Search results

In terms of science, technologies and end‐use applications, ‘lead‐free solders’ are a very broad subject. This paper is intended to highlight the key issues of this broad subject for its applications in electronic packaging and assembly. The areas covered include the main thrusts behind the research and development effort, the scientific approaches, economic and practical considerations, and regulatory and legislative perspectives of lead. In addition, this paper will present the new findings as a result of research and development effort on lead‐free solders. Technologically, in view of the current and future needs in interconnecting materials, the first objective of this work is to demonstrate the ability to provide lead‐free solders in lieu of conventional lead‐bearing solders without mandating significant changes in manufacturing processes and/or equipment which have been established and proven in the industry. The second objective is to design the lead‐free solders which are able to offer superior performance to their Sn/Pb counterparts, thus providing solder interconnections with higher reliability and endurance during service life. The fulfilment of these two objectives will concurrently address the two critical technological areas in the field of solders, namely the inherent vulnerability of conventional solders to temperature and stress, and the concerns of toxicity and health hazard of lead. Based on materials principles, ternary and quaternary lead‐free systems are at present being designed. The work conducted thus far demonstrates that two of the designed solder compositions show great promise. In comparison with 63Sn/37Pb eutectic solder, the new lead‐free compositions exhibit superior performance in shear strength, creep resistance and thermomechanical fatigue resistance. This paper will also cover the data in mechanical testings, the summary of microstructure evaluation (with detailed elucidation omitted), and low‐cycle fatigue fractoqraphy examination of the designed lead‐free compositions.

To provide further knowledge of the effect of solder composition and PCB surface finish on the creep properties of lead‐free SnAgCu solder joints.

Single‐overlap shear specimens were prepared for the creep testing. The test matrix included three different SnAgCu pastes with hypoeutectic, eutectic, and hypereutectic compositions. An Sn63Pb37 solder paste was used as a reference. The PCB finishes used were NiAu, organic solderability preservative (OSP) and immersion tin. The creep tests were performed at 85 and 105°C using a dead‐weight system.

According to the results, the SnAgCu solder with eutectic or near‐eutectic composition is the safest choice when the creep behaviour of solder joints is considered. Of the three different PCB surface finishes, immersion tin is the most favourable choice for use with SnAgCu joints when creep is the predominant deformation mechanism in the joints. On the NiAu finish the creep properties of SnAgCu solder joints were significantly weaker in eutectic and hypereutectic SnAgCu joints than on Sn and OSP.

The results can be used to enhance the reliability of SnAgCu joints in demanding conditions, when special attention is paid to the choice of PCB surface finish and SnAgCu solder composition.

Although the constitutive equations at elevated temperatures given in Eurocode 3 Part 1-2 are valid for steel grades up to S460, the experimental data base concerning the mechanical behaviour of this particular high strength fine-grained structural steel is very small. Especially its short time creep behaviour at high temperatures has not yet been analysed. A current research project, predominantly consisting of transient testing of numerous commercial S460 steels, targets a more precise determination of the stresss-train relationships at high temperatures. Additionally, the time-dependent strain components are extracted from test results gained at varying heating rates, and an empirical creep law, based on the concept of temperature-compensated time, is derived. As a result, the constitutive equations can be formulated taking the heating rate into account. The differences in the high temperature load-carrying capacity of steel structures made of S460 according to the research results and under the terms of Eurocode 3 Part 1-2 are assessed.

MAGNESIUM, because of its low density, has obvious possibilities as an aircraft structural material. The useful magnesium alloys have densities in the range 1·76 to 1·83, compared with the aluminium alloys range of about 2·5 to 2·8. The melting point of magnesium is 650 deg. C., almost identical with that of aluminium (660 deg. C.), so that generally the alloys of each of these base elements have applications in much the same temperature band.

In the first two parts of this paper attention has been directed to determination of the stresses in a disk as dependent upon elastic and plastic strain, including creep effects which may occur at the rim region. The problem has so far, however, been treated without complication introduced by the blade fastenings and therefore, in effect, the disk has been investigated as far as the base of the blade grooves or slots. It has, however, been realized that the projections forming blade root fastenings present their own problems, differing from those of the disk, and these problems have been left to be dealt with in this third portion of the paper. Nevertheless, in Parts I and II the fact that the root fastenings applied an interrupted radial loading at the outside, and not a perfectly distributed load as assumed in the analysis, was not overlooked.

Developments in higher temperature exchangers and other applications, require high strength as well as oxidation resistance at very high temperatures. In order to meet such requirements, Dour Metal has developed the ODM a new range of oxide dispersion strengthened (ODS) iron based materials. Compositional and thermomechanical manipulations can be used to develop the structure enabling the service temperature to be increased up to about 1200°C. Typical rupture strengths are presented for temperatures ranging from 900 to 1100°C. Data on oxidation and corrosion properties of various compositions are also presented. The emphasis is given to alloy ODM 751 on which most recent work has been performed. The composition of the alloy is 16.5% Cr‐4.5% Al‐0.6% Ti‐1.5% Mo‐0.5% Y2O3‐bal Fe and provides the best compromise between strength, formability and oxidation resistance. Measurements of longitudinal creep at 1100°C to 1200°C on ODM 751 tubes show the outstanding properties of the alloy in this range of temperatures.

This paper reviews the status of lead‐free solder development works. Some of the solder systems — Bi‐Sn, Bi‐Sn‐Fe, ln‐Sn, Sn, Sn‐Ag, Sn‐Ag‐Zn, Sn‐Ag‐Zn‐Cu, Sn‐Bi‐Ag, Sn‐Cu, Sn‐Cu‐Ag, Sn‐In‐Ag, Sn‐Sb, Sn‐Zn and Sn‐Zn‐ln — are discussed in more detail, while others are briefly commented on. In general, compared with eutectic Sn‐Pb solder, all the lead‐free solder alternatives investigated more or less exhibit some shortcomings, such as price, physical, metallurgical or mechanical properties. Relatively, Sn‐ln‐containing systems are more promising in terms of solder mechanical properties and soldering performance, although the price of ln may be a concern. Eutectic Sn‐Ag solder doped with Zn, Cu or Sb exhibits good mechanical strength and creep resistance, due to refined microstructure. The Bi‐Sn systems doped with other elements may have a niche in the low temperature soldering field. Eutectic Sn‐Cu has good potential due to its good fatigue resistance. The eutectic Sn‐Zn system modified with ln and/or Ag may be promising in terms of mechanical properties. Finding a lead‐free alternative for high temperature solders presents the biggest challenge to the industry.

Purpose — The purpose of this paper is to seek improved solution techniques for combined boundary‐initial value problems (IVPs) associated with the time‐dependent creep deformation and rupture of engineering structures at high temperatures and hence to reconfigure a parallel iterative preconditioned conjugate gradient (PCG) solver and the DAMAGE XXX software, for 3‐D finite element creep continuum damage mechanics (CDM) analysis.Design/methodology/approach — The potential to speed up the computer numerical solution of the combined BV‐IVPs is addressed using parallel computers. Since the computational bottleneck is associated with the matrix solver, the parallelisation of a direct and an iterative solver has been studied. The creep deformation and rupture of a tension bar has been computed for a range of the number of degrees of freedom (ndf), and the performance of the two solvers is compared and assessed.Findings — The results show the superior scalability of the iterative solver compared to the direct solver, with larger speed‐ups gained by the PCG solver for higher degrees of freedom. Also, a new algorithm for the first trial solution of the PCG solver provides additional speed‐ups.Research limitations/implications — The results show that the ideal parallel speed‐up of the iterative solver of 16, relative to two processors, is achieved when using 32 processors for a mesh of ndf = 153,238. Originality/value — Techniques have been established in this paper for the parallelisation of CDM creep analysis software using an iterative equation solver. The significant computational speed‐ups achieved will enable the analysis of failures in weldments of industrial significance.

This paper aims to identify a variety of binary system solders by alloying, and relevantly derive multiple system Pb-free solders from the former, attempting to replace the high temperature Sn-Pb solder.

The basis of the paper is the synthesis of previous studies. In terms of some binary high temperature solder alloys, such as Au-20Sn, Bi-2.5Ag, Sn-5Sb, Au-12.5Ge, Zn-6Al and Zn-Sn, taking the alloy phase diagram as the starting point, the melting characteristics, microstructure, mechanical properties, wetting ability and reliability of solder joint are analysed and the prospect is consequently indicated.

Based on the analysis of the six groups of Pb-free solders, the present binary system solder alloys, from the perspective of melting properties, mechanical properties, soldering or reliability of solder joint, rarely meet the comprehensive requirements of replacing the high-temperature Sn-Pb solder. It is assumed to be a solution that multiple-system Pb-free solders derive from a variety of binary system solders by means of alloying. The future development of high temperature Pb-free solder may focus on some factors such as physical properties, mechanical properties, processing, reliability of solder joint, environmental performance and expense.

The paper concentrates on the issue of Pb-free solders at high temperature. From a specific perspective of binary system solders, the presently available Pb-free solders are suggested from the starting point of the alloy phase diagram and the prospect of alternatives of Sn-Pb solders at high temperature are indicated.

This paper presents highlights of on-going research, which aims at developing analytical, computational and experimental predictions of the phenomenon of creep buckling in steel columns subjected to fire. Analytical solutions using the concept of time-dependent tangent modulus are developed to model time-dependent buckling behavior of steel columns at elevated temperatures. Results from computational creep buckling studies using Abaqus are also presented, and compared with analytical predictions. Material creep data on ASTM A992M steel is also presented and compared to existing creep models for structural steel. Both analytical and computational methods utilize material creep models for structural steel developed by Harmathy, by Fields and Fields, and by the authors. Predictions from this study are also compared against those from Eurocode 3 and the AISC Specification. Results of this work show that neglecting creep effects can lead to erroneous and potentially unsafe predictions of the strength of steel columns subjected to fire.