Materials Sciences and Applications, 2015, 6, 1008-1013
Published Online November 2015 in SciRes.
How to cite this paper: Wadud, M.A., Gafur, M.A., Qadir, M.R. and Rahman, M.O. (2015) Thermal and Electrical Properties
of Sn-Zn-Bi Ternary Soldering Alloys. Materials Sciences and Applications, 6, 1008-1013.
Thermal and Electrical Properties of
Sn-Zn-Bi Ternary Soldering Alloys
M. A. Wadud1, M. A. Gafur2*, M. R. Qadir2, M. O. Rahman1
1Department of Physics, Jahangirnagar University, Savar, Bangladesh
2PP&PDC, BCSIR, Dhaka, Bangladesh
Received 28 August 2015; accepted 17 November 2015; published 20 Novemb er 2015
Copyright © 2015 by authors and Scientific Research Publishing Inc.
This work is licensed under the Creative Commons Attribution International License (CC BY).
Sn-Zn based solder is a possible replacement of Pb solder because of its better mechanical proper-
ties. The alloys need to be studied and explored to get a usable solder alloy having better proper-
ties. In this work, eutectic Sn-9Zn and three Tin-Zin c-Bismuth ternary alloys were prepared and
investigated their thermal and electrical properties. Thermo-mechanical Analysis and Differential
Thermal Analysis were used to investigate thermal properties. Microstructural study is carried
out with Scanning Electron Microscope. The alloys have single melting point. The co-efficient of
thermal expansion and co-efficient of thermal contraction varies with alloy composition and tem-
perature range. Electrical conductivity changes with Bi addition.
Lead Free Solder Alloy, Eutectic Alloy, DTA, TMA, Conductivity
1. Introduction
Tin-Lead solders have been widely used in electronic and optoelectronic packaging due to their low cost and
low melting temperature and good soldering properties. But lead is an aggressive threat for human health and
the environment due to its toxicity. Many countries banned using lead and lead alloys [1] [2] for their use in
packaging. Waste Electronic and Electronic Equipment (WEEE) and Restriction of Hazardous Substances
(RoHS) approved banning the use of lead in European Union countries effective July 2006. The USA, the EU
and Japan forbade the use of Lead containing products [3] [4]. Because of the toxicity of lead , traditional Sn-Pb
solders are now being replaced with Sn-base soldering alloys containing additions of other metals (Ag, Bi, In,
etc.) [5]-[9]. Au-Sn is thought to be alternative but its mechanical properties are not sufficient [10] [11]. SAC
Corresponding author.
M. A. Wadud et al.
solder also an alternative has eutectic point at 217˚C [12] and good wetting properties [13]. The search for a
perfect Lead-free solder alloy with equivalent mechanical and thermal properties to Sn-Pb solder is an urgent
task. Targeting the deadline (July 1, 2006), a large number of studies on Lead-free solders were being conducted
worldwide to find an appropriate replacement. Nowadays, Sn-Ag base alloys containing one or more additional
metal are being used as alternating soldering alloy [5] [6] [8] [14]. Soldering alloy is the p ri me material for elec-
tronic packaging. Melting temperature, mechanical properties, wetting properties, thermal expansion and elec-
trical conductivity, etc. are very important issue for selecting a solder material. Recently Sn-Zn eutectic alloys
have received particular attention due to its low melting temperature [15] [16]. Sn-Zn eutectic alloy is consi-
dered as a good candidate for the replacement of traditional Pb-containing solder alloy [17]-[19]. However,
available infor matio n in literatur e abo ut the evolutio n and pro pe rties of Sn-Zn solder alloy is not enough [20]. In
this study eutectic S n-Zn allo y and t hree Sn-Zn-Bi ternary alloys containing 1%, 2% and 3% Bi were developed
and their thermal and electrical properties were measured. Melting temperature was studied with differential
thermal analyzer (DTA). Thermo-mechanical analysis (TMA) was carried to study thermal expansion. Electrical
conductivity was measured with Edd y current method.
2. Experimental Work
Sn-Zn-Bi solder alloys were prepared by using Tin, Zinc and Bi with 99.9% purity. Four alloys having different
compositions were prepared and studied here. Sn-Zn with desire compositio n was melted in an electrical fur nace
in a clay-graphite crucible at 450˚C temperature for 30 minutes. Then Bi was added and the mixture again kept
in furnace for 20 minutes. Then the liquid alloy poured in a cast iron mould having dimensions 300 mm × 10
mm × 10 mm and 10 mm mould thickness. The as cast alloys were sectioned and polished with emery paper and
then wet polis hed. Polished s amples then cleaned and etched by ethanol with 5% HNO3 to observe microstruc-
ture. Prepared samples were investigated by JOEL JSM-7600F scanning electron microscope. The co-efficient
of thermal expansion was studied up to 170˚C with TMA/SS6300, SII Nanotechnology Inc; Japan at a heating
rate of 10˚C/min. Melting behavior was studied with TG/DT A6300 SII nanotechnology, Japan at a heating rate
of 20˚C/min in a nitrogen environment. In this paper four alloys are referred as Sn-9Zn, Sn-8Zn-1Bi,
Sn-7Zn-2Bi and Sn-6Zn-3Bi. The pouring temp erature o f the liq uid sold er allo ys and the preheatin g te mperature
of the metal mold are 450˚C and 220˚C, respectively.
3. Results and Discussion
3.1. Differential Thermal Analysis
Melting temperature is the main characteristic of a solder alloy while it determines the maximum operating
tempe rat ure o f the s yste m and the mini mum pr oce ssin g te mpe rat ure it s compo ne nts must s urvi ve [21]. Figure 1
shows the superimposed Differential Thermal Analysis (DTA) curves of Sn-Zn-Bi allo ys. Meltin g temperature,
solidification temperature and pasty range of the alloys are presented in Table 1. They are extracted from the
DTA curves. The melting temperature is one of the most important considerations for the development of the
solder alloy because the high melting temperature of the solder alloy increases the reflo wing temperature in the
electronic packaging process and causes thermal damage to the polymer substrate. Melting temperature of
Sn-9Zn alloy is 199.4˚C. It is seen t hat Bi additio n decreases the melti ng temperature o f Sn-9Zn alloy. From the
SEM microstructure it is seen that Bi rich platelets are observed in Sn-6Zn-3Bi alloy (Figure 2(d)). Bi forms
43Sn-57Bi eutectic composition with the addition of Bi in Sn-9Zn alloy which has a relatively low melting
temperature. This eutectic formation contributes in lowering the melting temperature of Sn-9Zn alloy. It is
thought that some of high concentration Bi area might have formed 43Sn-57Bi eutectic. Similar criteria have
also been reported by other authors [22] [23]. All the alloys show single melting temper a ture.
3.2. Thermo-Mechanical Analysis
T h e r mo-mechanical Analysis (TMA) data o btai ned for Sn(9 -x)Zn-xBi alloys at different temperature range is
shown in Figure 3. It is observed that co-efficient of thermal expansion (CTE) changes with temperature.
This is why CTE presented for two temperature range. The coefficient of thermal expansion of Sn-9 Zn alloy
is 23.39867 × 106/˚C which is a good agreement with the literature value [24] . At low temperature range
CT E o f S n-9Zn alloy increases with Bi ad dition. At high temperature range nature of CTE is similar with low
M. A. Wadud et al.
Figure 1 . Endoth er mic p eaks of the Sn-(9 -x)Zn-xBi alloy.
(a) (b )
(c) (d)
Figure 2 . S EM micrograph of (a) Sn-9Zn alloy, (b) Sn-8Zn-1Bi alloy, (c) Sn-7 Zn-2Bi alloy and (d) Sn-6Zn-3Bi alloy.
M. A. Wadud et al.
Table 1. Liquidus and solidus point.
Allo y Solidus Temperature
˚C Liquids Temp era t u re
˚C Pasty Range
˚C Endothermic Peak
Sn-9Zn 199.4 224 24.6 204.1
Sn-8Zn-1Bi 198.1 223.5 25.4 203.1
Sn-7Zn-2Bi 197.3 224.8 27.5 202.4
Sn-6Zn-3Bi 196.1 225.5 29.4 203.1
Figure 3 . CTC/CTE of Sn-(9-x)Zn-xBi alloy.
Fig ure 4. Electrical conductivity of Sn-Zn-Bi alloy.
temperature but the numerical value of CTE is less than that of the low temperature. Co-efficient of thermal
contraction (CTC) of Sn-9Zn alloy increases with add ition of Bi. Thermal expansion depends on bonding
energy which also affects the melting point of the solid. High melting point materials likely to have lower
thermal expansion [25]. From DTA it is observed that Bi decreases the melting temperature of Sn-9Zn alloy.
Low melting po ints means low b onding energ y which decreases the CTE.
3.3. Electrical Conductivity of Sn-(9-x) Zn-xBi
Figure 4 shows the electrical conductivity of Sn-(9-x) Zn-xBi alloy. It shows that the conductivity is conti-
nuously decreases with Bi addition. Bi is soluble in β-Sn and up to 3wt.% Bi can remain in solid solution. The
M. A. Wadud et al.
implication of this would be to expect some contribution to hardening from solid solution [13] [22]. Bi rich
platelets are observed in SEM microstructure of Sn-6Zn-3Bi alloy (Figure 2(d)).The continuous decrease of
conductivity can be attributed to solid solution of high resistance Bi phase in solder matrix, which acts as scat-
tering ce nters for cond uction ele ctrons in crystals [26]. Similar criteria were reported by other author [27].
4. Conclusion
Eutectic Sn-Zn alloy and t hr e e Sn -Zn-Bi ternary alloys were cast. Melting behavior, thermal expansion and con-
traction and electrical conductivity were investigated. Thermal properties of Sn-Zn alloy changes with Bi addi-
tion. Melting po int decreases with Bi a ddition. Coefficie nt of thermal expansion a nd coefficient of thermal co n-
traction increase with Bi add ition. Electrical conductivity decreases with Bi add ition.
The autho r s ar e gr a teful to B anglad e s h Co unc il of Sc ienti fic and I nd ustr ia l Re sea r ch fo r p ro vid in g t he m wit h the
research facilities to carry out the work.
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