THE PHYSICAL PROPERTIES of Sn Bi and Sn Ag MELTS NEAR the EUTECTIC COMPOSITIONS

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

1UralState Pedagogical University, Ekaterinburg, Russia

2Institute 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 alloysof near eutectic compositions. The former are low-temperaturesolders, 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 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.

The positive deviation from the Raoult law(Fig.2) indicates that the interaction of identical atoms is predominant.

Fig.3shows that the electrical resistanceof Sn–Bi alloys dependsstronglyon 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 42wt.% 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 diamagneticand 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

of temperaturein 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 alloyswith 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 lawindicates 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, themagnetic susceptibilityis practically independent on the alloycomposition in liquid state andhas the largest value for Sn–3.5%Agin solid state. In Fig.5 the electrical resistance and magnetic susceptibility dependences ofSn–Ag alloys on Ag content are presented.

Summary

New experimental data on the physical properties (density,electrical resistance andmagnetic susceptibility) ofSn–Bi andSn–Ag alloys near the eutectic compositions over a wide temperature rangewere obtained. The temperature range of anomalous change of properties has been discovered. The overheating of the melt above thistemperature 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 jointssoldered with them.

This work is supported by RFBR (grants06-03-90568 and 07-03-96116).

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