Advances in lead-free electronics soldering

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Abstract

Lead-free soldering has emerged as one of the key technologies for assembling in environmental-conscious electronics. Among several candidate alloys, the Sn–Ag–Cu alloy family is believed to be the first choice with the combination of other alloys such as Sn–Zn–Bi, Sn–Cu and Sn–Bi–Ag. Phase diagrams of lead-free alloy systems have been intensively examined by using careful thermal and microstructural analysis combined with the thermodynamic calculation such as the CLAPHAD method. The Cu6Sn5/Cu3Sn layers are formed at most lead-free solder alloy/Cu interfaces, while Cu–Zn compound layers are formed in the Sn–Zn/Cu system. Growth kinetics of intermetallic layers both in solid-state and in soldering are also discussed. Creep and fatigue phenomena are also reviewed. In many aspects of lead-free soldering, much more work is required to establish a sound scientific basis to promote their applications.

Introduction

Sn–Pb solders for metal interconnections have a long history, dating back 2000 years. This solder and the alloys developed with it have long provided and continue to provide many benefits, such as ease of handling, low melting temperatures, good workability, ductility, and excellent wetting on Cu and its alloys. At present, soldering technology has become indispensable for the interconnection and packaging of virtually all electronic devices and circuits. Lead-bearing solders, and especially the eutectic or near-eutectic Sn–Pb alloys, have been used extensively in the assembly of modern electronic circuits. However, increasing environmental and health concerns about the toxicity of lead, as well as the possibility of legislation (as described in the WEEE proposal [1]) limiting the usage of lead-bearing solders, have stimulated substantial research and development efforts to discover substitute, lead-free solder alloys for electronic applications.

Although several commercial and experimental Sn-based lead-free solder alloys exist, none meets all standards, which includes the required material properties (e.g. low melting temperature, wettability, mechanical integrity), good manufacturability, and affordable cost. Current processing equipment and conditions (involving fluxes) have been optimized for Sn–Pb solder alloys over the last 30 years. The development of proper alloy compositions for the new solder systems — with suitable fluxes and assembly processes for lead-free solders — is also needed. Already, several big projects on developing lead-free solders have been carried out, such as the NCMS project in the US [2], the IDEALS project in the EU [3], and the NEDO project in Japan [4]. The reports on these projects have been made available to the scientific community, providing us with much useful information on processing electronic products with lead-free solders. In addition to the practical usage of lead-free solders, however, we need scientific information that enables us to understand the various phenomena occurring in electronic packaging employing lead-free solders. The development of a lead-free solder alloy that has all the aforementioned desirable properties and that allows easy assembly will be a formidable task unless we have established the scientific basis.

To answer these issues, we need to make clear the scientific bases of lead-free solders as well as their surrounding technologies. This paper reviews the current status of lead-free soldering in the following categories: the requirements in selection, current alloy candidates, metallurgical aspects, and mechanical properties. It is hoped that this paper can serve as a valuable source of information to those interested in environmentally conscious electronic packaging.

Section snippets

Resources and costs

In 1995, the world Pb consumption was about five million tons, of which more than half was used in recycled batteries. In contrast, electronic consumption was about 2% of the total waste discarded into landfills after usage. The first step in finding suitable alloy candidates is, therefore, to search for some nontoxic, low-melting-temperature alloys that can replace this amount of Pb [2], [5], [6]. The candidate alloy components involve Sn as the base element, Ag, Bi, Cu, and Zn as the major

Phase diagrams and microstructures

Since the early 1900s, a number of investigators have carried out studies on the thermodynamics and phase diagrams of Sn base alloys. Tin alloys have long been recognized as important alloys for metal–metal interconnection — and not only in electronics. Past work with Sn alloys are well summarized in books on phase diagrams such as Ref. [10]. For ternary systems, Ref. [11] provides a comprehensive summary of research before 1990. Much misleading information appears in past research because of

Summary and future of lead-free soldering

This paper reviewed recent investigations of lead-free solder materials and the soldered joints that use these materials. Intensive research and development on lead-free alloys has been carried out since 1990 because of the environmental hazard potential from Pb in solders carried in electronic waste streams. Three strong lead-free solder candidates have arisen from those investigations: Sn–Ag–Cu near eutectic, Sn–Zn eutectic, and Sn–Bi eutectic alloys. Among them, Sn–(3–4) wt% Ag–(0.5–0.9) wt%

Acknowledgements

The author wishes to thank Dr A.P. Tomsia (Lawrence Berkeley Laboratory) for his helpful advice on interfaces and wetting.

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