Sn-Pb Solder Alloys Research Articles

Thermodynamics of Oxidation on Pb-Free Solders at Elevated Temperature

Based on the available thermodynamic and phase equilibria data, the thermodynamic criteria for oxidation in tin-based lead-free solders under soldering condition was deduced. The dependence of Gibbs free energy on temperature in Pb-free solder oxidation reaction was calculated by applying MATLAB program. The characteristics of oxidation reaction of a varity of solder alloy systems such as Sn-Ag, Sn-Cu, Sn-Sb, Sn-Zn, Sn-Ag-Cu and Sn-Pb eutectic alloys at elevated temperature were analyzed. The results suggested that zinc preferentially oxidized in Sn-Zn solder alloys in the elevated temperature state, while tin preferentially oxidized in the other alloys. The oxidation potential of the Sn-Zn eutectic alloys was higher than that of the pure tin at elevated temperature, whereas the oxidation potentials of Sn-Ag, Sn-Cu, Sn-Sb and Sn-Ag-Cu eutectic alloys were approxiately equal to that of the pure tin. All tin-based Pb-free solder alloys more easily oxidized than the Sn-Pb solder alloys. Oxidizability of these alloys followed in a decreasing order: Sn-Zn＞Sn-Sb＞Sn-Cu＞Sn-Ag＞Sn-Ag-Cu＞Sn-Pb.

Microstructural evolution and tensile properties of Sn–Ag–Cu mixed with Sn–Pb solder alloys

The effect of incorporating eutectic Sn–Pb solder with Sn–3.0Ag–0.5Cu (SAC) Pb-free solder on the microstructure and tensile properties of the mixed alloys was investigated. Alloys containing 100, 75, 50, 25, 20, 15, 10, 5 and 0 wt% SAC, with the balance being Sn–37Pb eutectic solder alloy, were prepared and characterized. Optical and scanning electron microscopy were used to analyze the microstructures while “mini-tensile” test specimens were fabricated and tested to determine mechanical properties at the mm length scale, more closely matching that of the solder joints. Microstructural analysis indicated that a Pb-rich phase formed and was uniformly distributed at the boundary between the Sn-rich grains or between the Sn-rich and the intermetallic compounds in the solder. Tensile results showed that mixing of the alloys resulted in an increase in both the yield and the ultimate tensile strength compared to the original solders, with the 50% SAC–50% Sn–Pb mixture having the highest measured strength. Initial investigations indicate the formation and distribution of a Pb-rich phase in the mixed solder alloys as the source of the strengthening mechanism.

Electrical and mechanical properties of Sn-5wt.%Sb alloy with annealing temperature

A binary Sn-5wt.%Sb solder alloy was chosen as a potential alternative to Sn-Pb solder alloy to be subjected to many studies. It was casted from the liquid state, cold drawn into wires of 1 mm diameters. The study includes the structure, electrical resistivity, tensile strength, hardness and indentation creep behavior using XRD, four probes electrical circuit, conventional tensile testing machine, Vickers microhardness tester, respectively. These properties were carried out for the cold worked alloy and after annealing at 393 and 473 K for 60 min. It was found that annealed samples exhibit more precipitations of the intermetallic compounds SnSb, higher lattice parameters and higher crystallite size, while have lower lattice-strain induced due to the cold working process. These structural changes greatly affect the electrical resistivity and mechanical properties of this alloy.

A Nonlinear Fracture Mechanics Approach to Modeling Fatigue Crack Growth in Solder Joints

Predicting the fatigue life of solder interconnections is a challenge due to the complex nonlinear behavior of solder alloys and the importance of the load history. Long experience with Sn–Pb solder alloys together with empirical fatigue life models such as the Coffin–Manson rule have helped us identify reliable choices among package design alternatives. However, for the currently popular Pb-free choice of SnAgCu solder joints, designing accelerated thermal cycling tests and estimating the fatigue life are challenged by the significantly different creep behavior relative to Sn–Pb alloys. In this paper, a hybrid fatigue modeling approach inspired by nonlinear fracture mechanics is developed to predict the crack trajectory and fatigue life of a solder interconnection. The model is shown to be similar to well accepted cohesive zone models in its theoretical development and application and is anticipated to be computationally more efficient compared to cohesive zone models in a finite element setting. The approach goes beyond empirical modeling in accurately predicting crack trajectories and is validated against experiments performed on lead-free as well as Sn–Pb solder joint containing microelectronic packages. Material parameters relevant to the model are estimated via a coupled experimental and numerical technique.

Microstructure Characterization and Creep Behavior of Pb-Free Sn-Rich Solder Alloys: Part I. Microstructure Characterization of Bulk Solder and Solder/Copper Joints

Solders are used as interconnect materials in microelectronic packaging. Traditionally, eutectic and near-eutectic Pb-Sn solder alloys have been used. Due to the toxic nature of lead, environmentally-benign Sn-rich (Pb-free) solders are being developed. In this two part series of articles, we have focused on elucidating the relationship between microstructure and creep behavior of Sn-rich alloys, both in bulk form and at the solder sphere joint level. Part I of our study focuses on a detailed quantitative analysis of the effect of controlled cooling rates on solder microstructures. Bulk solder microstructures from cast bars were compared to smaller solder joint microstructures, reflowed from single solder spheres about 1 mm in diameter. Faster cooling rates resulted in much finer secondary dendrite size and spacing, as well as a much finer distribution of Ag3Sn and Cu6Sn5 particles. The companion article, Part II, provides a unified mechanistic understanding of the creep behavior for the Sn-rich solder alloys, intimately linked to the microstructure characterization work reported in Part I.

Creep Rupture of Sn-Ag-Cu Pb-Free Solder Alloy

The eutectic Sn3.8Ag0.7Cu alloy is widely considered a leading Pb-free replacement for the eutectic Pb-Sn solder alloy in electronic packaging where creep deformation and rupture is a major concern. In this study, creep rupture behavior of Sn-Ag-Cu solder alloy was investigated under the isothermal condition. Creep tests were conducted under a range of stresses and temperatures. Creep lifetime data were analyzed by the combined time-temperature equations following the Sherby, Larsen-Miller, and Manson-Haferd approaches. From these analyses, a series of material parameters were obtained from the experimental data. The results showed that the Manson-Hanferd method provided a better correlation with the creep rupture data. The mechanisms of creep deformation and rupture at different time-temperature combinations are discussed.

Rate dependent strengths of some solder joints

The shear strengths of three lead-free solder joints have been measured over the range of loading rates 10−3 to ∼105 mm min−1. Binary (SnAg), ternary (SnAgCu) and quaternary (Castin: SnAgCuSb) alloys have been compared to a conventional binary SnPb solder alloy. Results show that at loading rates from 10−3 to 102 mm min−1, all four materials exhibit a linear relationship between the shear strength and the loading rate when the data are plotted on a log–log plot. At the highest loading rate of 105 mm min−1, the strengths of the binary alloys were in agreement with extrapolations made from the lower loading rate data. In contrast, the strengths of the higher order alloys were found to be significantly lower than those predicted by extrapolation. This is explained by a change in failure mechanism on the part of the higher order alloys. Similar behaviour was found in measurements of the tensile strengths of solder joints using a novel high-rate loading tensile test. Optical and electron microscopy were used to examine the microstructures of interest in conjunction with energy dispersive x-ray analysis for elemental identification. The effect of artificial aging and reflow of the solder joints is also reported.

Materials behaviour and intermetallics characteristics in the reaction between SnAgCu and Sn–Pb solder alloys

This paper illustrates the application of thermodynamic modelling techniques that are capable of simulating and predicting a range of interacting physical phenomena associated with the thermodynamic behaviour of multicomponent Sn-Pb/Pb-free solders in relation to solder joint manufacture and long term reliability. The paper compares theoretical calculations with experimental data, to identify the metallurgical mechanisms with regard to the rework or repair that may be encountered in the transition period from Sn-Pb to Pb-free soldering. Thermodynamic calculations have been carried out to study the material behaviour and possible formation of intermetallic precipitates during the reaction between Sn-Pb and Sn-Ag-Cu Pb-free alloys. Two Sn-Ag-Cu alloys that are relevant to current industrial interests, namely Sn-3.9Ag-0.6Cu and Sn-3.0Ag-0.5Cu were reacted with different contamination levels of eutectic Sn-37Pb solder. The variables related to the materials and processes, such as composition, temperature and cooling rate, are the primary concerns with respect to resultant microstructures, which have also been studied experimentally using optical and scanning electron microscopy (SEM). The results from this work provide guidance as to the consequence for microstructural evolution and hence mechanical integrity when small amounts of Pb exist in Pb-free alloys.

Forward and Backward Compatibility of Solder Alloys With Component and Board Finishes

The primary objective of the research presented in this paper is to qualify the reliability of mixed assemblies by comparing them to the conventional Sn-Pb assembly and completely Pb-free assembly. The research investigates both forward and backward compatibility in electronic assemblies using a design of experiments (DOE) approach. The investigation utilized a test vehicle containing an area array component (BGA169) and chip components (0603 resistors). Hot air solder leveling (HASL) and organic solderability preservative (OSP) surface finishes were used on the test vehicles to represent Sn-Pb and Pb-free alternatives, respectively. The assembled test vehicles were cut into two panels - one containing a resistor section for isothermal aging and the other containing a BGA and another resistor section for thermal shock. The assemblies were subjected to isothermal aging and thermal shock tests as per Interconnecting and Packaging Electronic Circuits/Joint Electronic Device Engineering Council (IPC/JEDEC) standards. The resistors were sheared in the as-soldered condition, and at various isothermal aging intervals and thermal shock cycles. In order to simulate an intermetallic failure in isothermal aging, a reduced shear height (20 mum) was used for the shear test. The performance of the resistor solder joints were quantified in terms of the shear force. The performance of the ball grid array (BGA) solder joint during thermal shock testing was quantified in terms of the number of cycles to failure. Experimental results from shear analysis of resistor solder joints show that Pb-free alloy assemblies' performance is superior to those assembled using Sn-Pb alloy. Also, the ductile nature of the Sn-Ag-Cu (SAC) alloy provides the joints a better fatigue life. With the area array components, Pb-free assembly joints outlived the traditional Sn-Pb joints when subjected to thermal shock loading. Among the mixed assemblies, backward-compatible assemblies (Pb-free bumps and Sn-Pb solder alloy) showed superior performance. The microstructural analysis of the solder joints indicate a uniform distribution of lead in the solder joint matrix.

Multi-Scale Numerical-Experimental Analysis of Failure in Solder Alloys

The past years have triggered considerable scientific efforts towards the predictive analysis of the reliability of solder connections in micro-electronics. Undoubtedly, the replacement of the classical Sn-Pb solder alloy by a lead-free alternative constitutes the main motivation for this. This paper concentrates on the theoretical, computational and experimental multi-scale analysis of the microstructure evolution and degradation of the conventional solder material Sn-Pb and its most promising lead-free alternative, a Sn-Ag-Cu (SAC) alloy. Special attention is given to the thermal anisotropy of bulk SAC and the interfacial fatigue failure of SAC interconnects.

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.

Influence of single-wall carbon nanotube addition on the microstructural and tensile properties of Sn–Pb solder alloy

Single-wall carbon nanotubes (SWCNTs) functionalized 63Sn–37Pb solders was synthesized. Composite solders up to 1 wt.% reinforcement of SWCNTs were prepared. The composite specimens were characterized by X-ray diffraction (XRD), Field-emission scanning electron microscope (FE-SEM). Melting characteristics of the SWCNT based solders were determined by differential scanning calorimetry. Microstructures of the composite solders were examined by FE-SEM at higher magnification for the observation of the CNT distribution. Mechanical properties of the composite specimens were compared with those of the pure 63Sn–37Pb solder specimens and found that the ultimate tensile strength increased by 30% with the 0.3 wt.% addition of CNTs. Hardness values of the CNT based composites have shown higher Vickers hardness compared to un-reinforced counterparts. The enhanced mechanical properties are resulting from the possible load-transfer mechanisms of the CNTs, which is discussed in detail. The fractrographic examination conducted by FE-SEM indicated the existence of ductile features in the form of dimples in all the composite solders with different weight proportions; it is evident from the micrographs that much reduced ductility is observed in the specimens reinforced with the higher wt.% of CNTs.

Effect of copper addition on the microstructure and mechanical properties of lead free solder alloy

The microstructure and shear strength of a transition joint consisting of Sn–Ag–Cu solder alloy and Cu substrate were evaluated in reflowed condition. The results were compared with those obtained from a similar joint made with conventional eutectic Sn–Pb solder alloy. The reaction products at the interface are mainly Cu 6Sn 5 and Cu 3Sn along with Cu 6Sn 5 for Sn–Pb and Sn–Ag–Cu to Cu joint, respectively. The shear strengths of the two systems are ∼55 and ∼68 MPa, respectively. The fracture takes place through the Cu 6Sn 5 intermetallic phase as well as through the solder alloy and the strength depends on the thickness of the reaction layer. The width of the brittle reaction layer at the interface for Sn–Ag–Cu to Cu transition joint is thinner with respect to Sn–Pb to Cu joint; hence the shear strength of the former is superior to the latter.

Effect of SO42- Ion on Corrosion and Electrochemical MigrationCharacteristics of Eutectic SnPb Solder Alloy

Electrochemical migration phenomenon is correlated with ionization of anode electrode, and ionization of anode metal has similar mechanism with corrosion phenomenon. In this work, in-situ water drop test and evaluation of corrosion characteristics for SnPb solder alloys in solutions were carried out to understand the fundamental electrochemical migration characteristics and to correlate each other. It was revealed that electrochemical migration behavior of SnPb solder alloys was closely related to the corrosion characteristics, and Sn Ivas primarily ionized in solutions. The quality of passive film formed at film surface seems to be critical not only for corrosion resistance but also for electrochemical migration resistance of solder alloys.ጊ吀Ѐ㘹〻Ԁ䭃䑎䴀

Multiscale Analysis of Microstructural Evolution and Degradation in Solder Alloys

The past years have triggered considerable scientific efforts towards the predictive analysis of the reliability of solder connections in micro-electronics. Evidently, the replacement of theclassical Sn-Pb solder alloy by a lead-free alternative constitutes the main motivation for this. This paper concentrates on the theoretical, computational and experimental multi-scale analysis ofthe microstructure evolution and degradation of the reference material Sn-Pb and the most promising alternative, a Sn-Ag-Cu (SAC) alloy. The microstructure evolution of Sn-Pb is analysed on thebasis of a representative volume element (RVE) of the underlying microstructure. At this level, a phase field model is used to incorporate the thermally driven diffusion, thereby accounting for anonlocal interfacial free energy. Starting from this phase field description of the microstructure, the intrinsic viscoplastic response and the damage developing in the phases and interfaces areanalysed. The correlation with experimentally found results is highlighted, whereby the microstructural dependence is the key issue. The lead-free SAC alloy is investigated at the material level by considering the mechanical and thermal anisotropy of theSn-rich grains. It is shown that experimental results indicate severe grain boundary damage upon thermal cycling. Using detailed microstructural information obtained through orientation imaging microscopy, the elaborated microstructural model reflects patternsof localized plastic strains and damage, that show remarkable correlation with the experimentally found patterns. At the meso-scale, the numerical-experimental analysis concentrates at theinternal and external interfaces in the material. A cohesive zone methodology is followed here, which represents the homogenized response of the underlying complex interfacial intermetallic microstructure. The motivation, qualification and quantification ofthe cohesive zone parameters are briefly addressed. The paper concludes by emphasizing the importance of collecting and exploiting different computational and experimental techniques in a multi-scale setting, for which the case studied here constitutes a relevantexample.

Effect of trace elements on the interface reactions between two lead-free solders and copper or nickel substrates

Traditional Sn-Pb solder alloys are being replaced, because of environmental and health concerns about lead toxicity. Among some alternative alloy systems, the Sn-Zn and Sn-Cu base alloy systems have been studied and reveal promising properties. The reliability of a solder joint is affected by the solder/substrate interaction and the nature of the layers formed at the interface. The solder/substrate reactions, for Sn-Zn and Sn-Cu base solder alloys, were evaluated in what concerns the morphology and chemical composition of the interface layers. The effect of the addition of P, at low levels, on the chemical composition of the layers present at the interface was studied. The phases formed at the interface between the Cu or Ni substrate and a molten lead-free solder at 250?C, were studied for different stage times and alloy compositions. The melting temperatures, of the studied alloys, were determined by Differential Scanning Calorimetry (DSC). Identification of equilibrium phases formed at the interface layer, and the evaluation of their chemical composition were performed by Scanning Electron Microscopy (SEM/EDS). Different interface characteristics were obtained, namely for the alloys containing Zn. The obtained IML layer thickness was compared, for both types of alloy systems.

New lead-containing solder alloy with improved properties

Structure, hardness, mechanical and electrical transport properties of the rapidly solidified Pb–25Sn alloy with Cd additions have been investigated. The results show that, a continuous increase in Cd content causes more precipitations of Cd as a third phase, which in turn causes continuous enhancement of the wetting, electrical and mechanical properties of this alloy. In spite of the higher value of TCR for the alloy Pb–25Sn–20Cd, it has the most enhanced properties when compared with the others. It has a lower electrical resistivity, lower wetting angle, higher Vickers microhardness values and yield strength when compared with the eutectic Pb–Sn solder alloy.

A highly stabilized–inhibited nitric acid/ferric nitrate-based solder stripping solution

The effect of ammonium sulfamate and benzotriazole on the prevention of exothermic conditions, the evolution of toxic NO x gases and the reduction of copper attack during solder stripping were studied. The effect of these additives on the rate of temperature rise for solutions treating 63Sn37Pb solder alloy and pure copper were investigated. Weight loss analysis was also performed in order to evaluate the rate of copper etching in various compositions. Scanning electron microscopy was employed to investigate the nature of the attack on the copper surface. In order to identify the role of ammonium sulfamate and benzotriazole on the stripping process, anodic polarization measurements were performed for the copper electrodes. The results revealed that ammonium sulfamate is capable of eliminating the reduction of nitric acid and the evolution of toxic NO x gases, can prevent excessive temperature rise, and can inhibit the rate of copper etching. On the other hand, benzotriazole is capable of providing a mirror bright copper surface after the solder stripping. Simultaneous addition of ammonium sulfamate and benzotriazole to the stripping solution led to a unique solder stripping composition which is substantially secure in terms of the temperature rise, evolution of toxic NO x gases and copper attack.

High strain rate testing of solder interconnections

PurposeThis paper aims to present a new micro‐impact tester developed for characterizing the impact properties of solder joints and micro‐structures at high‐strain rates, for the microelectronic industry, and the results evaluated for different solder ball materials, pad finishes and thermal histories by using this new tester. Knowledge of impact force is essential for quantifying the strength of the interconnection and allows quantitative design against failure. It also allows one‐to‐one comparison with the failure force measured in a standard quasi‐static shear test.Design/methodology/approachAn innovative micro‐impact head has been designed to precisely strike the specimen at high speed and the force and displacements are measured simultaneously and accurately during the impact, from which the failure energy may be calculated.FindingsThe paper demonstrates that, peak loads obtained from the impact tests are between 30 and 100 percent higher than those obtained from static shear tests for all combinations of solder alloy and pad finish. The SnPb solder alloy had the maximum energy to failure for all pad finishes. Of all the lead‐free solders, the SnAg solder alloy had the highest energy to failure. Static shearing induces only bulk solder failure for all combinations of solder alloy and pad finish. Impact testing tends to induce bulk solder failure for SnPb solder and a mixture of bulk and intermetallic failure in all the lead‐free solder alloys for all pad finishes. In general, the peak loads obtained for solder mask defined pads are significantly higher than those for non‐SMD (NSMD) pads. The results obtained so far have highlighted the vulnerability of NSMD pads to drop impact.Practical implicationsThe work provides a new solution to the microelectronics industry for characterizing the impact properties of materials and micro‐structures and provides an easy‐to‐use tool for research or process quality control.Originality/valueThe new micro‐impact tester developed is able to perform solder ball shear testing at high speeds, of up to 1,000 mm/s, and to obtain fracture characteristics similar to those found in drop impact testing using the JEDEC board level testing method JESD22‐B111 – but without the complexity of preparing specialized boards. This is not achievable using standard low‐speed shear testers.

In-situ observation on microfracture behavior ahead of the crack tip in 63Sn37Pb solder alloy

In-situ observation on microfracture behavior ahead of the crack tip in 63Sn37Pb solder alloy