THE PHYSICAL PROPERTIES OF Sn–Bi AND Sn–Ag MELTS
NEAR THE EUTECTIC COMPOSITIONS
A.A. Sabirzyanov1, V.E. Sidorov1, D.A. Yagodin1, S.A. Uporov1, K.I. Grushevskii1,
K.Yu. Shunyaev2
1
2
Ural State Pedagogical University, Ekaterinburg, Russia
Institute of Metallurgy, Ural Branch of RAS, Ekaterinburg, Russia
Abstract. Some properties of lead-free solders, such as Sn–Bi and Sn–Ag near the
eutectic compositions, have been studied. The density, magnetic susceptibility and
electrical resistance were measured as a function of temperature during heating to
1100°C and subsequent cooling. The positive deviation from the Raoult law indicates
that the interaction of identical atoms is predominant. The temperature range of
anomalous change of properties has been discovered. The overheating of the melt
above this temperature range leads to hysteresis of properties in solid state, i.e.
noncoincidence of heating and cooling curves. The heat treatment of the melt is
supposed to lead to essential change of properties of lead-free solders in solid state.
In the last years the movement for the replacement of lead-bearing solders with
lead-free ones is in progress. Its purpose is to prevent the contamination of the
environment with lead from wasted electronic equipment. The European Council
Directive prohibits the sale of electronic products containing lead (with the exception
of medical and some other devices) in the European Union from 1 July, 2006.
However it is difficult to find an alternative to the traditional tin-lead solders because
the solder is bound to match many requirements, such as optimal melting temperature,
high durability of soldered joints, good wetting of joining surfaces and low cost price.
With regard to all characteristics a perfect equivalent of the traditional solders is not
found still.
One of the main criteria in selecting a solder is soldering temperature. All
solders can be classified into four groups: low-temperature (melting point below
180°), with melting temperature equal to that of Sn63/Pb37 eutectic (180−200°),
medium-temperature (200−230°) and high-temperature (above 230°). The great
majority of solders contain tin as one of the components; copper, silver, bismuth or
antimony as second and third components. The solders, containing more than 3
components, are rarely used due to economical reason. The components
concentrations are usually chosen close to eutectic because in this case the
crystallization occurs within narrow temperature range and is not followed by the
components shift, which results in higher strength of soldered joint. It is essential to
avoid the intermetallic compounds arising during the crystallization, since the strength
of the joint decreases due to stress concentration at the phase border of intermetallic
compound with tin matrix. Up to now more than one hundred patents have been
granted for lead-free solders of various compositions, but it is difficult to say which
solder is the best because each of them has the unique combination of properties.
Can the quality of a solder be improved without changing its chemical
composition? The possible approach is a heat treatment of a melt. In this connection
we have studied some properties of Sn–Bi and Sn–Ag alloys of near eutectic
compositions. The former are low-temperature solders, the latter medium-temperature
ones. The density, magnetic susceptibility and electrical resistance were measured as
a function of temperature during heating and subsequent cooling in a helium
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atmosphere. The temperature varied at a rate 3−5° to 1000−1100°C. The density was
measured by absolute gamma-penetration method with an error less than 1%. The
magnetic susceptibility was measured by Faraday method in a field less than 1 T with
an error less than 5%. The electrical resistance was measured by contactless method
of rotating magnetic field by comparison with a reference single crystal tungsten
specimen with an error less than 1%.
The Sn–Bi alloys with 37, 42 (eutectic composition) and 47 wt.% Sn were
studied. The weak density dependence on temperature in solid state and its rapid
linear dependence on temperature in liquid state were observed (Fig.1). The
anomalous behavior of these alloys, i.e. the density increase on melting, which is
inherited from pure Bi, takes place. The melt density linearly increases with Bi
content. The heating and cooling curves coincides with each other within the limits of
random error which is less than 0.3%. The dependence of melt density throughout the
crucible height was not observed.
d, kg/m3
Bi
heating Bi-47wt%Sn
cooling Bi-47wt%Sn
heating Bi-42wt%Sn
cooling Bi-42wt%Sn
heating Bi-37wt%Sn
cooling Bi-37wt%Sn
Sn
10000
9500
9000
8500
8000
7500
7000
6500
0
100
200
300
400
500
600
700
800
900
о
t,1000
С
Fig. 1. Density as a function of temperature for Sn-Bi alloys.
The positive deviation from the Raoult law (Fig.2) indicates that the interaction
of identical atoms is predominant.
Fig.3 shows that the electrical resistance of Sn–Bi alloys depends strongly on
Bi content especially near the melting temperature. Noteworthy is the semiconductor
behavior of the temperature dependence of electrical resistance of Sn–Bi alloys with
37 and 42 wt.% Sn below 220°C.
The study of magnetic susceptibility shows that all investigated Sn–Bi alloys
are diamagnetic except Bi–42%Sn at high temperatures. The Bi–42%Sn alloy is a
weak diamagnetic and has inherited the general form of the temperature dependence
of magnetic susceptibility from Bi. Its magnetic susceptibility depends strongly on
temperature in solid state, drops practically to zero at melting point and is independent
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Vm, 10-5 m3/mol
2,2
2,1
2,0
1,9
1,8
1,7
0
10
20
30
40
50
60
70
80
90
100
at.%Bi
Fig. 2. Molar volume as a function of Bi content for Sn-Bi alloys at 500°.
-8
r*10
, Ом*м
ρ, 810
Ohm·m
140
120
100
Temperature dependences of Bi-Sn alloys.
, kg/m 3
80
00
heating Bi-47wt%Sn
00
60
00
40
нагрев Bi-37wt%Sn
cooling Bi-47wt%Sn
охлаждение Bi-37wt%Sn
heating
Bi-42wt%Sn
нагрев
Bi-42wt%Sn
охлаждение
Bi-42wt%Sn
cooling Bi-42wt%Sn
нагрев
Bi-47wt%Sn
heating Bi-37wt%Sn
охлаждение Bi-47wt%Sn
cooling Bi-37wt%Sn
00
20
0
00
200
400
600
800
1000
t, 0C
Fig. 3. Electrical resistance as a function of temperature for Sn-Bi alloys.
00
00
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00
0
100
200
300
400
500
600
700
800
900
о
t,1000
С
d, kg/m
3
7400
heating Sn-4wt%Ag
cooling Sn-4wt%Ag
heating Sn-3,5wt%Ag
cooling Sn-3,5wt%Ag
heating Sn-3wt%Ag
7300
7200
7100
7000
6900
6800
6700
6600
6500
6400
0
100
200
300
400
500
600
700
800
900
о
t,1000
С
Fig. 4. Density as a function of temperature for Sn-Ag alloys.
ρ, 10-8 Ohm·m
χ, 10-7 emu/g
Ag, wt.%
Ag, wt.%
Fig. 5. Electrical resistance and magnetic susceptibility as a function of Ag content
for Sn-Ag alloys.
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of temperature in liquid state within the experimental accuracy in the covered
temperature range.
It has been established that overheating of the melt above a particular
temperature leads to noncoincidence of heating and cooling curves for the properties
sensitive to electronic structure (electrical resistance and magnetic susceptibility).
Similar measurements have been carried out for Sn–Ag alloys with 3, 3.5
(eutectic composition) and 4 wt.% Ag. A weak linear dependence of density on Ag
content (Fig.4) is observed. As in case of Sn–Bi, the positive deviation from the
Raoult law indicates that the interaction of identical atoms is predominant.
The electrical resistance is the same for all alloys in solid state and has the
largest value for Sn–3.5%Ag in liquid state. On the contrary, the magnetic
susceptibility is practically independent on the alloy composition in liquid state and
has the largest value for Sn–3.5%Ag in solid state. In Fig.5 the electrical resistance
and magnetic susceptibility dependences of Sn–Ag alloys on Ag content are
presented.
SUMMARY
New experimental data on the physical properties (density, electrical resistance
and magnetic susceptibility) of Sn–Bi and Sn–Ag alloys near the eutectic
compositions over a wide temperature range were obtained. The temperature range of
anomalous change of properties has been discovered. The overheating of the melt
above this temperature range leads to hysteresis of properties in solid state, i.e.
noncoincidence of heating and cooling curves. There is reason to suppose that heat
treatment of the melt can lead to essential change of properties of lead-free solders in
solid state as well as joints soldered with them.
This work is supported by RFBR (grants 06-03-90568 and 07-03-96116).
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