Pb-containing Solders Research Articles

Calculation of Phase Equilibria in Candidate Solder Alloys

Abstract New solder alloys are being developed for electronic assemblies as replacements for traditional Pb-containing solder materials as a result of international pressure to remove Pb from the waste stream. In the transition period from Pb-containing to Pb-free solder alloys, component leads and copper interconnections on circuit board will continue to be pre-tinned with a Pb-containing solder to improve solderability. Phase equilibria information is needed for the evaluation of candidate Pb-free solder alloys in terms of their melting behavior and of the possible effects of Pb-contamination on manufacturing and reliability. The CALPHAD method of phase equilibria calculation using thermodynamic descriptions from databases provides a powerful tool for obtaining such information. The reliability of the calculation, however, depends on the quality of the thermodynamic descriptions of the individual phases. Criteria for the evaluation of these descriptions and the available thermodynamic databases are discussed. The effect of Pb-contamination on four solder alloys is analyzed.

Ductile-to-brittle transition in fracture behaviors of common solder alloys over a temperature range down to −150 °C

Detailed comprehensive comparison of temperature variations of ductility over common solders forms the sound basis of designing reliable solder joints that operate across a wide range of low temperatures, but the cryogenic fracture mechanism needs further investigation. Here, we investigate fracture behaviors of typical solders at different temperatures from −150 to 20 °C. Using standardized Charpy impact tests and fractography, we discover that all Pb-free Sn-based solders (Sn, Sn0.7Cu and SAC305) make abrupt transitions from ductile to brittle with decreasing temperature. Instead, Pb-containing solders (Sn37Pb and Sn36Pb2Ag) gradually change their fracture mode across the broad temperature range; and furthermore, a Pb-based solder (Pb10Sn) undergoes ductile deformation even at −150 °C. Sn exhibits deformation twinning below the DBTT and the cryogenic intergranular fracture is due to the strain incompatibility. Intermetallics in Sn matrix facilitate the transgranular cleavage fracture and the cleavage plane is {211}. Pb-rich phase can block cleavage crack propagation to prevent brittle failure. Estimated toughness together with presented cryogenic fracture mechanisms would be useful to inspire the design of robust joints that survive cryogenic temperatures.

Investigations of high-temperature tensile properties of Zn–25Sn–x(0.1–0.2)Cu–y(0.01–0.02)Ti high-temperature Pb-free solders

The use of Pb-containing solders in electronic products has been restricted due to their harm to both human health and the environment. Although the Sn–37Pb solder has been well replaced by Sn–3.0Ag–0.5Cu or other Pb-free solders, there is no drop in replacement for high-temperature Pb-free solders. In this study, the microstructure and high-temperature tensile properties of the Zn–25Sn–x(0.1–0.2)Cu–y(0.01–0.02)Ti high-temperature Pb-free solders were investigated. The design of the moderate alloy composition prevented undesirable Cu- and Ti-containing intermetallic compound formation that may cause alloy embrittlement. The solders exhibit superior tensile strength and competitive elongation compared with the conventional high-Pb solders and the other potential candidates. The Zn–25Sn–xCu–yTi solders can be strengthened in terms of the tensile strength enhancement without a loss of ductility under both room-temperature and high-temperature testing conditions. The minor Cu and Ti elements served as heterogeneous nucleation sites for inducing the microstructure refinement of the primary (Zn) phase. The Cu-in-Zn solid solution phenomenon as well as the formation of deformation twins during the high-temperature tensile test also contributed to the superior tensile properties of the designed alloys.

Influence of Thermal Aging on Lead-Free Solder Joints Reliability: A Review

The handheld electronic products currently turning to high density, high performances, and multifunctionality, which sets higher demands on the solder joints reliability in electronic packaging. Because of environmental conscious towards Pb-containing solders, Pb-free solder get attracted wide attention in the electronic industry. Challenges of the Pb-free solders in the microelectronics device are the microstructural evolution influence the mechanical behavior of the solder reliability. This paper analyses a short overview on the progress of the study on the relationship between structure and property, and possible concerns regarding reliability of Pb-free solders joints. Furthermore, the measures to enhance the solder joint reliability are evaluated which contribute to a further theoretical foundation for the research on the solder joints reliability of electronic devices in service life.

Influence of cooling conditions on the interfacial Cu6Sn5 intermetallic compound in Sn-37Pb/Cu solder joints during reflow

Despite a shift to Pb-free solders in the majority of electronic assemblies, Pb-containing solders are still used in a variety of applications. Recent research has revealed the complexities of the intermetallic Cu6Sn5 in terms of its crystal structure and stability when present as the reaction product between Pb-free solders and common substrates. In light of these findings, the effect of multiple reflow cycles with different cooling conditions on the microstructure of the interfacial Cu6Sn5 intermetallic compound (IMC) between Sn-Pb solders and Cu substrates was re- examined. For this purpose, the Sn-37Pb (in wt.%) solder was reflowed on a Cu substrate at a temperature of around 240 °C and cooled at a rate of around 30 °C/min under each of two conditions; (i) direct-cooling to room temperature, and (ii) interrupted-cooling with extended isothermal holding periods of 30 and 180 seconds at 140 °C. This second condition was chosen to encourage the polymorphic transformation that occurs in the Cu6Sn5 (at an equilibrium temperature of 186 °C) at a temperature which minimises the stresses involved. Smaller cracks and a thinner interfacial IMC layer were observed in the solder joints that were reflowed repeatedly with the interrupted-cooling conditions. However, despite the higher number of cracks in the samples undergoing conventional reflow, the shear strength was not significantly affected.

The Effect of Thermal Ageing on Solder/Substrate Interfacial Microstructures During Reflow of Sn–37Pb and Sn–3Ag–0.5Cu

In the current study, the influence of thermal ageing on evolution of microstructures in the interfacial region between solders (Sn–37Pb, Sn–3.5Ag–0.5Cu) and copper substrates was investigated. Pb-containing and Pb-free solders were reflowed on Cu substrates at 230 °C for 15 min and were isothermally aged at 100 °C for 24 h. As-reflowed Sn–Pb solder/Cu substrate interfacial region exhibited continuous and layered type of IMC at the interface, and this IMC morphology changed to scallop type with isothermal ageing. SAC solder/Cu as-reflowed samples showed continuous and needle-shaped Cu6Sn5 and Ag3Sn IMCs at the interface. However, in an isothermally aged condition, plate-shaped Cu6Sn5 and flower-shaped Ag3Sn IMCs were found inside the solder matrix. Scanning electron microscopic (SEM) study showed that the thickness of Cu6Sn5 IMC was higher in reflowed Sn–Pb/Cu region than in SAC/Cu region.

Experimental Determination of the Sn-Cu-Ni Phase Diagram for Pb-Free Solder Applications

The rapid increase of scrapped electrical products disposed in landfill sites is significantly influencing the environment because of the composition of the associated components. This situation has been the stimulus for the international community to issue a directive to prohibit the use of Pb-containing solders in electronic devices and effectively ban such products on the market. The Sn-Cu-Ni has become increasingly significant as it is used in several solder alloys and more generally both Cu and Ni are common substrates that interact with Sn-based solders in microelectronic applications. However, there is currently an insufficient understanding of the phase equilibria and the associated phase diagrams of many Sn alloys. The present work implemented an experimental methodology to obtain high accuracy results for the phase equilibria between one liquid and one or two intermetallic compounds. The Sn-Cu-Ni and the associated Sn-Cu and Sn-Ni binary systems have been re-evaluated and compared with the literature. The liquidus and the invariant lines were determined for the Sn-Cu-Ni ternary at the temperatures ranges of between 240 °C and 270 °C. The result will contribute to filling phase equilibria data gaps in the Sn-rich corner of the Sn-Cu-Ni alloy system.

Wetting and IMC Growth Behavior Between Cu Substrate and Zn-25Sn-xCu-yTi High-Temperature Solder Alloys

One of the suitable candidates to replace conventional high-temperature Pb-containing solders is Zn-xSn solder alloy. However, its high Zn content may cause poor wettability. Addition of Cu and Ti has been proposed to improve the wettability of Zn-25Sn solder alloy. This study investigated the wetting and intermetallic compound (IMC) formation behavior between Zn-25Sn-xCu-yTi solder alloys (x = 0.1 wt.% to 0.3 wt.%, y = 0.01 wt.% to 0.03 wt.%) and Cu substrate. The wetting behavior was investigated using a wetting balance with flux at a temperature ∼ 40°C above the liquidus temperature of Zn-25Sn solder. The results indicated that combined addition of 0.3 wt.% Cu and 0.03 wt.% Ti gave rise to the best performance in terms of wetting time and wetting force. CuZn5 and Cu5Zn8 IMCs formed at the solder–Cu interface, with Cu5Zn8 being the predominant layer. Cu5Zn8 also grew much faster than CuZn5 with respect to dipping time. Combined addition of Cu and Ti also decreased the activation energy for Cu5Zn8 IMC formation in the Zn-25Sn system.

Development of Pb-Free Nanocomposite Solder Alloys

As an alternative to conventional Pb-containing solder material, Sn–Ag–Cu (SAC) based alloys are at the forefront despite limitations associated with relatively poor strength and coarsening of grains/intermetallic compounds (IMCs) during aging/reflow. Accordingly, this study examines the improvement of properties of SAC alloys by incorporating nanoparticles in it. Two different types of nanoparticles were added in monolithic SAC alloy: (1) Al2O3 or (2) Fe and their effect on microstructure and thermal properties were investigated. Addition of Fe nanoparticles leads to the formation of FeSn2 IMCs alongside Ag3Sn and Cu6Sn5 from monolithic SAC alloy. Addition of Al2O3 nano-particles do not contribute to phase formation, however, remains dispersed along primary β-Sn grain boundaries and act as a grain refiner. As the addition of either Fe or Al2O3 nano-particles do not make any significant effect on thermal behavior, these reinforced nanocomposites are foreseen to provide better mechanical characteristics with respect to conventional monolithic SAC solder alloys.

A Comparison Between Solders & Transient Liquid Phase Sintered Interconnects in High Temperature Multilayer Ceramic Capacitors

There is a long-established market for high temperature multilayer ceramic capacitors (MLCCs) that operate at 150°C and higher in downhole oil and gas exploration, and military and aerospace applications. To maximize the capacitance density and achieve a high degree of mechanical robustness, stacks and leaded form factors have been used with high melting point–Pb-containing solders as the preferred interconnects. However, Pb-containing solders are limited to temperatures below 300°C and are banned from many commercial and automotive applications with further legislation limiting their use planned in the future. Common Pb-free solders such as SAC 305 or SnSb alloys are in widespread use, but their performance at prolonged exposures at 200°C is limited. Exposures to high reflow temperatures during assembly, especially successive reflow operations, can also compromise interconnect integrity. Higher temperature gold-containing solders are widely available, but these are cost-prohibitive and so are not viable for emerging high temperature electronics including higher volume, price-sensitive automotive and power markets. The development of more energy-efficient power converters and inverters based on wide bandgap semiconductors is driving the adoption of higher temperature electronics in these markets because these operate at higher junction temperatures than traditional silicon. This has led to the development of nonsolder interconnects based on sintered silver, nanometal sintering, and transient liquid phase sintering (TLPS) technologies capable of higher temperature performance than common solder-based interconnects. The availability of discrete components, such as capacitors, that can operate under these conditions is a key barrier to the development and adoption of high temperature electronics. In this article, the key property differences between solders and TLPS interconnect technologies are compared in detail for MLCC interconnects. The development of a new range of nickel base metal electrode C0G MLCC stacks rated for 200°C is described and performance compared with traditional precious metal electrode stacks. Thermal cycling performance to 200°C of BME X7R stacks made with 10Sn/88Pb/2Ag solders are compared with similar stacks made with TLPS interconnects of CuSn and InAg. The development of leadless stacks, a new bulk capacitance form factor enabled by TLPS technology, is described and their properties compared with traditional stacks.

Influence of ENIG defects on shear strength of pressureless Ag nanoparticle sintered joint under isothermal aging

An Ag nanoparticle sintering is a promising die attach method for use in high-temperature electronics and could potentially be substituted for conventional high Pb-containing solders. An electroless Ni/immersion Au (ENIG) is a common surface finish for solder, wire bonding, and Ag nanoparticle sintering. However, there is no report on the influence of ENIG defects on the shear strength and bonding quality of Ag nanoparticle sintered joint. In this study, the relationship between the presence of ENIG defects and shear strength of Ag nanoparticle sintered joint after bonding and isothermal aging test using two types of ENIG surface finished Cu substrates (with ENIG defects and without defects) was investigated. The initial shear strength of Ag sintered joint showed similar values of approximately 36 MPa despite the presence of ENIG defects. However, the ENIG defects could affect the shear strength degradation of Ag sintered joint after thermal aging at 250 °C. This degradation came from the Ni oxidation by the presence of ENIG defects and subsequent brittle fracture of joint alone the Ni oxidation layer.

A Comparison Between Solders & Transient Liquid Phase Sintered Interconnects in High Temperature Multi-Layer Ceramic Capacitors

Abstract There is a long-established market for high temperature multi-layer ceramic capacitors (MLCC) that operate at 150°C and higher in down-hole oil & gas exploration, military and aerospace applications. To maximize the capacitance density and achieve a high degree of mechanical robustness, stacks and leaded form factors have been used with High Melting Point (HMP) Pb-containing solders as the preferred interconnects. However, Pb-containing solders are limited to temperatures below 300°C and are banned from many commercial and automotive applications with further legislation limiting their use planned in the future. Common Pb-free solders such as SAC 305 or SnSb alloys are in widespread use but their performance at prolonged exposures at 200°C is limited. Exposures to high reflow temperatures during assembly, especially successive reflow operations, can also compromise interconnect integrity. Higher temperature gold-containing solders are widely available but these are cost prohibitive and so are not viable for emerging high temperature electronics including higher volume, price sensitive Automotive and Power markets. The development of more energy efficient power converters and inverters based on Wide Band Gap (WBG) semiconductors is driving the adoption of higher temperature electronics in these markets since these operate at higher junction temperatures than traditional silicon. This has led to the development of non-solder interconnects based on sintered silver, nano-metal sintering and transient liquid phase sintering (TLPS) technologies capable of higher temperature performance than common solder based interconnects. The availability of discrete components, such as capacitors, that can operate under these conditions is a key barrier to the development and adoption of high temperature electronics. In this paper the key property differences between solders and TLPS interconnect technologies are compared in detail for MLCC interconnects. The development of a new range of nickel Base Metal Electrode C0G MLCC stacks rated for 200°C is described and performance compared to traditional Precious Metal Electrode (PME) stacks. Thermal cycling performance to 200°C of BME X7R stacks made with 10Sn/88Pb/2Ag solders are compared to similar stacks made with TLPS interconnects of Cu-Sn and In-Ag. The development of leadless stacks, a new bulk capacitance form factor enabled by TLPS technology, is described and their properties compared to traditional stacks.

Effect of aluminum addition on the electrochemical corrosion behavior of Sn–3Ag–0.5Cu solder alloy in 3.5 wt% NaCl solution

The ternary eutectic Sn–Ag–Cu (SAC) solder alloys has become the most promising Pb-containing solders for the application in electronic packaging. The effect of Aluminum on the electrochemical corrosion behavior of Sn–3.0Ag–0.5Cu (SAC 305) solder alloy was investigated in 3.5 wt% NaCl solution in terms of potentiodynamic polarization and electrochemical impedance spectroscopy. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and atomic force microscopy (AFM) were used to characterize the samples after the electrochemical tests. Polarization studies indicated that an addition of 0.1 and 0.5 wt% Al in the Sn–3.0Ag–0.5Cu solder alloy shifted the corrosion potential (Ecorr) towards more negative values. The Impedance results revealed that the addition of Al with Sn–3.0Ag–0.5Cu solder alloy led to a lower corrosion resistance in terms of impedance values and the capacitance. The SEM images revealed existence of openings and pores in the corrosion product of solders containing Al. EDX and XRD analysis confirmed that the oxides and hydroxides of Sn and Al were the major corrosion products. AFM images exhibited that the surface becomes more rough and non-uniform with Al addition to SAC 305 solder alloy.

Microstructure, thermal behavior, and wettability of Zn4Al3Mg–xIn solders

The potential of Zn4Al3Mg–xIn alloys to replace Pb-containing solders by high-temperature soldering was studied. The microstructure, thermal behavior, and wettability of Zn4Al3Mg–xIn solders were investigated by scanning electron microscopy, X-ray diffraction, differential scanning calorimetry (DSC), and spreading tests. The influence of In content on the microstructure and characteristics of the solders was explored. Small additions of In result in grain boundary segregation because of the limited solid solubility of In in other phases, and thereby lead to more complicated microstructure of Zn4Al3Mg–xIn alloys. DSC measurements showed that the onset of melting of the Zn4Al3Mg–xIn alloys decreased with increasing In content, while the melting peak became broader. The electrical conductivity values of ZnAlMg–xIn solders were of the same order of magnitude as those of a conventional Pb-5(wt%)Sn solders. The wettability of Zn4Al3Mg–xIn solders over Cu substrate was also improved with increasing In content.

CORRELAÇÃO ENTRE MICROESTRUTURA, RESISTÊNCIAS MECÂNICA E À CORROSÃO DA LIGA DE SOLDAGEM LIVRE DE CHUMBO Sn-0,7%Cu*

Sn-Cu alloys are promising alternatives to the replacement of Pb-containing solder alloys. However, the effects of the cooling rate on the solidification microstructures of these alloys and the corresponding influence on the mechanical and corrosion resistances are not well known. In the present study, the transient directional solidification technique has been used to obtain a Sn-0,7wt.%Cu ingot. The experimental results include: solidification thermal parameters (cooling rate, Ṫ, growth rate, v, and metal/substrate heat transfer coefficient, hi ), cellular spacing, λc , and primary dendritic arm spacing, λ1 , corrosion rate, corrosion potential and polarization resistance and mechanical strength and ductility. The results show a cellular/dendritic transition with eutectic cells prevailing for Ṫ< 0,9°C/s. Lower corrosion resistances have been associated with dendritic regions compared with regions characterized by eutectic cells. In the interdendritic regions extremely fine and fibrous Cu6 Sn5 intermetallic particles can be observed

Effect of Hold Time on Crack Growth Behavior of Pb-Containing and Pb-Free Solders

Four types of loading waveforms were adopted to investigate the effect of hold time on crack growth behavior of Sn-37Pb and Sn-3.0Ag-0.5Cu solders: a sinusoidal waveform, triangular waveform, and trapezoidal waveform with hold times of 1 s, 5 s, and 10 s, and a triangular waveform with hold time of 10 s at maximum tensile load. The experimental results showed that the crack growth behavior of both solders tested under sinusoidal and triangular waveforms without any hold time was predominantly cycle dependent. On the other hand, the crack growth behavior of both solders tested under trapezoidal waveforms with hold times of 1 s, 5 s, and 10 s and triangular waveforms with hold time of 10 s was predominantly time dependent. Under cycle-dependent conditions the crack growth data were a function of J-integral range, while under time-dependent conditions the crack growth data were not a function of the creep J-integral range but rather of the C* parameter.

Thermomigration in solder joints

In 3D IC technology, the vertical interconnection consists of through-Si-vias (TSV) and micro solder bumps. The size of the micro-bump is approaching 10μm, which is the diameter of TSV. Since joule heating is expected to be the most serious issue in 3D IC, heat flux must be conducted away by temperature gradient. If there is a temperature difference of 1°C across a micro-bump, the temperature gradient will be 1000°C/cm, which can cause thermomigration at the device operation temperature around 100°C. Thus thermomigration will become a very serious reliability problem in 3D IC technology. We review here the fundamentals of thermomigration of atoms in microbump materials; both molten state and solid state thermomigration in solder alloys will be considered. The thermomigration in Pb-containing solder joints is discussed first. The Pb atoms move to the cold end while Sn atoms move to the hot end. Then thermomigration in Pb-free SnAg solder joints is reviewed. The Sn atoms move to the hot end, but the Ag atoms migrate to the cold end. Thermomigration of other metallization elements, such as Cu, Ti and Ni is also presented in this paper. In solid state, copper atoms diffuse rapidly via interstitially to the cold end, forming voids in the hot end. In molten state, Cu thermomigration affects the formation of intermetallic compounds.

Creep–Fatigue Crack Growth Behavior of Pb-Containing and Pb-Free Solders at Room and Elevated Temperatures

Fatigue crack growth tests of lead-containing (Sn-37Pb) and lead-free (Sn-3.0Ag-0.5Cu) solders were conducted at frequencies ranging from 0.1 Hz to 10 Hz at stress ratio of 0.1, at room temperature and at 70°C. The J-integral range (ΔJ) and the modified J-integral (C *) were used in assessing the cycle-dependent and time-dependent crack growth behavior for both solders. The experimental results showed that the crack growth behavior of both solders at the lower frequency and higher temperature was predominantly time dependent, whereas the crack growth behavior of both solders at the higher frequency and lower temperature was predominantly cycle dependent, with the transition in fatigue crack growth behavior from cycle dependent to time dependent expressed as f + 6500exp(1/T) = 6520. In both the cycle-dependent and time-dependent regions, the crack growth resistance of the lead-free solder was higher than that of lead-containing solder. Fracture surface observations showed that, as the frequency decreased and/or the temperature increased, the fracture path changed from transgranular to intergranular for Sn-37Pb solder, and from transgranular to mixed transgranular–intergranular for Sn-3.0Ag-0.5Cu solder.

Constitutive Behavior of Mixed Sn-Pb/Sn-3.0Ag-0.5Cu Solder Alloys

During the transition from Pb-containing solders to Pb-free solders, joints composed of a mixture of Sn-Pb and Sn-Ag-Cu often result from either mixed assemblies or rework. Comprehensive characterization of the mechanical behavior of these mixed solder alloys resulting in a deformationally complete constitutive description is necessary to predict failure of mixed alloy solder joints. Three alloys with 1 wt.%, 5 wt.%, and 20 wt.% Pb were selected so as to represent reasonable ranges of Pb contamination expected from different 63Sn-37Pb components mixed with Sn-3.0Ag-0.5Cu. Creep and displacement-controlled tests were performed on specially designed assemblies at temperatures of 25°C, 75°C, and 125°C using a double lap shear test setup that ensures a nearly homogeneous state of plastic strain at the joint interface. The observed changes in creep and tensile behavior with Pb additions were related to phase equilibria and microstructure differences observed through differential scanning calorimetric and scanning electron microscopic cross-sectional analysis. As Pb content increased, the steady-state creep strain rates increased, and primary creep decreased. Even 1 wt.% Pb addition was sufficient to induce substantially large creep strains relative to the Sn-3.0Ag-0.5Cu alloy. We describe rate-dependent constitutive models for Pb-contaminated Sn-Ag-Cu solder alloys, ranging from the traditional time-hardening creep model to the viscoplastic Anand model. We illustrate the utility of these constitutive models by examining the inelastic response of a chip-scale package (CSP) under thermomechanical loading through finite-element analysis. The models predict that, as Pb content increases, total inelastic dissipation decreases.

Uniaxial Ratchetting Behavior of Solder Alloys and Its Simulation by an Elasto-Plastic-Creep Constitutive Model

Ratchetting deformation occurring at solder joints in electronic packaging is a concern for electronic devices. Therefore, ratchetting deformation due to thermal cycling at solder joints should be simulated by structural analysis employing tools such as the finite-element method (FEM). However, simulation of ratchetting deformation is difficult, and little modeling to simulate ratchetting deformation accurately has been reported. This work experimentally examines uniaxial ratchetting deformation of Pb-free and Pb-containing solder alloys to elucidate the effect of rate on uniaxial ratchetting. An elasto-plastic-creep constitutive model is developed to simulate uniaxial ratchetting deformation. The constitutive model incorporates a method to determine the material constants simply from a small number of pure tensile tests and subsequent stress relaxation tests. Uniaxial ratchetting deformation of solder alloys was successfully simulated using this constitutive model and simple method for material constant determination.