SnAgCu Alloys Research Articles

Effect of Creep, Fatigue and Random Vibration on the Integrity of Solder Joints in BGA Package

This study employs Finite Element Analysis (FEA) to investigate the effects of creep, fatigue, and random vibration on the integrity of solder joints in BGA packages in electronic modules. Evaluating the response of lead-based Sn63Pb37 and lead-free SnAgCu alloy solders (SAC305, SAC387, SAC396, and SAC405) to induced loads - stress, strain, strain energy density, and displacement in the joints is obtained and studied better to understand the mechanism of the joints' degradation. SAC305 and SAC405 are modelled with linear stress-temperature relationships σ = 3.152T–68.167 and σ = 1.543T–34.983, respectively. The magnitude of the strain energy density in the joints is a key failure driver. SAC387 and SAC396 solder joints display lower values of strain energy density and thus have higher durability. Displacement analysis indicates that SAC305 and Sn63Pb37 are prone to deformation-induced failure. SAC387 exhibits the highest fatigue yield stress at 58 MPa, while SAC405 displays the lowest stress at 22 MPa. Analysis of the results of random vibration shows that Sn63Pb37 developed the highest stress at 34.62 MPa and is thus susceptible to stress-induced failure. The robust stress, strain, and strain energy responses of SAC405 and SAC396 provide key insights into improving the mechanical reliability of future electronic devices towards better sustainability.

Leadfree SnAgCu Solder Materials Characterization at High Strain Rates at Low Test Temperatures and Drop and Shock Simulation Using Input-G Method

Abstract Electronics will experience high and low working temperatures during operations, handling, and storage in severe environments applications such as download drilling, aircraft, and transportation. Temperatures in the vehicle underhood applications can range from −65 °C to +200 °C. Lead-free solder materials continue to evolve under varying thermal workloads. Material characteristics may deteriorate if operating conditions are harsh or heavy. Nonetheless, lead-free solders are susceptible to high strains, which can lead to electronic device failure. A better understanding of solder alloys is needed to ensure reliable operation in harsh environments. New doped solder alloys have recently been created by adding Ni, Co, Au, P, Ga, Cu, and Sb to SnAgCu (SAC) solder alloys to improve mechanical, thermal, and other qualities. SAC-Q has recently been made using Sn–Ag–Cu and the addition of Bi (SAC+Bi). It was discovered that adding dopants to SAC alloys may enhance mechanical characteristics and reduce aging damage. There is no published data on SAC solder alloys after prolonged storage at high strain rates and low functioning temperatures. The materials characterization of SAC (SAC105 and SAC-Q) solder after extended storage at low working temperatures (−65 °C–0 °C) and high strain rates (10–75 per sec) is investigated in this article. To characterize the material constitutive behavior, the Anand viscoplastic model was utilized to derive nine Anand parameters from recorded Tensile data. The generated nine Anand parameters were used to validate the Anand model's reliability. A strong correlation was established between experimental data and Anand's predicted data. The Anand parameters were used in a finite element framework to simulate drop events for a ball-grid array package on printed circuit board assembly to calculate hysteresis loop and plastic work density. The plastic work per shock event measures the damage progression of the solder interconnects. Thermal aging effects have been studied in terms of the hysteresis loop and the evolution of PWD.

Investigation of Microstructure and Mechanical Properties of SAC105 Solders with Sb, In, Ni, and Bi Additions.

Low Ag lead-free Sn-Ag-Cu (SAC) solders have attracted great interest due to their good drop resistance, high welding reliability, and low melting point. However, low Ag may lead to the degradation of the mechanical properties. Micro-alloying is an effective approach to improving the properties of SAC alloys. In this paper, the effects of minor additions of Sb, In, Ni, and Bi on microstructure, thermal and mechanical properties of Sn-1 wt.%Ag-0.5 wt.%Cu (SAC105) were systematically investigated. It is found that the microstructure can be refined with intermetallic compounds (IMCs) distributed more evenly in the Sn matrix with additions of Sb, In, and Ni, which brings a combined strengthening mechanism, i.e., solid solution strengthening and precipitation strengthening, leading to the tensile strength improved of SAC105. When Ni is substituted by Bi, the tensile strength is further enhanced with a considerable tensile ductility higher than 25%, which still meets the practical demands. At the same time, the melting point is reduced, the wettability is improved, and the creep resistance is enhanced. Among all the investigated solders, SAC105-2Sb-4.4In-0.3Bi alloy possesses the optimized properties, i.e., the lowest melting point, the best wettability, and the highest creep resistance at room temperature, implying that element alloying plays a vital role in improving the performance of SAC105 solders.

Effect of Bi, Sb, and Ti on Microstructure and Mechanical Properties of SAC105 Alloys.

The Sn-Ag-Cu (SAC) solder alloys with a low Ag (Ag < 3 wt.%) content have attracted great attention owing to their low cost, increased ability in bulk compliance, and plastic energy dissipation. However, some of their mechanical properties are generally lower than the SAC alloys with a higher Ag content. Adding alloying elements is an effective approach for improving the mechanical properties of the SAC alloys. In this study, the effect of Bi, Sb, and Ti on Sn-1 wt.%Ag-0.5 wt.%Cu (SAC105) solder alloys was investigated. The SAC solders with four compositions: SAC105-1 wt.%Bi, SAC105-1 wt.%Sb, SAC105-1 wt.%Bi-1 wt.%Sb, SAC105-1 wt.%Bi-1 wt.%Sb-0.4 wt.%Ti were prepared. The microstructure and phase compositions were characterized using electron scanning microscopy, and X-ray diffraction. The thermal properties and wettability were also examined. Uniaxial tensile tests and nano-indentation tests were conducted to evaluate the mechanical properties. The results show that adding Bi or Sb could increase the strength of SAC105 alloys mainly due to the solid solution strengthening effect. The creep resistance of SAC105 alloys was also improved with the additions of Bi and Sb. The co-additions of Bi and Sb into SAC105 alloys exhibit an enhanced creep resistance than that calculated by the theoretical calculation. The further addition of Ti into SAC105-1Bi-1Sb alloys demonstrated a much-improved creep resistance, which could be attributed to the synergistic effects of both solid solution strengthening and the precipitation hardening effects.

Influences of Impurity Incorporation in Electroplated Cu on the SnAgCu and Ni-Containing SnAgCu Solder Joints

SnAgCu and Ni-containing SnAgCu alloys are Pb-free solders widely used to join with Cu to construct the solder joints. Electrodeposition is a technology commonly used to fabricate Cu but co-deposition of organic impurities originating from additives is an inevitable reliability issue. This study investigates the impurity effect on the voiding propensity in the two solder joints (SnAgCu/Cu and SnAgCu-Ni/Cu) subjected to thermal aging at 200 °C. Results show that a high level of impurity incorporation causes massive void propagation along the SnAgCu/Cu and SnAgCu-Ni/Cu interface. Reduction of the impurity concentration by precise control of the additive formulas can weaken the impurity effect and effectively suppress the void propagation. The weakening phenomenon of the impurity effect is more pronounced in the SnAgCu-Ni/Cu joint, indicating that suppression of the Cu3Sn growth as well as Kirkendall voids by Ni addition is also helpful in reducing the influences of impurities.

Evolution of Anand Parameters for Thermally Aged Sn-Ag-Cu Lead-Free Alloys at Low Operating Temperature

Abstract In extreme environmental applications, such as aerospace and automotive, electronics may endure high or low operating temperatures during service, handling, and storage. An electronic assembly may experience strain rates of 1–100 per sec of strain and ambient temperatures of –65 to +200 °C. Electronic assembly's temperature depends mainly on location, energy dissipation, and thermal architecture. Electronic assemblies in automotive applications may be located underhood, on engine, on-transmission, and or in wheel well. Study of property evolution of solders used for interconnection is important for assurance of reliability. Degradation in material properties for lead-free solder alloys can be caused by change in microstructure due to variation in temperatures. There is need for data on the effect of operating period and operating conditions on the material properties. Addition of dopants in Sn-Ag-Cu (SAC) alloys has been shown to improve mechanical properties and minimize deterioration due to aging at lower strain rates. SAC-Q is formulated with Sn-Ag-Cu with addition of Bi (SAC+Bi). It has been observed that adding Bismuth (Bi) to SAC alloy can play an important role to make the solder alloy resistant to aging-induced degradations. In author's prior research the evolution of Anand parameters and materials properties for SAC solder (SAC105, SAC305, and SAC-Q) at high temperatures and high strain rates has been studied. However, data on thermally aged SAC solder alloys at high strain rate levels at low operating temperatures are not available in published literature. In this paper, materials characterization of thermally aged SAC (SAC105 and SAC-Q) solder at low operating temperatures (−65 °C to 0 °C) and at high strain rates (10–75 per sec) has been studied. Stress–strain curves have been measured at low operating temperatures using impact hammer-based tensile tests with cooling chamber. The fabricated SAC lead-free solder specimen was isothermally aged up to 6 months at 50 °C before testing. Anand viscoplastic model has been used to compute nine Anand parameters to describe the material constitutive behavior. Anand Model parameters evolution due to thermal aging has been studied for SAC solders. The computed nine Anand parameters from experimental data then were used to simulate the tensile test to predict the stress–strain curve and compared to experimental stress–strain curves to verify the accuracy of the model. A good correlation was found between experimental data and Anand-predicted data.

Droplet relaxation of molten metals in polypropylene matrix: Measurement of the interfacial tension

Using the retraction method of a deformed drop, the interfacial tension between polypropylene (PP) and two different molten metals (tin and the eutectic SnAgCu alloy) has been measured at 250 and 230 °C, respectively. Systematic rheological investigations of the materials under study enabled us to measure the viscosity of the polymer matrix and that of the metal liquids. A viscosity of 1.9 and 2 mPa s in the melt is reported for the tin and SnAgCu alloys, respectively. Based on these investigations, the interfacial tension between the molten metals and the polymer matrix is determined for the first time. The influence of the polymer matrix viscoelasticity and liquid metal surface oxidation is analyzed in order to determine the most accurate conditions for measuring the interfacial tension between the polymer and the liquid metal. Values as high as 350 mN m−1 (for PP/SnAgCu) and 140 mN m−1 (PP/tin) are reported, indicating a poor affinity between the molten metal and the polymer matrix. By temperature quenching, the dispersion of the liquid metal drops in the polypropylene under constant shear, and the formation of solid metal ellipsoids is obtained, with a characteristic size that is limited by the critical deformation parameter defined by Taylor. Our study, thus, offers an experimental approach of measuring the interfacial tension between two highly incompatible liquids, opening at the same time a promising perspective for obtaining, by a rheological approach, metal particles with a non-negligible form factor.

Effects of bismuth and nickel on the microstructure evolution of Sn-Ag-Cu (SAC)-based solders

The addition of alloying elements to the SAC alloy composition changes the growth behavior of thick Cu6Sn5 and Cu3Sn intermetallic compounds (IMCs) and their influences on mechanical responses. The purpose of this study is to investigate the effects of Bi and Ni on the growth kinetics of interfacial IMCs (Cu6Sn5 and Cu3Sn) and the mechanical strength of the bulk solders during high temperature aging. Reflowed samples of the SAC405 solder balls and the SAC + Bi,Ni solder balls were stored in high-temperature storage (HTS) chamber at 150 °C and 175 °C each for 0 h, 500 hs, 1000 hs, and 2000 hs. The microstructure evolutions of bulk solder and its interfacial areas were evaluated and the associated microhardness values were measured on the solder balls. During aging, the presence of Bi was seen only in the bulk solder region, resulting in increased hardness by a factor of ∼ 2 and resistance to the age softening due to solid solution strengthening. On the other hand, Ni was observed in both the solder ball and IMC regions. After aging at 150 °C, SAC + Bi,Ni solder balls had a thinner Cu3Sn IMC layer at its interface when compared to that in SAC solders with no Ni. At 175 °C, however, Ni addition did not decrease the growth of Cu3Sn. Long aging (2000 h) of the SAC405 sample at 175 °C resulted in the complete consumption of the Cu layer, which was slowed in SAC + Bi,Ni. The presence of Bi and Ni, therefore, show the potential to improve the reliability of the SAC solder ball joint during aging by restricting the solder deformation and the growth of the Cu3Sn layer.

Evolution of Anand Parameters With Elevated Temperature Aging for SnAgCu Lead-free Alloys

Abstract Electronic components in downhole oil drilling and gas industry applications, automotive, and avionics may be exposed to high temperatures (&amp;gt;150 °C) and high strain rates (1–100 per sec) during storage, operation, and handling, which can contribute to the failures of electronic devices. Temperatures in these applications can exceed 200 °C, which is close to melting point for SnAgCu (SAC) alloys. Prior studies at low strain rates have shown property evolution even under moderate exposure to high temperature. In this paper, the evolution of Anand parameters for unaged and aged SAC (SAC105 and SAC-Q) lead-free solder alloys at high strain rates has been investigated induced under sustained periods of thermal aging. The thermal aged lead-free SAC solder alloys specimen has been tested at high strain rates (10–75 per sec) at elevated temperatures of (25 °C–200 °C). The SAC lead-free solder samples were subjected to isothermal aging at 50 °C up to 1-year before testing. To describe the material constitutive behavior, the Anand Viscoplastic model has been used. The effect of thermal aging on Anand parameters also has been investigated. In order to verify the accuracy of the model, the computed Anand parameters have been used to simulate the uni-axial tensile test. The material constitutive behavior has been implemented in a finite element framework to simulate the drop events using the Anand constitutive model and determine the plastic work per shock event. The plastic work per shock event is a measure of the damage progression in the solder interconnects. The constitutive model has been used to simulate the shock event of a ball-grid array package on printed circuit board assembly.

A New Electrodeposition Approach for Multicomponent Solder Alloys

Enactment and enforcement of international regulations restricting the use of lead-bearing solders has triggered plenty of research on Pb-free alloys for Integrated Circuits (ICs) packaging [1]. Nonetheless, Sn-Pb (63/37) eutectic still possesses unique features (e.g. low melting point, optimum mechanical properties, ideal surface tension allowing for holes and gaps wetting, capability of reducing, inhibiting or eliminating tin whiskers formation, etc.) hardly matched by lead-free solders.Alloys belonging to the SAC (Sn-Ag-Cu) category constitute the most widespread alternative to tin-lead solder because of their relatively low melting point [2], “composition-tunable” wettability [3], and high tensile strength (compared to the Sn-Pb eutectic). Particularly, SAC-305 (Sn = 96.5, Ag = 3, and Cu = 0.5 wt%) and SAC-405 (Sn = 95.5, Ag = 4, and Cu = 0.5 wt%) are by far the most employed solders compatible with the PTH (Plating Through Hole) layers connection method, the SMT (Surface Mounted Technology) components placement approach, and the surface finishes routinely used in PCBs manufacturing (e.g. organic solder preservative, lead-free hot air solder levelling, immersion silver, immersion tin, electroless nickel immersion gold, etc.). Furthermore, near eutectic SAC alloys (i.e. comprising 3-4 wt% of silver and 0.5-1 wt% of copper) also benefit from improved temperature cycling reliability mainly due to the high Ag content.The present work puts forward a new multi-layer electrodeposition technique amenable to wafer level chip scale package. By taking advantage of multiple Dynamic Liquid Drop/Meniscus (DLD/DLM), it is possible to plate binary, ternary and even quaternary solders at high-throughput, high precision and low cost [4]. The DLD/DLM exposes a constantly refreshed confined electrolyte volume in contact with the substrate surface [5]. The core of the method consists in interposing a rinse DLD/DLM into a sequential array of multiple independent electroplating DLD/DLMs (one for each component to deposited). Such a configuration results in a synergistic system composed of different plating outputs (plating head). Thereby, scanning the plating head over the wafer surface allows to coat the substrate with a thin layer of each component. Thus, each plating head pass leads to the deposition of a multilayered structure reflecting the targeted composition of the alloy. In other words, the current density is regulated in each DLD/DLM in order to obtain the desired wt% of every single component in the final solder deposit. Obviously, the total film thickness depends on the number of layers and can be varied in a wide range to allow an homogenous structures even without reflow.As a proof-of-concept, SAC-305 and SAC-405 bumps and caps on top of copper pillars [6] were deposited: 30 μm thick structures were electroplated through more than 350 layers of each element. Bumps as high as 10 µm (attained for net electroplating times as low as 14 seconds) were sectioned by FIB (Focused Ion Beam) to verify the agreement between the expected composition and the actual one. EDX analysis performed before and after reflow proved that the high number of layers produces a compositionally homogenous solder regardless of the metal deposition sequence and reflow temperature. A detailed investigation of SAC and quaternary SAC+Me (e.g. Ni, Zn) alloys composition will be presented (e.g. thermal, mechanical and wetting properties); EDX analysis before and after reflow proving that the high number of layers homogenous solder is obtained independently of metal deposition sequence and even without reflow. Furthermore, the nozzle sizes of the multiple DLD/DLM equipment will be correlated to the aspect ratio of the dry film opening.[1] A. Lancaster and M. Keswani, Integration, 75, 77 (2017).[2] A. B. Frazier, R. O. Warrington and C. Friedrich, IEEE Transactions on Industrial Electronics, 42(5), 423 (1995).[3] X. Xu, A. S. Gurav, P. M. Lessner and C. A. Randall, IEEE Transactions on Industrial Electronics, 58(7), 2636 (2011).[4] K. Kholostov, L. Serenelli, M. Izzi, M. Tucci, D. Bernardi, M.Balucani, IEEE Journal of Photovoltaics 5(2), 404 (2016).[5] M. Balucani, K. Kholostov, P. Nenzi, R. Crescenzi, D. Ciarniello, D. Bernardi, L. Serenelli, M. Izzi, M. Tucci,Energy Procedia, 43, 54 (2013).[6] K. Kholostov, A. Klyshko, D. Ciarniello, P. Nenzi, R. Pagliucci, R. Crescenzi, D. Bernardi and M. Balucani, 43, 2014 IEEE 64th Electronic Components and Technology Conference.

Physics of failure of die-attach joints in IGBTs under accelerated aging: Evolution of micro-defects in lead-free solder alloys

Fatigue of die-attach solder joints is quite critical to reliability of IGBT modules. In the present work, physics of failure of die-attach joints in IGBTs was discussed based on the evolution of micro-defects in materials under accelerated aging through experiments and Finite element (FE) simulations. It shows that voids, cracks of Sn-Ag-Cu (SAC) alloys and detachment of Si/SAC interfaces are three major micro-defects in the die-attach solder. Voids could accelerate evolution of other two micro-defects. Cracking of SAC alloys is the major failure mode of die-attach solder under power cycling and detachment of Si/SAC interfaces turns to be the dominant failure mode under transient electrical/thermal shocks. FE simulations indicate that the shear thermal stress τshear at Si/SAC interfaces is the decisive factor for switching between the two failure modes. When ΔTj > 105 °C, the average shear stress τshear_ave at Si/SAC interfaces exceeds the bonding strength and detachment of Si/SAC interfaces plays a major role in failure of die-attach solder. Based on physics of failure, two physical life models for die-attach joints under various accelerated aging conditions were proposed.

Investigation of the preparation and tribological behavior of a frictional interface covered with sinusoidal microchannels containing SnAgCu and Ti3SiC2

Tribological behavior of 37NbCrCo steel is a key factor with regard to the service life of the gearing systems. We prepared the frictional interfaces covered with sinusoidal microchannels in 37NbCrCo material, using laser additive manufacturing. Subsequently, lubricants of SnAgCu and Ti3SiC2 were filled into the microchannels using high-temperature infiltration. On a MFT-5000 tribometer, tribological behaviors of 37NbCrCo–SnAgCu–Ti3SiC2 were superior to 37NbCrCo and 37NbCrCo–SnAgCu. Moreover, the Ti3SiC2 was wrapped into the SnAgCu alloys to ensure the formation of soft tribo-film on the high-hardness supporting layer. The following factors were responsible for the excellent friction and wear behaviors, namely: the Ti3SiC2-induced the formation of the tribo-film, plastic deformation of SnAgCu, the rolling friction of Ti3SiC2, and their cooperation functions.

Studies of the solidification characteristics in Sn-Ag-Cu-Bi solder alloys.

Because of the environmental health implications of Pb, and legislation backing Restriction on Hazardous Substances (RoHS) in electronic devices, Sn-based lead-free solders are developed. Near eutectic Sn-Ag-Cu (SAC) compositions is the most widely used Pb-free solder in the electronic industries. However, its high melting temperatures (>217 ℃) remains a concern. The addition of Bi to a Sn-based alloy can be used to lower melting temperature because of the presence of a deep valley eutectic in the Bi-Sn system. The effects of such additions in SAC on solidification path are rarely considered. The effects of Bi substitution for Sn on solidification of near eutectic SAC alloy are followed with differential thermal analysis (DSC). It is found that the solidus temperature of SAC-Bi alloy mixes is considerably lowered. For low concentrations of Bi, melting range (ΔTr) were generally less than 4 ℃. However, (ΔTr) considerably widens when the Bi content exceeds 2 wt. %. The DSC and X-ray diffraction (XRD) studies suggested that a higher concentration of Bi in SAC would lead to far eutectic solidification that is attended by a formation of primary Bi phase. The existence of primary Bi phase in a solidified solder would lead to embrittlement and thus would affect the integrity of soldered joints. This work suggests that the use of Bi in SAC solders should be restricted to less than 2 wt. % (by varying %Wt of Bi).

Fabrication and Characteristics of SnAgCu Alloy Nanowires for Electrical Connection Application.

As electronic products become more functional, the devices are required to provide better performances and meet ever smaller form factor requirements. To achieve a higher I/O density within the smallest form factor package, applying nanotechniques to electronic packaging can be regarded as a possible approach in microelectronic technology. Sn-3.0 wt% Ag-0.5 wt% Cu (SAC305) is a common solder material of electrical connections in microelectronic devices. In this study, SAC305 alloy nanowire was fabricated in a porous alumina membrane with a pore diameter of 50 nm by the pressure casting method. The crystal structure and composition analyses of SAC305 nanowires show that the main structure of the nanowire is β-Sn, and the intermetallic compound, Ag3Sn, locates randomly but always appears on the top of the nanowire. Furthermore, differential scanning calorimetry (DSC) results indicate the melting point of SAC305 alloy nanowire is around 227.7 °C. The melting point of SAC305 alloy nanowire is significantly higher than that of SAC305 bulk alloy (219.4 °C). It is supposed that the non-uniform phase distribution and composite difference between the nanowires causes the change of melting temperature.

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.

Wettability of SnAgCu–Cr alloys on graphite with different Cr contents

The wettability of brazing alloy on graphite surface is a key factor affecting the successful brazing of graphite to metals. By introducing active Cr, the traditional Sn0.3Ag0.7Cu brazing alloy can wet the graphite effectively. The design of active Cr introduction into SnAgCu alloy caused a significant decrease in contact angle, and the contact angle was decreased from 120° to 20° with the increasing content of active Cr. The influence of active Cr content on the wettability of SnAgCu/graphite system was investigated by sessile drop way at elevating and isothermal process in this work. Cr3C2 reaction layer formed at the interface of liquid alloy and graphite substrate, which enhanced the spreading of liquid brazing alloy and then decreased the contact angle. The thickness of Cr–C reaction layer kept identical of 2.5 μm with the further increasing content of active Cr. Finally, the wetting mechanism of SnAgCu–Cr/graphite system was proposed.

Formation of tin-based crystals from a SnAgCu alloy under formic acid vapor

A new method is developed to crystallize tin formates in a formic acid-enriched nitrogen atmosphere. The SAC alloy is used.

Effect of Strain Rate on Joint Strength and Failure Mode of Lead-Free Solder Joints

In surface mount technology, the Sn-3.0Ag-0.5Cu solder joint has a shorter impact lifetime than a traditional lead–tin solder joint. In order to improve the impact property of SnAgCu lead-free solder joints and identify the effect of silver content on tensile strength and impact property, impact experiments were conducted at various strain rates on three selected SnAgCu based solder joints. It was found that joint failure mainly occurred in the solder material with large plastic deformation under low strain rate, while joint failure occurred at the brittle intermetallic compound layer without any plastic deformation at a high strain rate. Joint strength increased with the silver content in SnAgCu alloys in static tensile tests, while the impact property of the solder joint decreased with increasing silver content. When the strain rate was low, plastic deformation occurred with failure and the tensile strength of the Sn-3.0Ag-0.5Cu solder joint was higher than that of Sn-0.3Ag-0.7Cu; when the strain rate was high, joint failure mainly occurred at the brittle interface layer and the Sn-0.3Ag-0.7Cu solder joint had a better impact resistance with a thinner intermetallic compound layer.

Synthesis of SnAgCu nanoparticles with low melting point by the chemical reduction method

SnAgCu alloy with low melting point and good soldering property is a good candidate for the Sn/Pb eutectic. In this paper, SnAgCu nanoparticles were synthesized by a chemical reduction method. The particle size and the melting point are controlled by modifying the process parameters, including reaction temperature, surfactant concentration and dropping speed of precursor. The lowest melting onset temperature is observed at 199.1°C, which is 18°C lower than that of commercially available SnAgCu solder alloy. The tensile strength of the as-synthesized reaches 34.3MPa, which reveals a good solderability property.

Critical evaluation of creep behavior of Sn-Ag-Cu solder alloys over wide range of temperatures

A critical survey of the current literature on creep behavior of Pb-free Sn based Sn-Ag-Cu (SAC) alloys show that there is considerable discrepancy in the creep parameters, such as activation energy, Qc, and stress exponent, n. Accordingly, this study addresses the underlying reasons for such variability in the existing data by studying the creep behavior in compression of two most commonly used SAC alloys, namely Sn-1wt.%Ag-0.5wt.%Cu and Sn-3wt.% Ag-0.5wt.% Cu, over wide range of temperatures, from 60 to 200°C, and stresses from 7 to 14MPa. Analysis of microstructure of SAC alloys reveals that both temperature and stress have pronounced effects on coarsening of precipitates during creep. The steady state strain rate increases with both temperature and stress as well as with decrease in Ag content of the alloys. Irrespective of the composition, SAC alloys show a transition in the creep behavior around 150°C, with Qc and n changing from 55kJ/mol and 7, respectively, at low temperatures to 100kJ/mol and 5, respectively, at higher temperatures. Such a simultaneous change in both Qc and n suggests that creep mechanism in SAC alloys is dislocation climb controlled by the core diffusion at low temperatures and the lattice diffusion at high temperatures.