SnAgCu Solder Joints Research Articles

Elastic Behavior of Coarse-Grained SnAgCu (SAC) Solder Joints Based on an Anisotropic Multi-scale Predictive Modeling Approach

Lead-free electronic interconnections made of Sn-rich solder alloys usually contain only a few highly anisotropic grains, rendering their overall thermo-mechanical behavior highly dependent on each crystal’s morphology and orientation. Miniaturization and heterogeneous integration will further reduce the length-scale of interconnections and exacerbate this piece-to-piece variation among different solder joints. The objective of this paper is to predict the elastic response and intergranular stresses in coarse-grained SAC (SnAgCu) solder joints, starting with orientation sensitive anisotropic predictive models of the elastic behavior of single crystals. The properties of the single crystal are, in turn, predicted by a multi-scale method, where each grain is modeled as a collection of Sn dendrites embedded in a eutectic Sn-Ag matrix. The eutectic phase, in turn, is modeled as an effective homogenized composite material consisting of nano-scale Ag3Sn particles embedded in a highly anisotropic tin matrix. All homogenization for prediction of single-crystal elastic properties is based on composite micromechanics, using Eshelby’s method and Mori–Tanaka homogenization. Elastic response of a few-grained solder joints is then numerically predicted from the anisotropic behavior of the single crystal, by explicitly modeling the grain morphology in finite element analysis (FEA). Stress distributions along grain boundaries are simulated and analyzed in terms of the misorientation angle between neighboring grains. The mismatch in von Mises stress along the boundaries varies up to 55% for grain structures considered here. This multi-scale predictive modeling approach is explicitly sensitive to microstructural features such as the morphology of: (1) the intermetallic compound (IMC) reinforcements in the eutectic phase; (2) dendrites; and (3) grains. Thus, this modeling approach is ideally suited for: (1) parametric studies of the effect of microstructural tailoring on solder joint thermo-mechanical behavior and (2) examining the effects of microstructural evolution caused by static and cyclic thermo-mechanical aging. This work forms the starting point for future development of microstructurally based anisotropic models of crystal-scale inelastic behavior.

Effects of Thermal Cycling on Creep of SnAgCu Solder Joints

The long-term life of properly optimized high-end microelectronics assemblies is commonly limited by fatigue of the solder joints. The most credible assessments of this rely on modeling-based extrapolation and generalization of accelerated test results. As the validity of extrapolations to long-term service can rarely if ever be verified experimentally, there is a strong need for mechanistically justified models. The models all rely on predictions of stresses, strains, and perhaps functions of those versus time and temperature in cycling. Finite-element-based predictions of these require the assumption of relatively complex constitutive relations capable of predicting inelastic strain rates under continuously varying stresses and temperatures. These are usually extracted from a series of creep or stress relaxation experiments and generalized under the implicit assumption that the inelastic deformation is dominated by dislocation creep. However, the resulting constitutive relations were recently shown to be strongly misleading. It was suggested that this is a result of the inelastic deformation at the stresses and temperatures of primary concern actually being dominated by dislocation core diffusion. This paper lends further support for this picture, showing additional contributions from diffusion on cycling induced dislocation structures to make for ongoing, usually nonmonotonic, variations in deformation rates from cycle to cycle. These effects are certain to have major consequences for modeling. In fact, it has been argued that the combination of wrong constitutive relations with a wrong damage function has led to a partial cancellation of errors across a limited parameter range, but that this cannot be counted on for more detailed predictions of accelerated test results and does not allow for interpretation of such results in terms of even qualitative trends in service.

Effect of Cycling Amplitude Variations on SnAgCu Solder Joint Fatigue Life

Reliability of conventional electronic assemblies in real service applications is typically limited by fatigue failure of a single solder joint in reversed cyclic loading. Fatigue damage accumulation in a lead-free solder joint is still not well understood, especially not for cycling with a varying amplitude; i.e., random vibration testing is not representative of realistic service conditions and the common reliability models fail to offer relatively accurate life predictions. Developing or modifying a fatigue life model should, of course, consider the interactive effect of stress amplitude variations, which is found to be substantial. Cycling real individual SnAgCu solder joints with systematically varying stress amplitudes allowed understanding of the fatigue damage accumulation through monitoring of the plastic work dissipation during cycling. The results show a significant amplification in the work dissipation compared to what was expected based on fixed stress amplitude cycling results. Cycling with combinations of 16- and 20-MPa stress amplitudes leads to amplification of the work (more fatigue damage) at the 16-MPa amplitude, but no significant change in damage accumulation at the 20-MPa amplitude. Cycling with combinations of three or more stress amplitudes leads to work amplification at the lowest amplitude, no change at the higher amplitude, and slight to moderate amplifications at the intermediate amplitudes. In addition, the value of the work amplification at any stress amplitude depends on the sequence of higher amplitudes applied immediately before it. It is also found that any high amplitude does not affect subsequent work at a lower amplitude if an even higher amplitude was applied just before it.

Fatigue measurement setup under combined thermal and vibration loading on electronic SMT assembly

In this work we introduce an experimental setup for experiments on combined thermal and vibration loadings applied to surface mount technology electronic systems. The setup provides well-defined load conditions and the ability of in-situ load measurement. Furthermore, rigging-free or known-rigging specimen mounting is featured.A test vehicle and an according mount as well as in-situ measurement approaches were developed and tested. This novel setup offers test vehicle mounting options which either reproduce system conditions including rigging for qualification purposes or enable fatigue research which rely on constant rigging-free load conditions. Specimens are made of standard or high temperature FR4 with assembled SMT components. Typically, vibration experiments will be conducted as a sine dwell with constant amplitude and frequencies in the range of 50 Hz…1 kHz. The setup may be used for isothermal vibration experiments at room, lower or higher temperatures (e.g. −40 °C or 125 °C) but it enables vibrations experiments at temperature cycling conditions as well. A low weight mount and proper PCB design targeting temperature cycling with reasonable gradients of about 1 K/min. A certain specimen design with integrated PCB heaters even enables local component heating. Dependent on the temperature conditions either optical or capacitive contact-free in-situ measurements of the specimen displacement and deflection can be utilised respectively.The ability of conducting contact-free measurements is very important for small or low weight specimens. Bulky and heavy acceleration sensors add to much mass and change the deformation behaviour. Strain gages do not come with these drawbacks but require large mounting area and are difficult to mount reproducible. Using the contact-free measurement systems high spatial and time measurement resolution can be achieved. First experiments on solder joints of chip resistor components show the effect of temperature on solder joint damage under vibration loading. Increased temperatures increase the risk of vibration caused damage in SnAgCu solder joints.

Effect of Long-Term Room Temperature Aging on the Fatigue Properties of SnAgCu Solder Joint

Solder joints in electronic assemblies are subjected to mechanical and thermal cycling. These cyclic loadings lead to the fatigue failure of solder joints involving damage accumulation, crack initiation, crack propagation, and failure. Aging leads to significant changes on the microstructure and mechanical behavior of solder joints. While the effect of thermal aging on solder behavior has been examined, no prior studies have focused on the effect of long-term room temperature aging (25 °C) on the solder failure and fatigue behavior. In this paper, the effects of long-term room temperature aging on the fatigue behavior of five common lead-free solder alloys, i.e., SAC305, SAC105, SAC-Ni, SAC-X-Plus, and Innolot, have been investigated. Several individual lead-free solder joints on printed circuited boards with two aging conditions (no aging and 4 years of aging) have been prepared and subjected to shear cyclic stress–strain loadings until the complete failure. Fatigue life was recorded for each solder alloy. From the stress–strain hysteresis loop, inelastic work and plastic strain ranges were measured and empirically modeled with the fatigue life. The results indicated that 4 years of room temperature aging significantly decreases the fatigue life of the solder joints. Also, inelastic work per cycle and plastic strain range are increased after 4 years of room temperature aging. The fatigue life degradation for the solder alloys with doped elements (Ni, Bi, Sb) was relatively less compared to the traditional SAC105 and SAC305.

Effects of Amplitude Variations on Deformation and Damage Evolution in SnAgCu Solder in Isothermal Cycling

Although apparently simpler than in thermal cycling, the behavior of SnAgCu (SAC) solder joints in cyclic bending or vibration is not currently well understood. The rate of damage has been shown to scale with the inelastic work per cycle, and excursions to higher amplitudes lead to an apparent softening, some of which remains so that damage accumulation is faster in subsequent cycling at lower amplitudes. This frequently leads to a dramatic breakdown of current damage accumulation rules. An empirical damage accumulation rule has been proposed to account for this, but any applicability to the extrapolation of accelerated test results to life under realistic long-term service conditions remains to be validated. This will require a better understanding of the underlying mechanisms. The present work provides experimental evidence to support recent suggestions that the observed behavior is a result of cycling-induced dislocation structures providing for increased diffusion creep. It is argued that this means that the measured work is an indicator of the instantaneous dislocation density, rather than necessarily reflecting the actual work involved in the creation of the damage.

A Mechanistic Thermal Fatigue Model for SnAgCu Solder Joints

The present work offers both a complete, quantitative model and a conservative acceleration factor expression for the life span of SnAgCu solder joints in thermal cycling. A broad range of thermal cycling experiments, conducted over many years, has revealed a series of systematic trends that are not compatible with common damage functions or constitutive relations. Complementary mechanical testing and systematic studies of the evolution of the microstructure and damage have led to a fundamental understanding of the progression of thermal fatigue and failure. A special experiment was developed to allow the effective deconstruction of conventional thermal cycling experiments and the finalization of our model. According to this model, the evolution of damage and failure in thermal cycling is controlled by a continuous recrystallization process which is dominated by the coalescence and rotation of dislocation cell structures continuously added to during the high-temperature dwell. The dominance of this dynamic recrystallization contribution is not consistent with the common assumption of a correlation between the number of cycles to failure and the total work done on the solder joint in question in each cycle. It is, however, consistent with an apparent dependence on the work done during the high-temperature dwell. Importantly, the onset of this recrystallization is delayed by pinning on the Ag3Sn precipitates until these have coarsened sufficiently, leading to a model with two terms where one tends to dominate in service and the other in accelerated thermal cycling tests. Accumulation of damage under realistic service conditions with varying dwell temperatures and times is also addressed.

Evaluation of thermo-mechanical damage and fatigue life of solar cell solder interconnections

The soldering process of interconnecting crystalline silicon solar cells to form photovoltaic (PV) module is a key manufacturing process. However, during the soldering process, stress is induced in the solar cell solder joints and remains in the joint as residual stress after soldering. Furthermore, during the module service life time, thermo-mechanical degradation of the solder joints occurs due to thermal cycling of the joints which induce stress, creep strain and strain energy. The resultant effect of damage on the solder joint is premature failure, hence shortened fatigue life. This study seeks to determine accumulated thermo-mechanical damage and fatigue life of solder interconnection in solar cell assembly under thermo-mechanical cycling conditions. In this investigation, finite element modelling (FEM) and simulations are carried out in order to determine nonlinear degradation of SnAgCu solder joints. The degradation of the solder material is simulated using Garofalo-Arrhenius creep model. A three dimensional (3D) geometric model is subjected to six accelerated thermal cycles (ATCs) utilising IEC 61215 standard for photovoltaic panels. The results demonstrate that induced stress, strain and strain energy impacts the solder joints during operations. Furthermore, the larger the accumulated creep strain and creep strain energy in the joints, the shorter the fatigue life. This indicates that creep strain and creep strain energy in the solder joints significantly impacts the thermo-mechanical reliability of the assembly joints. Regions of solder joint with critical stress, strain and strain energy values including their distribution are determined. Analysis of results demonstrates that creep energy density is a better parameter than creep strain in predicting interconnection fatigue life. The use of six ATCs yields significant data which enable better understanding of the response of the solder joints to the induced loads. Moreover, information obtained from this study can be used for improved design and better-quality fabrication of solder interconnections in solar cell assembly for enhanced thermo-mechanical reliability. Thermo-mechanical damage of solder joints in Si solar cell assembly evaluated.FEM and simulations carried out to determine nonlinear degradation of solder joints.Results show that induced stress and strain impacts solder joints reliability.Creep energy density better than creep strain in solder joint life prediction.Results useful for improved design of solder joints and better-quality fabrication.

Retardation of thermomigration-induced Cu substrate consumption in Pb-free solder joints by Zn addition

Thermomigration (TM) caused by temperature gradient in micro solder joints is recently recognized as a crucial reliability concern for 3D packaging technologies. In the present study, the interfacial reactions in Cu/Pb-free solder/Cu solder joints have been in situ investigated under a simulated temperature gradient of 508 °C/cm by synchrotron radiation real-time imaging technology. Serious TM-induced Cu substrate consumption at hot end and abnormally fast TM-enhanced intermetallic compound (IMC) growth at cold end are clearly observed in pure Sn and SnAgCu solder joints. However, significant retardation of the TM-induced Cu substrate consumption and the TM-enhanced IMC growth is realized by introducing Zn into the solders. The dissolution rate of the hot end Cu substrate and the growth rate of the cold end IMCs both decrease about three times as the Zn content increases from 0 wt% to 9.0 wt%. The improvement in TM resistance by Zn addition is attributed to the formation of Cu5Zn8 or Cu6(Sn,Zn)5 IMC at the hot end, the weakened diffusion of Cu atoms and the lowered driving force for IMC precipitation at the cold end. The molar heat of transport Q* of Zn is calculated to be 2.31 kJ/mol.

Reliability of SnAgCuFe Solder Joints in WLCSP30 Device

Anand constitutive model of SnAgCuFe solder joints was studied, and 9 parameters were determined based on tensile testing. And the model was incorporated in finite element code for analyzing the stress-strain response of SnAgCuFe solder joints in WLCSP30 device. The results indicate that the maximum stress concentrates on the top surface of corner solder joints, and the stress-strain of SnAgCuFe solder joints is lower than that of SnAgCu solder joints. Based on the fatigue life model, the addition of Fe can enhance the fatigue life of SnAgCu solder joints. Therefore, the SnAgCuFe solders can replace the traditional SnPb to be used in electronic packaging

Anand constitutive model of lead-free solder joints in 3D IC device

Anand constitutive relation of SnAgCu and SnAgCu-nano Al solders were studied under uniaxial tension, and the constitutive model was used in the finite element simulation to analyze the stress-strain response of lead-free solder joints in 3D IC devices. The results showed that the nine parameters of the Anand model can be determined from separated constitutive relations and experimental results. Based on Anand model, the finite element method was selected to calculate the stress-strain response of lead-free solder joints, it was found that in the 3D IC device the maximum stress-strain concentrated in the concern solder joints, the stress-strain of SnAgCu-nano Al solder joints was lower than that of SnAgCu solder joints, which represented that the addition of nano Al particles can enhance the reliability of lead-free solder joints in 3D IC devices.

Effect of Solder-Joint Geometry on the Low-Cycle Fatigue Behavior of Sn-xAg-0.7Cu

Low-cycle fatigue tests of Sn-Ag-Cu (SAC) Pb-free solder joints under fixed displacement were performed to evaluate the influence of Ag content (0–3 wt.%) and solder-joint geometry (barrel and hourglass types) on solder-joint fatigue behavior and reliability. The solder joints were composed of fine particles of Ag3Sn and Cu6Sn5, which aggregated as an eutectic constituent at grain boundaries of the primary β-Sn phase and formed a dense network structure. A decrease in the Ag content resulted in coarsening of the β-Sn and eutectic phases, which, in turn, decreased the strength of the joint and caused earlier failure. Solder joints in the hourglass form exhibited better fatigue performance with longer life than barrel-type joints. The sharp contact angle formed between the solder and the Cu substrate by the barrel-type joints concentrated stress, which compromised fatigue reliability. The addition of Ag to the solder, however, enhanced fatigue performance because of strengthening caused by Ag3Sn formation. The cracks of the barrel-type SAC solder joints originated mostly at the contact corner and propagated along the interfacial layer between the interfacial intermetallic compound (IMC) and solder matrix. Hourglass-type solder joints, however, demonstrated both crack initiation and propagation in the solder matrix (solder mode). The addition of 1.5–2.0 wt.% Ag to SAC solder appears to enhance the fatigue performance of solder joints while maintaining sufficient strength.

Creep Constitutive Models Suitable for Solder Alloys in Electronic Assemblies

Most solders used in electronic systems have low-melting temperature and hence experience significant amount of creep deformation throughout their life-cycle because typical operational and test conditions represent high homologous temperature. Phenomenological and mechanistic models used in the literature for predicting creep response of both bulk and grain scale specimens are reviewed in this paper. The phenomenological models reviewed in this paper are based on purely empirical observations of the creep deformation behavior or derived from qualitative interpretation of the underlying microscale mechanisms. These models have some intrinsic disadvantages since they do not have explicit mechanistic dependence on microstructural features. Therefore, the constitutive relations derived using the above models are difficult to extrapolate beyond the test conditions. This paper also reviews how some of the above limitations can be mitigated by using mechanistic or microstructurally motivated models. Mechanistic models are capable of estimating the material creep response based on the detailed physics of the underlying mechanisms and microstructure. The microstructure and constitutive response of the most popular family of lead-free solders, namely, SnAgCu (SAC) solders, are significantly different from those of previously used eutectic Sn37Pb solder. The creep deformation in Sn37Pb solder occurs primarily through diffusion-assisted grain-boundary sliding. In SAC solder joints, dislocation-based creep deformation mechanisms such as glide, climb, detachment, and cross-slip appear to be the dominant mechanisms in coarse-grained joints. Mechanistic creep models are therefore based on the deformation mechanisms listed above.

Mechanistic Prediction of the Effect of Microstructural Coarsening on Creep Response of SnAgCu Solder Joints

Mechanistic microstructural models have been developed to capture the effect of isothermal aging on time dependent viscoplastic response of Sn3.0Ag0.5Cu (SAC305) solders. SnAgCu (SAC) solders undergo continuous microstructural coarsening during both storage and service because of their high homologous temperature. The microstructures of these low melting point alloys continuously evolve during service. This results in evolution of creep properties of the joint over time, thereby influencing the long term reliability of microelectronic packages. It is well documented that isothermal aging degrades the creep resistance of SAC solder. SAC305 alloy is aged for (24–1000) h at (25–100)°C (~0.6–0.8 × Tmelt). Cross-sectioning and image processing techniques were used to periodically quantify the effect of isothermal aging on phase coarsening and evolution. The parameters monitored during isothermal aging include size, area fraction, and inter-particle spacing of nanoscale Ag3Sn intermetallic compounds (IMCs) and the volume fraction of micronscale Cu6Sn5 IMCs, as well as the area fraction of pure tin dendrites. Effects of microstructural evolution on secondary creep constitutive response of SAC305 solder joints were then modeled using a mechanistic multiscale creep model. The mechanistic phenomena modeled include: (1) dispersion strengthening by coarsened nanoscale Ag3Sn IMCs in the eutectic phase; and (2) load sharing between pro-eutectic Sn dendrites and the surrounding coarsened eutectic Sn-Ag phase and microscale Cu6Sn5 IMCs. The coarse-grained polycrystalline Sn microstructure in SAC305 solder was not captured in the above model because isothermal aging does not cause any significant change in the initial grain size and orientation of SAC305 solder joints. The above mechanistic model can successfully capture the drop in creep resistance due to the influence of isothermal aging on SAC305 single crystals. Contribution of grain boundary sliding to the creep strain of coarse grained joints has not been modeled in this study.

Effects of Strain Rate and Amplitude Variations on Solder Joint Fatigue Life in Isothermal Cycling

The behavior of lead-free solder alloys under realistic service conditions is still not well understood. Life prediction of solder joints relies on conducting accelerated tests and extrapolating results to service conditions. This can be very misleading without proper constitutive relations and without understanding the effects of cycling parameter variations common under realistic service conditions. It has been shown that the fatigue life depends on the inelastic work accumulation, independently of cycling-induced material property variations, which explains the breakdown of damage accumulation rules and allows the development of a modified Miner's rule. This paper discusses the interacting effects of strain rate and amplitude variations on solder joint fatigue life. Individual SnAgCu solder joints with two different Ag contents (SAC305 and SAC105) were tested in low cycling shear fatigue under single and varying amplitudes with different strain rates. Such a shear fatigue experiment allows the measurement of work accumulation and the evolution of solder deformation properties during cycling. The results showed that cycling with a lower strain rate at fixed amplitude causes more damage per cycle. Alternating between mild amplitude at a high strain rate and harsh amplitude at a low strain rate leads to ongoing increases in the rate of damage at the mild amplitude and thus relatively rapid failure. In comparing SAC305 with SAC105, the effect of strain rate on both alloys is almost the same, and SAC305 is still more fatigue resistant than SAC105 in varying amplitude cycling with any strain rate.

Multiscale modeling of the anisotropic transient creep response of heterogeneous single crystal SnAgCu solder

The lack of statistical homogeneity in functional SnAgCu (SAC) solder joints due to their coarse grained microstructure, in conjunction with the severe anisotropy exhibited by single crystal Sn, renders each joint unique in terms of mechanical behavior. A mechanistic multi-scale modeling framework is proposed in this study to predict the influence of composition and microstructure on the anisotropic transient creep response of single crystal SnAgCu (SAC) solder. Tier I consist of single-crystal eutectic Sn–Ag alloy, with nanoscale Ag3Sn particles embedded in a single-crystal Sn matrix. Tier II consists of single crystal SAC solder which is composed of Sn dendrites surrounded by the eutectic Sn–Ag phase of Tier I. The Tier I anisotropic transient creep model is based on dislocation mechanics. The Tier II model uses the results of Tier I as an input and is based on anisotropic composite micro-mechanics.In Tier I, creep deformation is governed by dislocation impediment and recovery at nanoscale Ag3Sn particles, with recovery being the rate controlling mechanism. Dislocation climb and dislocation detachment at the Ag3Sn particles are proposed to be the competing rate controlling recovery mechanisms. Line tension and mobility of dislocations in dominant slip systems of single crystal Sn are estimated based on the elastic crystal anisotropy of body centered tetragonal (BCT) Sn. The anisotropic transient creep rate of the eutectic Sn–Ag phase of Tier I is then modeled using above inputs and the evolving dislocation density calculated for dominant glide systems during the transient stage of creep. The dominant slip systems are determined based on the dislocation mobility and on the orientation angle between the crystal principal axes and the loading direction. The creep response of the eutectic phase (from Tier 1) is combined with the creep response of Sn lobes at Tier 2, using the anisotropic Mori-Tanaka homogenization theory, to obtain the transient creep response of a SAC305 single crystal along global specimen directions. This model has been calibrated using experimentally obtained transient creep response of a SAC305 single crystal specimen. The above multiscale calibrated model is then used to predict (i) the transient creep response of another SAC305 single crystal specimen and (ii) the effect of orientation (by changing one of the Euler angles) on the transient creep response of SAC305 single crystal. The grain orientation of above two SAC single crystal specimens (with respect to loading direction) were identified with orientation image mapping and then utilized in the model to estimate the resolved shear stress along the dominant slip directions. Parametric studies have also been conducted to predict the effects of the volume fraction, aspect ratio, and orientation of ellipsoidal Sn inclusions on the anisotropic transient creep response of SAC single crystals.

TEM observation of interfacial compounds of SnAgCu/ENIG solder bump after laser soldering and subsequent hot air reflows

In three-dimensional (3D) packaging and assembly technology, Lead-free SnAgCu solder joints inevitably experience multiple reflows. The interfacial compounds of Sn3.0Ag0.5Cu (SAC305)/ Electroless Ni–P/Immersion Au (ENIG) solder bump after laser soldering and subsequent hot air reflows were observed by transmission electron microscopy (TEM), respectively. The initial laser soldering played a key role in interfacial compound evolution during the subsequent multiple reflows. After laser soldering, a thin P-rich layer and an ultrafine scallop-type (Ni, Cu)3Sn4 layer formed at the interface of SAC305/ENIG. The valleys of ultrafine (Ni, Cu)3Sn4 grains served as rapid channels for Ni diffusion, which contributed to the formation of dendritic η-(Cu, Ni)6Sn5+(Ni, Cu)3Sn2 intermetallic compounds after the first hot air reflow. However, with the number of reflow times increasing, the η-(Cu, Ni)6Sn5 totally transformed to noodle-like (Ni, Cu)3Sn2 owing to their similarly hexagonal lattice structure and the limited Cu supply in solder.

Effects of graphene nanosheets on interfacial reaction of Sn–Ag–Cu solder joints

In this work, varying amount of graphene nanosheets (GNSs) were introduced as reinforcements into Sn–Ag–Cu (SAC) solder to get composite solders. Then, the formation and growth kinetics of the intermetallic compounds (IMC) were investigated during the liquid–solid reactions and solid-state aging. Experimental results demonstrated that the morphologies of the interfacial IMC were transformed from scalloped to platelike after liquid–solid reactions and solid-state aging. The IMC thickness of composite solder joints was thinner than that of the unreinforced solder joints. The calculations revealed that the growth rate constant of composite solder was lower than that of bulk SAC solder. It's worth to mention that the composite solder joints exhibited lower diffusion coefficients (ranging from 0.6 × 10−12 m2/s to 1.07 × 10−12 m2/s) than that of the SAC solder joints (1.81 × 10−12 m2/s) after liquid–solid reactions and the composite solder joints also exhibited lower diffusion coefficients (ranging from 1.42 × 10−12 m2/s to 1.78 × 10−12 m2/s), as compared to that of the monolithic SAC solder joint (2.46 × 10−12 m2/s) after aging studies. It indicates that the addition of GNSs can effectively suppress the growth of the overall IMC layers, which can also hinder the IMC's growth and enhance solder joints' reliability.

Improved drop reliability of Sn–Ag–Cu solder joints by Zn addition to a Cu wetting layer

Cu–20 wt% Zn solder wetting layers were fabricated by electroplating in cyanide and non-cyanide solutions, and board-level drop impact reliability was evaluated for Sn–Ag–Cu (SAC) solder joints formed on Cu–Zn layers. Adding Zn to the Cu wetting layer suppressed the formation of a large Ag3Sn plate in the solder and delayed interfacial intermetallic compound (IMC) growth in the solder/Cu–Zn interfaces. Formation of Cu3Sn IMC and microvoids was not observed in the Cu6Sn5/Cu–Zn interface during thermal aging. The drop impact reliability of the Cu–Zn/SAC/Cu–Zn solder joints was superior to that of Cu/SAC/Cu solder joints, both before and after aging. The drop impact performance of the Cu–Zn specimens was similar whether their Cu–Zn layers were electroplated in cyanide or non-cyanide solution. Drop impact reliability of both Cu and Cu–Zn specimens decreased after aging due to IMC growth and/or microvoid formation in the solder interfaces. Interfacial microstructure influenced the failure mode of the solder joints. For Cu specimens, the dominant failure sites were a Cu6Sn5/Cu interface before aging and a Cu3Sn/Cu interface after aging. The formation of a large Ag3Sn plate facilitated crack growth. For Cu–Zn specimens, the crack always propagated inside the Cu6Sn5 layer. Microstructural changes in the solder interfaces arising from the addition of Zn contributed to the improvement of drop impact reliability of SAC solder joints.

IMC and creep behavior in Lead-free solder joints of Sn-Ag and Sn-Ag-Cu alloy system by SP method

The creep properties of Lead-free solder joints were evaluated by Shear Punch (SP) method. Lead-free solder joints with Sn-4Ag, Sn-4Ag-0.5Cu and Sn-4Ag-1.0Cu solder tested and compared to the lead solder joint with Sn-37Pb. SP creep tests were performed at temperature of 30, 50 and 80°C. For the SP method, constant shear stresses from 3 to 22 MPa were applied into the SP specimen(10 × 10 × 0.5 mm). From the SEM, IMC layer of Cu6Sn5 were observed on the interface between solder and Cu. From the SEM and EDX, the Lead-free solder joints have much thicker interfacial IMC layer than the Sn-37Pb solder joint. From the power law relationships, the Lead-free solder joints show superior creep resistance than the Sn-37Pb solder joint. And among Lead-free solder joints, at 30 and 50°C, the ternary SnAgCu solder joints have high creep resistance than the binary SnAg solder joint. Sn-4Ag-1.0Cu solder joint shows the best creep resistance at 30°C, and Sn-4Ag-0.5Cu solder joint shows the best creep resistance at 50°C. However, at high temperature 80°C, all of the Lead-free solder joints have similar creep properties. The activation energy were expressed with creep strain rate and temperature. The ternary SnAgCu solder joints have the higher activation energy than the binary SnAg solder joint at 17.68 and 18.72 MPa. Average activation energy of Lead-free solder joints were lower than that of Sn-37Pb 75.11 kJ/mol. The average activation energy of Sn-4Ag-1.0Cu, Sn-4Ag-0.5Cu and Sn-4Ag solder joint at high stress is 102.68, 62.52 and 58.61 kJ/mol, respectively.