Electronic Solder Research Articles

Environmentally friendly assembly of robust electronics without solder

PurposeTin‐lead solder has been the primary method for connecting electronic components to printed circuit boards since near the time of its inception. Over the last 60 years, solder has proven a viable assembly method over that time and there is a deep understanding of the technology won over years of practice. However, the European Union has banned the use of lead in electronic solder, based on the misguided assumption that lead in electronic solder represented a risk to human health. Aims to describe a new approach to manufacturing electronic assemblies without the use of solder.Design/methodology/approachThe paper discusses how the new era of lead‐free solder has resulted in a host of new problems for the electronics industry, many of which had not been experienced when elemental lead was included in the solder alloy.FindingsElectronics assembly technology literature is rife with articles and papers citing the problems or challenges of lead‐free assembly and proposing new or improved solutions or investigative tool to better unearth the problems of lead‐free. The new process has come to be known as the Occam process, named to honor the fourteenth century English philosopher and logician, William of Occam, whose rigorous thinking and arguments in favor of finding the simplest possible solution served as the inspiration and catalyst for the new approach.Originality/valueThe paper describes a new approach to manufacturing electronic assemblies without the use of solder.

X-ray based methods for non-destructive testing and material characterization

The increasing complexity and miniaturization in the field of new materials as well as in micro-production requires in the same way improvements and technical advances in the field of micro-NDT to provide better quality data and more detailed knowledge about the internal structures of micro-components. Therefore, non-destructive methods like radioscopy, ultrasound, optical or thermal imaging increasingly gain in importance with respect to ongoing product and material development in the different phases like material characterization, production control or module reliability testing. Because of the manifold different application fields, i.e., certain physical NDT methods applied to material inspection, characterization or reliability testing, this contribution will focus on the radioscopic-based methods related to their most important applications. Today, in modern industrial quality control, X-ray transmission is used in two different ways: • Two-dimensional radioscopic transmission imaging (projection technique), usually applied to inline inspection tasks in application fields like lightweight material production, electronic component soldering or food production. • Computed tomography (CT) for generation of three-dimensional data, representing spatial information and density distribution of objects. CT application fields are on the one hand the understanding of production process failure or component and module inspection (completeness) and on the other hand the dimensional measuring of hidden geometrical outlines (metrology). This paper demonstrates the methods including technical set-ups (X-ray source and detector), imaging and reconstruction results and the methods for high speed and high-resolution volume data generation and evaluation.

Chemical Reaction: The U.S. Response to REACH

Last summer ushered in a new era in the regulation of chemicals. On 1 June 2007, REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals), the expansive scheme by the European Union (EU) to regulate chemicals used in commerce and consumer products, took effect. REACH applies to chemicals manufactured or marketed in Europe, and its regulations affect companies exporting chemicals to Europe as well as those located there. REACH puts the burden on chemical companies to provide information on how the chemicals they make affect human health and the environment. REACH has two parts: the collection and sharing of data throughout supply chains, and the authorization of chemicals of higher concern to human and environmental health.

Three-dimensional (3D) microstructure visualization of LaSn 3 intermetallics in a novel Sn-rich rare-earth-containing solder

Global concern over the environmental impact and healtheffects of Pb-based solders in consumer electronics has led tothedevelopmentofPb-freesolderalternatives.Sn-richsolderssuch as Sn–Ag and Sn–Ag–Cu are the most promising Pb-freealloysforreplacementofPb–Snsolder[1].Manyissuesrelatingto their mechanical properties and reliability still need to beaddressed. This includes, in particular, their higher meltingpoint andlower ductility[2,3]. Thelower ductilityofthe solderhas important implications because it is directly related tomechanical shock and drop resistance [4].Recently, small additions of rare-earth elements (RE), suchas Ce and La, have been shown to refine the microstructuresand improve the mechanical response of Sn-rich solders [5–8].Due to the complexity of these ternary and quaternarysystems, very little information is available on Sn–RE inter-metallics formed during solidification. Dudek et al. [7] showedthat small La additions (0.1 wt.% and 0.5 wt.%) to Sn–Ag–Curesults in the formation of a stable, homogeneously distrib-utedLaSn

Lead-free solders: Reliability and other issues a year after the legislation

July 1, 2006 is in the rearview mirror and the legislated implementation of a ban on lead in electrical and electronic equipment throughout the European Union according to the directives of Restriction of the use of Certain Hazardous Substances in Electrical and Electronic Equipment (RoHS) is almost a year old. Although legislation in Europe was implemented according to the plan, a number of questions remain regarding lead-free technology as the drop-in replacement for lead-bearing alloys. To compound the confusion, China is only in the drafting stage of its “Management Methods for Prevention and Control of Pollution from Production of Electronic and Information Products,” and with the exception of California, the United States has yet to legislate lead-free solders in electronics. In light of this legislation many electronic products manufacturers are making haste to implement lead-free solders and to market reliable green products. Thus, in response to growing interest and want of technical information on lead-free solders, the Electronic Packaging and Interconnection Materials Committee of the TMS Electronic, Magnetic & Photonic Materials Division organized a series of successful symposia on lead-free and lead-bearing solders in the past few years. The symposia were held at TMS annual and fall meetings and results were published. Continuing the trend, the recent “Pbfree Electronic Solders: Alloy Design, Characterization and Service Reliability” symposium held from February 25 to March 1, 2007 at the TMS Annual Lead-Free Solders: Reliability and Other Issues a Year after the Legislation

Influence of corrosion properties on electrochemical migration susceptibility of SnPb solders for PCBs

Electrochemical migration is caused by the adsorption of water and the bias voltages between the electrodes or pads which form an electric circuit or in the solders used to connect the parts. This work focused on the elucidation of the mechanisms of electrochemical migration of pure Sn, Sn37Pb, and Sn55Pb solders in electronics. The electrochemical migration behavior was discussed on the basis of the polarization behavior of SnPb solders. After the water drop test, the time to failure decreased with increasing Pb content in Cl − solution and increased with increasing Pb content in SO4 2− solution. In the case of the SnPb solder alloys, the pitting potential, the passive current density, the cathodic current density and the efficiency of cathodic deposition of the alloying elements were closely related to the resistance of the electrochemical migration.

Tin pest issues in lead-free electronic solders

Tin pest is the product of the β → α allotropic transition at 13.2°C in pure tin. It is a brittle crumbly material, often responsible for the total disintegration of the sample. The transformation involves nucleation and growth, with an incubation period requiring months or years for completion. Experimental observations reveal a substantial inconsistency and an incomplete understanding of the process. Some alloy additions promote tin pest by reducing the incubation time, whereas others retard or inhibit its formation. Traditional solder alloys have generally been immune to tin pest in service due to the presence of lead, and bismuth and antimony as common impurities. However, the new generation of lead-free solders are more dilute—closely resembling tin. A much debated question is the susceptibility of these alloys to tin pest. Bulk samples of tin-0.5 copper solder undergo the transition at −18°C although not at −40°C after five years exposure. Other lead-free alloys (Sn–3.5Ag, Sn–3.8Cu–0.7Cu and Sn–Zn–Bi) are immune from tin pest after a similar period. Large scale model joints exhibit tin pest but it appears that actual joints may be resistant due to the limited free solder surface available and the constraint of intermetallic compounds and components. It seems likely that impurities are essential protection against tin pest, but for long term applications there is no certainty that tin pest and joint deterioration will never occur.

Development of Sn–Ag–Cu and Sn–Ag–Cu–X alloys for Pb-free electronic solder applications

The global electronic assembly community is striving to accommodate the replacement of Pb-containing solders, primarily Sn-Pb alloys, with Pb-free solders due to environmental regulations and market pressures. Of the Pb-free choices, a family of solder alloys based on the Sn-Ag-Cu (SAC) ternary eutectic (T eut. = 217°C) composition have emerged with the most potential for broad use across the industry, but the preferred (typically near-eutectic) composition is still in debate. This review will attempt to clarify the characteristic microstructures and mechanical properties of the current candidates and recommend alloy choices, a maximum operating temperature limit, and directions for future work. Also included in this review will be an exploration of several SAC + X candidates, i.e., 4th element modifications of SAC solder alloys, that are intended to control solder alloy undercooling and solidification product phases and to improve the resistance of SAC solder joints to high temperature thermal aging effects. Again, preliminary alloy recommendations will be offered, along with suggestions for future work.

Composite lead-free electronic solders

Composite approaches have been developed in lead-free solder research in an effort to improve the service temperature capabilities and thermal stability of the solder joints. This article overviews the background for composite lead-free solder development, the roles of reinforcements and their requirements, composite solder processing techniques, as well as current status of composite solder research. Examples of several representative lead-free composite solders produced with various methods and reinforcement types are presented. The effects of reinforcement addition on processing and mechanical properties are analyzed. Difficulties and problems that exist in composite solder research are proposed and tentative solutions are attempted with examples of newly emerging novel lead-free composite solders.

Interfacial reaction issues for lead-free electronic solders

The interfacial reactions between Sn-based solders and two common substrate materials, Cu and Ni, are the focuses of this paper. The reactions between Sn-based solders and Cu have been studied for several decades, and currently there are still many un-resolved issues. The reactions between Sn-based solders and Ni are equally challenging. Recent studies further pointed out that Cu and Ni interacted strongly when they were both present in the same solder joint. While this cross-interaction introduces complications, it offers opportunities for designing better solder joints. In this study, the Ni effect on the reactions between solders and Cu is discussed first. The presence of Ni can in fact reduce the growth rate of Cu3Sn. Excessive Cu3Sn growth can lead to the formation of Kirkendall voids, which is a leading factor responsible for poor drop test performance. The Cu effect on the reactions between solders and Ni is then covered in detail. The knowledge gained from the Cu and Ni effects is applied to explain the recently discovered intermetallic massive spalling, a process that can severely weaken a solder joint. It is pointed out that the massive spalling was caused by the shifting of the equilibrium phase as more and more Cu was extracted out of the solder by the growing intermetallic. Lastly, the problems and opportunities brought on by the cross-interaction of Cu and Ni across a solder joint is presented.

Rare-earth additions to lead-free electronic solders

The research in lead(Pb)-free solder alloy has been a popular topic in recent years, and has led to commercially available Pb-free alloys. Further research in certain properties to improve aspects such as manufacturability and long term reliability in many Pb-free alloys are currently undertaken. It was found by researchers that popular Pb-free solders such as Sn-Ag, Sn-Cu, Sn-Zn and Sn-Ag-Cu had improved their properties by doping with trace amounts of rare earth (RE) elements. The improvements include better wettability, creep strength and tensile strength. In particular, the increase in creep rupture time in Sn-Ag-Cu-RE was 7 times, when the RE elements were primarily Ce and La. Apart from these studies, other studies have also shown that the addition of RE elements to existing Pb-free could make it solderable to substrates such as semiconductors and optical materials. This paper summarizes the effect of RE elements on the microstructure, mechanical properties and wetting behavior of certain Pb-free solder alloys. It also demonstrates that the addition of RE elements would improve the reliability of the interconnections in electronic packaging. For example, when Pb-free-RE alloys were used as solder balls in a ball grid array (BGA) package, the intermetallic compound layer thickness and the amount of interfacial reaction were reduced.

Issues related to the implementation of Pb-free electronic solders in consumer electronics

Consumer electronic applications are the primary target of the Pb-free initiative and package assembly and performance is affected by the move from eutectic Sn-Pb to Pb-free solder alloys. This paper outlines the key issues and mitigation possibilities for package assembly using Pb-free solders: High temperature reflow, Interfacial reactions, and Reliability. At the high temperatures required to reflow Pb-free alloys, moisture absorbed into the package can result in delamination and failure. The reaction of the Pb-free solder with Ni and Cu metallizations results in interfacial intermetallics that are not significantly thicker than with Sn-Pb but provide a path for fracture under mechanical loading due to the increased strength of the Pb-free alloys. The reliability issues discussed include thermomechanical fatigue, mechanical shock, electromigration and whiskering. The Pb-free alloys tend to improve thermomechanical fatigue and electromigration performance but are detrimental to mechanical shock and whiskering. Design trade-offs must be made to successfully implement Pb-free alloys into consumer applications.

Theory of Endochronic Cyclic Viscoplasticity of Eutectic Tin/Lead Solder Alloy

AbstractIn this paper, a theory of Endochronic cyclic viscoplasticity of eutectic Tin/Lead (60Sn/40Pb) solder alloy under cyclically thermomechanical strain histories had been established. Under the conditions of isotropic and inelastically incompressible small deformation, the constitutive equation of deviatoric behavior was expressed as:here and the strain rate dependent intrinsic time scale and . Employing the experimental cyclic shear stress-strain curves of various testing temperature and frequency, all temp. dependent material parameters and ; and the temp.-freq. dependent material function were determined for temp. between 213K and 423K and freq. between 0.3Hz and 0.01Hz. Predicative capability of the theory were then challenged by a set of experiments with complicate strain history such as (i) Fast in tension/Slow in compression constant strain amplitude cyclic tests (ii) Slow-Fast-Slow constant amplitude cyclic tests. Through the excellent computational results, the present theory demonstrated that it can, not only play a vital role in the area of electronic solder mechanics, but also meet the needs of reliability analysis and life assessment in the electronic/photoelectronic packagings.

Wave Soldering Using Sn/3.0Ag/0.5Cu Solder and Water Soluble VOC-Free Flux

Environmental and health concerns, due to the leaching of lead from landfills into ground water, have necessitated legislation that restricts the use of lead in electronics. The transition from the eutectic tin-lead composition used in electronic solders to lead-free solder is imminent. Understanding the impact of this transition on lead-free wave soldering is crucial because a large segment of printed circuit boards (PCBs) used in electronic home appliances are wave soldered. The usage of volatile organic compound (VOC)-free flux chemistry is expected to gather momentum in conjunction with lead-free wave soldering because of process requirements and environmental considerations. A thorough review of published literature indicated that there is limited information available on the application of water-soluble VOC-free flux chemistries for lead-free wave soldering. Consequently, the objectives of this research were to select a preferred VOC-free water-soluble flux chemistry, understand the process window of wave soldering using a 96.5Sn/3.0Ag/0.5Cu lead-free alloy, and study the impact of the lead-free wave soldering process on different surface finishes. Many of the earlier process recommendations were based on a solder pot temperature of 260 degC and higher. However, the packaging of through hole components may not withstand the higher pot temperature and longer contact time. Hence, assembly at a lower solder pot temperature and shorter contact time are highly desired. Consequently, experiments were conducted to verify the feasibility of reducing the solder pot temperature (250 degC) and the contact time (2 s). Three VOC-free fluxes, from three different vendors, were evaluated and the best flux chosen was used to establish the process window. It was demonstrated through this study that it is possible to wave solder, using a lead-free solder with low silver content, different surface finishes with a wide process window. The effect of lead-free process temperatures on the cleaning of the VOC-free water-soluble flux residues was evaluated. Residues were washed and cleaned using the existing equipment sets without any change in the process parameters. The process developed based on the initial set of experiments has been validated by the successful assembly of a number of lead-free prototype assemblies

Evaluation of Lead Free Soldering Development by Means of a Holistic Consideration

Engineering is still mainly oriented towards step‐by‐step‐processes. Due to ongoing challenges such as increasing complexity or shortened time for product developments, connections and correlations have to be understood and used as an important part of work in engineering practice as well as in research. Therefore, a tool derived form a holistic model based on a material approach is explained. The subject of lead‐free electronic soldering is taken for illustrating its practical use and potential.

Electrochemical migration of Sn-Pb and lead free solder alloys under distilled water

Electrochemical migration (ECM) is a potential reliability problem in electronic soldering which might become more dangerous in lead free electronic devices. In this paper, electrochemical migration tests on Sn-Pb and lead free solder alloys were conducted under distilled water by applying constant voltages with a power supply. The susceptibility of the solder alloys to ECM and the effect of the composition on ECM behavior were studied. It is found that both Sn-Pb and lead free solders investigated in present research have susceptibility on ECM. Dendrites grow from cathode to anode and show different morphologies with the different migration elements involved. In Sn-37Pb and Sn-36Pb-2Ag solders, the main migration element is Pb. While for Sn-Ag and Sn-Ag-Cu solder alloys, Sn leads the migration. For Sn-8Zn-3Bi, both Sn and Zn can migrate. Furthermore, the effect of applied voltage on the time to short and short resistance was also investigated. As could be expected, the higher the voltage is, the shorter the failure time is. The electrochemical migration mechanism of the solder alloys was also discussed.

Electronic packaging solder joint reliablity assessment with a mechanics-based StrainGage methodology

It is essential to understand solder joint strains to improve package reliability. However, it is often difficult to measure the true solder joint strains directly. Strain gages have been increasingly used by component suppliers and original equipment manufacturers (OEMs) to indicate package mechanical stress levels. One of the most used strain gage locations is on the component side, right next to the package corner. A concern with this location alone is that the strain gages mounted close to the package pick up the local stress concentration near the package edges or corners. In this paper, appropriate strain gage locations are suggested based on mechanics principles and finite element simulation results. An analytical methodology is developed to determine the solder joint deformations from strain gage readings at various locations. This paper also proposes a new strain matching criterion. Most OEMs have been using maximum principal strain to match stress levels and define critical strain limits. A problem with maximum principal strain is that it does not indicate the direction along which the maximum bending occurs. Experimental data demonstrates that the new strain criterion has much better correlation to solder joint failure than maximum principal strains in various bend modes. The conclusions apply to any strain gage mounting metrology on packages attached to printed circuit boards.

Identification of issues with controversial technologies

Long-term and cumulative side-effects of technologies (e.g., health effects) are likely to become more relevant management issues in the future. Social processes play an important role and companies are forced to deal with such issues more intensively. Identification of such topics is the first and most important step to managing issues. We have conducted interviews with company representatives to understand the companies' perspective during these early phases. The 42 cases are arranged around the European phase-out of the use of lead-bearing solders in electronics manufacturing in 2006. We found that the outside-in perspective is the main source of information about upcoming issues. Sensing weak signals is the prevailing mechanism to become aware of these topics. Even though hints about the necessity to phase out a technology are known long before, customer push and strict regulatory requirements have been found to be the most important triggers for systematic actions by companies.

Are you ready for lead-free electronics?

An expedient transition to lead-free electronics has become necessary for most electronics industry sectors, considering the European directives [The Waste of Electrical and Electronic Equipment (WEEE) directive requires manufacturers to reduce the disposal waste of electrical and electronic products by reuse, recycling, and other forms of recovery. The Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment (RoHS) legislation restricts the use of lead, as well as cadmium, mercury, hexavalent chromium, and two halide-containing flame retardants, namely polybrominated biphenyls (PBB) and polybrominated diphenyl ethers (PBDE), in eight of the ten product categories identified in the WEEE directive. Unlike for WEEE, whereby EU member states are free to set more severe national legislation satisfying the WEEE directive requirements, RoHS is a single market directive. Both the WEEE and RoHS directives will become effective on July 1, 2006] , other possible legislative requirements, and market forces , . In fact, the consequences of not meeting the European July 2006 deadline for transition to lead-free electronics may translate into global market losses. Considering that lead-based electronics have been in use for over 40 years, the adoption of lead-free technology represents a dramatic change. The industry is being asked to adopt different electronic soldering materials , component termination metallurgies, and printed circuit board finishes. This challenge is accompanied by the need to requalify component-board assembly and rework processes, as well as implement test, inspection, and documentation procedures. In addition, lead-free technology is associated with increased materials, design, and manufacturing costs. [The cost of implementing the RoHS directive in the EU has been estimated to be US$ 20Bn . Intel Corporation's efforts to remove lead from its chips have been estimated to cost the company over US$ 100 million so far]. The use of lead-free materials and processes has also prompted new reliability concerns , as a result of different alloy metallurgies and higher assembly process temperatures relative to tin-lead soldering. This paper provides guidance to efficiently implement the lead-free transition process that accounts for the company's market share, associated exemptions, technological feasibility, product reliability requirements, and cost. Lead-free compliance, part and supplier selection, manufacturing, and education and training are addressed. The guidance is presented in the form of answers to key questions.

Development of nano-composite lead-free electronic solders

Inert, hybrid inorganic/organic, nano-structured chemicals can be incorporated into low melting metallic materials, such as lead-free electronic solders, to achieve desired levels of service performance. The nano-structured materials technology of polyhedral oligomeric silsesquioxanes (POSS), with appropriate organic groups, can produce suitable means to promote bonding between nano-reinforcements and the metallic matrix. The microstructures of lead-free solder reinforced with surface-active POSS tri-silanols were evaluated using scanning electron microscopy (SEM). Wettability of POSS-containing lead-free solders to copper substrate was also examined. Steady-state deformation of solder joints made of eutectic Sn-Ag solder containing varying weight fractions of POSS of different chemical moieties were evaluated at different temperatures (25°C, 100°C, and 150°C) using a rheometric solids analyzer (RSA-III). Mechanical properties such as shear stress versus simple shear-strain relationships, peak shear stress as a function of rate of simple shear strain, and testing temperature for such nano-composite solders are reported. The service reliability of joints made with these newly formulated nano-composite solders was evaluated using a realistic thermomechanical fatigue (TMF) test profile. Evolution of microstructures and residual mechanical property after different extents of TMF cycles were evaluated and compared with joints made of standard, unreinforced eutectic Sn-Ag solder.