 0.5 wt.%) to SnAgCu solder alloys refines the microstructure by
decreasing the length and spacing of the Sn dendrites and decreases the thickness of the Cu6Sn5
intermetallic layer at the Cu/solder interface [3]. Also Zhang et al. [2,4] reported that by adding a small
amount of Ce could refine the grain size of CuSn IMCs and make the microstructure more uniform.
Other studies have also shown, that with a small addition of La or Ce into the SnAg, SnCu, SnAgCu solder
alloys, the coarse β-Sn grains are refrained, the eutectic colony becomes rather narrower, and, at the same
time, Cu6Sn5 and Ag3Sn intermetallics are finer as a result of the adsorption affection of the active rare earth
elements. Due to the uniform and fine microstructure, the mechanical properties of these solders are better
compared with the solders without RE element addition. [4]
18. - 20. 5. 2011, Brno, Czech Republic, EU
In this paper we deal with the development of SnCu and Sn - eutectic solders doped with RE element
cerium. The presented study includes the effect of Ce on microstructure of solders, IMC layer formation on
the Cu surface, and thermal properties.
2.
EXPERIMENTAL
Four types of solders based on SnCu produced by casting, were chosen for experiment. After melting the
solder, all the components were cast on the cleaned and degreased metal plate. The first used was a solder
alloy without the addition of Ce and the other three were with various Ce contents. The chemical composition
of the solders is shown in Table 1.
Table 1. Chemical composition of used solders
Solder
Chemical composition [wt.%]
SnCu0.7
SnCu0.7 + 0.1 Ce
SnCu0.7 + 0.2 Ce
SnCu0.7 + 0.5 Ce
99.3%Sn; 0.7%Cu
99.22%Sn; 0.68%Cu; 0.1%Ce
99.13%Sn; 0.67%Cu; 0.2%Ce
98.91%Sn; 0.59%Cu; 0.5%Ce
Base material (substrate) Cu was of the thickness of 0.8 mm. Samples with dimensions 20 x 20 mm were cut
out from base metal. This was followed by grinding gradually to blade with a 600, 800 and 1200 grit. The
samples were then put into an ultrasonic cleaner to remove residuals. Subsequently, the samples were
chemically cleaned in 10% HCl, rinsed with water, put in alcohol and dried. Samples were preheated on a
hot plate. SnCu - Cu joints were prepared by the method of hot plate. Preheat should be short to avoid the
formation of an oxide layer on the surface of the base material. Solder bath temperature during the
experiment was 230 ± 10 °C, since the soldering tem perature is 20 to 30 °C above the solder liquidus.
Prepared samples were then annealed at 150 ºC during 50, 500 and 1000 hours. After soldering, the
samples were embedded and prepared for metallographic observation of the grinding rolls of a 600 - 1200
grit and polished with diamond pastes gradually from 9 to 1 µm. Chemical-mechanical suspension Master
Prep was used for polishing. Light microscopy was used for observation of IMC (shape and size) present in
the structure of solders and solder joints at the interface. The thickness of Cu6Sn5 Cu3Sn phases was
determined by means of Image - Pro program.
3. RESULT AND DISCUSSION
Fig. 1 shows the microstructure of SnCu based solders with and without Ce addition. The solder alloys after
addition of a small amount of Ce show finer microstructure.
The results of microscopic analysis of joints for SnCu 0.7 solder without and with the Ce addition of 0.1%,
0.2% and 0.5% Ce after heat affecting at different times are summarized in Fig. 2. Solder-substrate
interfaces were mainly observed.
18. - 20. 5. 2011, Brno, Czech Republic, EU
a)
b)
c)
d)
Fig. 1 Microstructure of solder:
a) SnCu0.7; b) SnCu0.7+0.1Ce;
c) SnCu0.7+0.2Ce; d) SnCu0.7+0.5Ce
18. - 20. 5. 2011, Brno, Czech Republic, EU
50 h
500 h
1000 h
SnCu0.7
Cu6Sn5
Cu6Sn5
Cu3Sn
10 µm
SnCu0.7+0.1Ce
10 µm
10 µm
Cu6Sn5
10 µm
SnCu0.7+0. 2Ce
10 µm
Cu3Sn
10 µm
Cu6Sn5
10 µm
Cu3Sn
SnCu0.7+0.5Ce
10 µm
10 µm
Cu6Sn5
10 µm
10 µm
Fig. 2 Intermetallic compounds present at Cu - solder interface
Cu3Sn
10 µm
18. - 20. 5. 2011, Brno, Czech Republic, EU
The IMC layers in the samples prepared for shorter times of 50 hours have a typical planar or undulating
morphology with Cu6Sn5 scallops.
Measurements of the thickness and Cu6Sn5 Cu3Sn phases are given in Fig. 3. The average thicknesses of
Cu3Sn, Cu3Sn5 and the total layer on both sides are listed in Table 2. With increasing the time to 500 hour,
the scallops were joined together and growing. Small parts of IMC were separated from the Cu6Sn5layer.
Cu3Sn layers observed at the interfaces of the samples prepared with SnCu0.7 solder and small addition of
Ce (0.1, 0.2 and 0.5 wt %) at the time of 1000 hours are described in Figs. 2 and 3.
10 µm
º
Fig. 3 Thickness measurement of Cu3Sn and Cu3Sn5 (150 C / 1000 h)
Table. 2 Measured IMC thickness on the both sides
Solder
SnCu0.7
SnCu0.7+0.1Ce
SnCu0.7+0.2Ce
SnCu0.7+0.5Ce
Time
[h]
50
500
1000
50
500
1000
50
500
1000
50
500
1000
Cu6Sn5
[ µm]
1.10793
4.04597
7.33404
1.82519
3.81412
5.15411
1.29504
4.03211
4.50910
1.27293
4.55703
4.98803
Cu3Sn
[ µm]
0.36240
0.71876
0.77883
0.46228
0.67284
0.77266
0.52431
0.58587
0.79273
0.30071
0.60009
0.78334
Total
1.47033
4.76467
8.11288
2.28747
4.48697
5.92677
1.81935
4.61799
5.30184
1.57365
5.15718
5.77137
In spite of that the addition of Ce into solder did not revealed evident effect in reaction of liquid solder/Cu
substrate [5] the morphology of IMC layers in solid state changed evidently in dependence on the Ce
addition. The layer is less continual with a broken relief. This effect was observed with the SnCu0.7 solders
after 500 and 1000 hours.
18. - 20. 5. 2011, Brno, Czech Republic, EU
4. CONCLUSIONS
The experiment was dealed with the growth of intermetallic compounds present in Sn-based solders – Cu0.7
with and without Ce addition. The results showed that the small amount of Ce in the Sn-Cu 0.7 solder was
caused the refinement of the microstructure. IMCs were in that case smaller. Uneven and rapid increase in
IMC was observed when using Ce free solder. Creation of larger IMCs caused the embrittlement of solder
joints. The thickness of Cu3Sn and Cu6Sn5 IMC present at the interface of solder joint produced during
annealing time of 1000 h was significantly greater than that, present in the solders with the Ce addition.
In comparison to the shape of the IMC at the interface of solder with the addition of Ce, it can be concluded
that the IMC is a compact layer.
ACKNOWLEDGEMENT:
This contribution was prepared under the support of GA VEGA MŠ SR and SAV No. 1/0842/09 and
1/1000/09 projects.
REFERENCES
[1]
SUBRABANIAN, K., N., LEAD-FREE ELECTRONIC SOLDERS, New York, 2007, Springer Science, ISBN 0-38748431-0
[2]
Wang, L., Yu, D., Q., Zhao, J., Huang, M., L., Improvement of wettability and tensile property In Sn–Ag–RE leadfree solder alloy, In Materials Letters, 2002, č. 56, s. 1039–1042.
[3]
DUDEK, M.A. et al., Microstructure an Mechanical Behavior of Novel Rare Earth-Containing Pb- Fre Solders. In
Journal of Electronic Materials, 2006, roč. 35, č. 12, s. 2088- 2097.
[4]
Yu, D., Q., Zhao, L. Wang Improvement on the microstructure stability, mechanical and wetting properties of Sn–
Ag–Cu lead-free solder with the addition of rare earth elements. In Journal of Alloys and Compounds, 2004, č.
376, s. 170-175.
[5]
Chriašteľová, J., Pocisková Dimová, K., Rízeková Trnková, L., Lokaj, J., Turňa, M., Ožvold, M., Intermetallic
compounds formed between Cu substrates and lead-free solders containing Ce, In METAL 2010, ISBN 978-8087294-15-4