Effects of small amount addition of rare earth Er on microstructure and property of SnAgCu solder

doi:10.1016/j.jallcom.2006.11.165

Journal of Alloys and Compounds

Volume 453, Issues 1–2, 3 April 2008, Pages 180-184

https://doi.org/10.1016/j.jallcom.2006.11.165 Get rights and content

Abstract

In the present work, effect of adding small amount of Er on the microstructure, physical and mechanical properties of Sn3.8Ag0.7Cu solder has been investigated. The purpose is to understand if the heavy rare earth Er has similar effect as light rare earth Ce or La on improving properties of SnAgCu lead-free solder. The results indicated that adding small amount of Er can evidently improve the wettability, mechanical strength and creep rupture life of the Sn3.8Ag0.7Cu solder alloy. It is pointed out that proper Er content in the Sn3.8Ag0.7Cu solder alloy should be in a range of 0.05–0.25 wt%. Moreover, the size of intermetallic compounds (IMC) was measured quantitatively for the Sn3.8Ag0.7Cu solder alloys with various Er contents. It is felt that the increase in mechanical strength may be related to the refining of IMCs of the solder due to the addition of small amount of Er.

Introduction

Electrical equipment that is placed on the EU market will be presumed to comply with the requirements of the directive of the restriction of the use of certain hazardous substances in electrical and electronic equipment (RoHS), 2002/95/EC [1]. Under the pressure of legislation and trade competitions, large quantities of work have been carried out for the substitutes of SnPb solders. Because of the excellent wetting and mechanical properties, the SnAgCu eutectic or near-eutectic alloy has recently been considered as one of the most promising lead-free solder materials that can replace the traditional SnPb solder. Moreover, under the drives of increasingly finer pitch and severe service conditions, novel lead-free solders with better creep and thermal fatigue performances are needed sometimes. Thus, it is expected that the addition of the fourth element will further improve the performance of the SnAgCu solders [2]. In the past investigation, adding minute amount of rare earth (RE) to the alloy is considered to be an effective way to improve the high temperature performance of solders [3], [4], [5], [6].

Rare earth elements have been regarded as the vitamin of metals, which means that a minute amount of RE elements may greatly enhance the properties of metals. RE elements are the surface-active element, which plays an important role in metallurgy of materials, such as refinement of microstructure, alloying and purification of materials and metamorphosis of inclusions.

It is well-known that there are two groups of rare earth elements. One is called light RE or Ce group of elements, which includes seven elements. They are La, Ce, Pr, Nd, Pm, Sm, Eu. The other is called heavy RE or Y group of elements, such as Er, Y, Sc, Gd, Tb, Yb, etc. In recent studies of the effect of RE on the lead-free solders, the RE used generally are the light RE, such as La, Ce or mixed rare earth of mainly Ce and La [3], [4], [5], [6], [7], [8], [9]. Even all the RE elements have very similar physical-chemical properties, we still do not know whether the Y group of elements have similar effect on improving properties of lead-free solder till now. Thus, in the present studies rare earth Er is selected for the investigation. The aim is to study the effect of adding small amount of Er on the microstructure, physical and mechanical properties of SnAgCu solder.

Section snippets

Alloy design and preparation

The pure Sn, Ag and Cu metals with purity of 99.9 wt% were used as raw materials. The RE was Er with purity of 99.9 wt%. The investigated solders are listed in Table 1. The pure metals were mixed and then melted in an Al₂O₃ ceramic crucible at 550 °C for 30 min. With the help of a stainless steel bell with holes on the sidewall, the RE was pushed into the melted Sn liquid alloy. After the RE melted, the melted alloy was held for 2 h. To homogenize the solder alloy, mechanical stirring was performed

Melting temperature

The onset point of the DSC heating curve corresponds to the solidus temperature. The onset temperature was formed by a computer software and listed in Table 2. The results indicate that the melting temperature of Sn3.8Ag0.7Cu solder determined in this investigation is consistent with the previous data [11]. It is indicated that with the addition of small amount of rare earth Er, the melting temperatures changed only slightly. The solidus of the Sn3.8Ag0.7CuEr is approximately within 217–219 °C.

Conclusion

(1)
Adding small amount of Er can evidently improve the wettability, mechanical strength and creep rupture life of the Sn3.8Ag0.7Cu solder alloy.
(2)
Proper Er content in the Sn3.8Ag0.7Cu solder alloy should be in a range of 0.05–0.25 wt%.
(3)
The increase in shear strength may be related to the refining of IMCs particles and solid solution strengthening of the solder due to the addition of small amount of Er.

Acknowledgement

The authors greatly appreciate the financial support of this work by the National 863 Hi-Tech Project (No. 2002AA322040).

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Effects of small amount addition of rare earth Er on microstructure and property of SnAgCu solder

Abstract

Introduction

Section snippets

Alloy design and preparation

Melting temperature

Conclusion

Acknowledgement

Microelectron. Reliability

J. Alloys Compd.

Mater. Sci. Eng.

Mater. Sci. Eng.

J. Electron. Mater.