Lead-free Solder Joints Research Articles

Interfacial reaction, wettability, and shear strength of ultrasonic-assisted lead-free solder joints prepared using Cu–GNSs-doped flux

Interfacial reaction, wettability, and shear strength of ultrasonic-assisted lead-free solder joints prepared using Cu–GNSs-doped flux

Reliability Analysis of Solder Joints on Rigid-Flexible Printed Circuit Board for MEMS Pressure Sensors Under Combined Temperature Cycle and Vibration Loads With Continuously Monitored Electrical Signals

Abstract The reliability of lead-free solder joints on flexible printed circuit board (PCB) has created significant new challenges in the industry, especially in automotive electronics, and possibly for future wearable electronics. In the automotive industry, thermal cycling test and random vibration test need to be conducted to certify every electronic product to be used in harsh automotive environments. In order to accelerate the testing time, we may need to subject the electronic components, in particular, sensors to both loadings such as thermal cycling and vibration. During all the experiments, the electrical signals of each specimen were continuously monitored by using an event detector. One advantage of this method is that any individual soldering interconnect failure will result in the diagnostic signal of the circuit, which could be detected in real-time. A simulation was used to confirm the possibility of the stress concentration location caused by the vibration and thermal cycling loads. The influence of vibration frequency and acceleration on the vibration fatigue life of solid joints was investigated. In this paper, the submodeling technique was used to construct the finite element model of the rigid-flexible printed circuit board (rigid-flexible PCB) coupled with a MEMS pressure sensor subjected to temperature cycle and random vibration loadings. The research results are helpful to effectively characterize the performance of the MEMS sensors under both thermal cycling test and vibration test. Two kinds of land shapes and two kinds of PCB assemblies were selected. In order to investigate the crack growth in the solder joint, the solder joint is sliced and the crack on the cross section of the solder joint was observed. Results of finite element modeling analysis were consistent with the experimental results. Two design parameters have been identified in our research as being important to soldering usage in automotive environments: pad type (teardrop versus nonteardrop) and pad size (big versus small, matching size for Cu-wire and pad). Experimental results also showed that the solder joint with a big land shape presented a relatively good thermal fatigue resistance.

RETRACTED ARTICLE: Effect of grain size on the interface structure and shear behavior of lead-free solder joints

RETRACTED ARTICLE: Effect of grain size on the interface structure and shear behavior of lead-free solder joints

Research on Microstructure and Shear Behavior of Au/Sn-Ag-Cu/Cu Lead-free Solder Joints at Different Soldering Temperatures

Research on Microstructure and Shear Behavior of Au/Sn-Ag-Cu/Cu Lead-free Solder Joints at Different Soldering Temperatures

Shear Strength Degradation Modeling of Lead-Free Solder Joints at Different Isothermal Aging Conditions

Abstract SAC-based alloys are one of the most common solder materials that are utilized to provide mechanical support and electrical connection between electronic components and the printed circuit board. Enhancing the mechanical properties of solder joints can improve the life of the components. One of the mechanical properties that define the solder joint structure integrity is the shear strength. The main objective of this study is to assess the shear strength behavior of SAC305 solder joints under different aging conditions. Instron 5948 Micromechanical Tester with a customized fixture is used to perform accelerated shear tests on individual solder joints. The shear strength of SAC305 solder joints with organic solderability preservative (OSP) surface finish is investigated at constant strain rate under different aging times (2, 10, 100, and 1,000 h) and different aging temperatures (50, 100, and 150°C). The nonaged solder joints are examined as well for comparison purposes. Analysis of variance (ANOVA) is accomplished to identify the contribution of each parameter on the shear strength. A general empirical model is developed to estimate the shear strength as a function of aging conditions using the Arrhenius term. Microstructure analysis is performed at different aging conditions using scanning electron microscope (SEM). The results revealed a significant reduction in the shear strength when the aging level is increased. An increase in the precipitates coarsening and intermetallic compound (IMC) layer thickness are observed with increased aging time and temperature.

Effect of Ni on the Suppression of Sn Whisker Formation in Sn-0.7Cu Solder Joint.

The evolution of internal compressive stress from the intermetallic compound (IMC) Cu6Sn5 growth is commonly acknowledged as the key inducement initiating the nucleation and growth of tin (Sn) whisker. This study investigates the effect of Sn-0.7Cu-0.05Ni on the nucleation and growth of Sn whisker under continuous mechanical stress induced. The Sn-0.7Cu-0.05Ni solder joint has a noticeable effect of suppression by diminishing the susceptibility of nucleation and growth of Sn whisker. By using a synchrotron micro X-ray fluorescence (µ-XRF) spectroscopy, it was found that a small amount of Ni alters the microstructure of Cu6Sn5 to form a (Cu,Ni)6Sn5 intermetallic layer. The morphology structure of the (Cu,Ni)6Sn5 interfacial intermetallic layer and Sn whisker growth were investigated by scanning electron microscope (SEM) in secondary and backscattered electron imaging mode, which showed that there is a strong correlation between the formation of Sn whisker and the composition of solder alloy. The thickness of the (Cu,Ni)6Sn5 IMC interfacial layer was relatively thinner and more refined, with a continuous fine scallop-shaped IMC interfacial layer, and consequently enhanced a greater incubation period for the nucleation and growth of the Sn whisker. These verification outcomes proposes a scientifically foundation to mitigate Sn whisker growth in lead-free solder joint.

Influence of Joint Arrangement on the Fracture Behavior of Lead-Free Solder Joints

The capability to standardize the fracture strength of solder joints is an effective tool to investigate the reliability of electronic devices. To achieve this purpose, in this research, the influences of joint arrangement (loading arm and load sharing) on the level of constraint imposed on joint deformation, fracture energy, and generally, fracture behavior of solder joints were investigated. Fracture behavior of solder joints using double-cantilever-beam (DCB) specimens as a function of loading arm and load sharing (i.e., the distance between two solder joints) was studied under mode I loading conditions at a strain rate of 0.03 s−1. By increasing the loading arm, the fracture force, Fci, decreased linearly, while the critical strain energy release rate for crack initiation, Jci, increased from a loading arm of 12.7 mm to 38.1 mm and then remained almost unchanged for loading arms of 38.1 mm to 71.1 mm. Plastic deformation in the solder layer and criteria such as opening stress (i.e., the predominant stress component in DCB specimen) and stress triaxiality factor were calculated. It was shown that for the larger loading arms (from 38.1 mm to 71.1 mm), the fracture behavior was as a normal DCB specimen (i.e., the normal stress caused by bending was predominant and normal stress caused by tensile loading could be ignored). For the loading arm of less than 38.1 mm, the fracture behavior was similar to that of tensile-type specimens (i.e., the normal stress caused by bending decreased significantly by decreasing the loading arm and normal stress caused by tensile loading became considerable). Variations in the distance between two solder joints did not affect the Jci value, while Fci was influenced by the joint arrangement. This behavior was attributed to stress distribution in the solder layer.

In Situ Observation of Electromigration-Induced Anomalous Precipitation of Ag3Sn Phase in Ag-Containing Solder Joints

The anomalous precipitation behavior of Ag3Sn phase in Sn-3.5Ag lead-free solder joints during liquid–solid electromigration at 1.0 × 104 A/cm2 has been observed using a real-time imaging method with synchrotron radiation. Large Ag3Sn phases precipitated at the anode instead of cathode side in the Ag-containing solder joints during cooling, due to directional diffusion of Ag atoms towards the anode under current stressing. The electromigration-induced directional diffusion behavior of Ag atoms can mainly be attributed to their negative effective charge number (Z Ag * ) of −0.48. Electromigration induced a localized Ag-rich area and thereby a large number of Ag3Sn-based clusters at the anode. This resulted in preferential nucleation and anomalous precipitation of Ag3Sn phases at the anode side in Ag-containing solder joints. The rapid growth of the Ag3Sn plates can be explained in terms of the net atomic flux driven by the electromigration (Jem) and chemical potential (Jchem) effects.

Effect of Soldering Temperature on the Reliability of Sn-Ag-Cu Lead-Free Solder Joints

This paper investigates the effect of soldering temperature on solder joint voids and reliability of flip-chip LED chips during reflow soldering. Lead-free solder SAC305 was used as solder paste. The void ratio of the flip-chip LED solder joint at 250°C, 260°C, 270°C, 280°C, and 290°C reflow soldering temperatures was detected by x-ray detector. Shear tests were conducted to evaluate the influence of interfacial reactions on the mechanical reliability of solder joints. The distribution of voids in the shear section was observed by scanning electron microscope (SEM). Next, the photoelectric and thermal properties of FC-LED filament were tested and analyzed. Finally, a high-temperature and high-humidity aging experiment was carried out to test the reliability of the LED filament. The results show that the void ratio of the LED filament soldering joint is the lowest when the soldering temperature is 270°C. The small void ratio of the solder joints results in lower steady-state voltage and junction temperature of the flip-chip LED filament. As the void density in the solder joint decreases, the shear strength of the solder joint increases. At this time, the shear resistance and mechanical reliability are the highest.

Effect of Bi addition on the shear strength and failure mechanism of low-Ag lead-free solder joints

To optimize the mechanical properties of low-Ag lead-free solder, different amount of Bi element was added into the Sn–1.0Ag–0.5Cu (SAC105) to form SAC105-xBi (0 ≤ x ≤ 4) solder alloy. The microstructural evolution and shear fracture of SAC105-xBi solder joints were systematically investigated to clarify the failure mechanism after reflow and after 175 °C aging compared to SAC305. It was found that Bi element could provide solution strengthening and second-phase strengthening in the low-Ag SAC solder joints depending on the amount of added Bi element. The shear strength after reflow increased from 34.79 to 70.16 MPa with the increase of Bi addition from x = 0.0 to x = 4.0, while that after aging at 175 °C for 168 h increased from 28.97 MPa (x = 0.0) to 55.02 MPa (x = 4.0). The thickness of IMC decreased with the addition of Bi, and excessive Bi would precipitate in the matrix when the Bi content reached 2 wt% or even form brittle Bi barrier layer after 4 wt% addition. During shear tests, three kinds of fracture mode including ductile fracture, quasi-cleavage fracture, and cleavage brittle fracture were observed, and three kinds of failure position including solder matrix, Cu6Sn5/solder interface, and Cu3Sn/Cu interface were revealed. The failure mechanism of different solder joints was closely related to the coupling effects among the Bi amount in the matrix, the thicknesses of IMC, and the status of Kirkendall voids, which significantly affected the reliability of SAC solder joints. Finally, SAC105-1.0Bi solder alloy was considered having the most suitable Bi addition with anticipated comprehensive properties compared with SAC305.

Effect of Isothermal Aging and Copper Substrate Roughness on the SAC305 Solder Joint Intermetallic Layer Growth of High Temperature Storage (HTS)

This study aims to evaluate the effect of copper (Cu) substrate surface roughness on the intermetallic compound (IMC) growth and interfacial reaction of SAC305 lead-free solder joint after undergone an aging process. Aging process was conducted using high temperature storage (HTS) at temperature of 150 °C and aging times of 200, 400, 600, 800, and 1000 h. IMC morphology and growth were examined using infinite focus microscope (IFM). Then, the SAC305 solder joint IMC growth kinetic was measured based on power law relationship and diffusion coefficient formula. It was noted that the morphology of IMC for the rougher Cu substrate has scallop-shaped and uniform layer as compared to that of smoother Cu substrate for the initial exposure to the HTS. In addition, Cu substrate with Ra of 579 nm is the turning point for the creation of Cu6Sn5 towards more Cu3Sn of IMC. In addition, Cu substrate with Ra of 579 nm also acts as the turning point for the IMC growth of SAC305 solder joint on Cu substrate for the solid-state diffusion to be happened during 150 °C of aging from grain boundary dominant toward volume diffusion dominant.

Impact of inter-metallic compound thickness on thermo-mechanical reliability of solder joints in solar cell assembly

This study evaluates the impact of intermetallic compound (IMC) thickness on thermo-mechanical reliability of lead-free SnAgCu solder joints in crystalline silicon solar cell assembly with regard to fatigue life. Finite element modelling is used to simulate the non-linear thermo-mechanical deformation of the joints. Five geometric models of solar cell assemblies with different IMC thickness layers in the range of 1 to 4 μm are utilized. The models were subjected to accelerated thermal cycling from 40 °C to 85 °C employing IEC 61215 standard for photovoltaic panels. Creep response of each of the assembly's solder joints to the induced thermal load were simulated using Garofalo-Arrhenius creep model. Simulation results indicate that when IMC thickness grows incrementally to 1, 2, 2.5, 3 and 4 μm, thermo-mechanical fatigue life of solder joints diminishes to 13,800, 11,800, 10,600, 9400 and 7800 cycles to failure respectively. Thus, solder joint fatigue life decreases as the IMC thickness increases during service lifetime. Therefore, proper design of solder joint in crystalline silicon solar cell assembly must include consideration of IMC layer thickness to prevent premature failure and to ensure fulfilment of desired functional lifetime of 13,688 cycles to failure (25 years).

Influence of microstructure inhomogeneity on the current density and temperature gradient in microscale line-type Sn58Bi solder joints under current stressing

Due to the increasing miniaturization of microelectronic packaging, the electromigration (EM) phenomenon generally exists in microscale lead-free solder joints, which is mainly caused by high current stressing. To address the limitation of experimental measurement at a microscale, this study performed a finite element analysis (FEA) based on microstructure simulation and image recognition to characterize the influence of microstructure inhomogeneity on the magnitude and distribution of current density and temperature gradient in microscale line-type Sn58Bi solder joints under current stressing. The simulation results indicate that the microstructure inhomogeneity has a significant influence on the magnitude and distribution of the current density and temperature gradient in Sn58Bi solder joints. Specifically, the current density and the total current-induced Joule heats in the Sn-rich phase are much higher than those in the Bi-rich phase. However, the temperature gradient in the Sn-rich phase is smaller than that in the Bi-rich phase. Moreover, similar results and conclusions can be obtained from both two- and three-dimensional FEA.

Impact of crystalline orientation of lead-free solder joints on thermomechanical response and reliability of ball grid array components

The microstructure of lead-free solder joints often consists of only one or a few randomly oriented tin grains as a result of cooling conditions and reactions that take place during the solidification. Due to severe anisotropy of the tin phase and this complex microstructure, the stress state of each joint will be unique, and lead to a dispersion in times to fail. This study aims at evaluating the impact of orientations of tin grains on the stress state of a BGA component. Different combinations of solder joint grain orientations are studied through thermo-mechanical simulations in order to assess the stress state of each joint. A significant effect of neighbouring joints has been observed for the different cases.

Superconducting Lead-free Solder Joint: A Short Review

Superconducting solders are widely used as joining material to connect superconducting wires such as Niobium-titanium (NbTi) and Niobium–tin (Nb3Sn) wires in commercial superconducting magnet applications. The physical and superconducting properties of the solder materials are very important since it influence the overall performance of the joint during service. Pb-Bi solder alloy widely used due to their good properties suitable for the superconducting application, however, the restriction on the use of Pb in industry lead to the development of new Pb-free solder material. This paper reviews the superconducting Pb-free solder alternatives, including In-Sn based solder. Besides that, the effect of bismuth (Bi) and antimony (Sb) addition to the binary In-Sn solder alloy were also discussed.

State of the Art of Lead-Free Solder Joint Reliability

AbstractThe state of the art of lead-free solder joint reliability is investigated in this study. Emphasis is placed on the design for reliability (DFR) and reliability testing and data analysis. For DFR: (a) the Norton power creep constitutive equations and examples for Au20Sn, Sn58Bi, Sn3.8Ag0.7Cu, and Sn3.8Ag0.7Cu0.03Ce, (b) the Wises two power creep constitutive equations and examples for Sn3.5Ag and Sn4Ag0.5Cu, (c) the Garofalo hyperbolic sine creep constitutive equations and examples for Sn3.5Ag, Sn3Ag0.5Cu, Sn3.9Ag0.6Cu, Sn3.8Ag0.7Cu, Sn3.5Ag0.5Cu, and Sn3.5Ag0.75Cu, Sn4Ag0.5Cu, Sn(3.5-3.9)Ag(0.5-0.8)Cu, 100In, Sn52In, Sn3.8Ag0.7Cu0.03Ce, and Au20Sn, and (d) the Anand viscoplasticity constitutive equations and examples for Sn3.5Ag, Sn3Ag0.5Cu, Sn3.8Ag0.7Cu, Sn3.8Ag0.7CuCe, Sn3.8Ag0.7CuAl, Au20Sn, Sn3.5Ag with temperature and strain rate-dependent parameters, and Sn1Ag0.5Cu, Sn2Ag0.5Cu, Sn3Ag0.5Cu, and Sn4Ag0.5Cu after extreme aging will be discussed. For reliability testing and data analysis: (a) the Weibull and lognormal life distributions for lead-free solder joints under thermal-cycling and drop tests, (b) the true Weibull slope, true characteristic life, and true mean life, and (c) the linear acceleration factors for various lead-free solder alloys based on: (i) frequency and maximum temperature, (ii) dwell time and maximum temperature, and (iii) frequency and mean temperature will be presented. Some recommendations will also be provided.

Microstructure and shear behavior of Sn58Bi/Cu solder joint enhanced by SnAgCuBiNi bump

In order to improve the mechanical behavior of the low-temperature [Formula: see text] (SnBi) lead-free solder joint, the [Formula: see text] (SACBN) solder ball with the diameter of 400 [Formula: see text]m was pre-soldered on Cu to obtain the SnBi/SACBN/Cu composite joint. The microstructure and shear behavior of the solder joints were investigated. Experimental results indicate that SnBi solder is well bonded on the SACBN bump due to the elemental diffusion and dissolution between the molten SnBi and solid SACBN bump during the soldering process. The addition of the SACBN bump shows a significant effect on the formation and growth of the [Formula: see text]-Sn grains in the SnBi bulk. Compared with the SnBi/Cu joint, the SnBi bulk in the composite joint shows enlarged [Formula: see text]-Sn dendritic grains. Meanwhile, the interfacial intermetallic compound (IMC) layer transforms from Cu6Sn5 into (Cu, Ni)6Sn5. Among these three solder joints, the shear strength of the SACBN/Cu joint is the highest, reaching 86.7 MPa. The shear strength of the SnBi/Cu solder joint is enhanced by the SACBN bump from 68.2 MPa to 75.2 MPa. Additionally, the addition of the SACBN bump shows a positive effect on suppressing the brittleness of the SnBi/Cu solder joint.

Microstructure, isothermal and thermomechanical fatigue behaviour of leaded and lead-free solder joints

The reliability of solder joints is a critical issue in the microelectronics industry. The requirement of permanent electrical and thermal connections between solder alloys and the various components of a surface mount device is dependent upon the mechanical integrity of the solder and its interfaces.Accordingly, in this paper, the reliability of lead-free, Sn-3.8Ag-0.7Cu, and leaded, Sn-37Pb, solder alloys was investigated under both thermal-mechanical fatigue (TMF) and isothermal mechanical fatigue (IF) conditions. The investigation included material characterisation and fatigue testing on 4-ball grid array (BGA) specimens. The IF tests were carried out under load control at three different temperatures including Room Temperature (RT), 35 °C and 75 °C. Also, a set of ‘not-in-phase’ (nIP), ‘in-phase’ (IP) and ‘out-of-phase’ (OoP) combined thermal and mechanical cycling tests were carried out to investigate the TMF behaviour of the solders. The stress-life curves for each test condition were generated and then compared taking into account the observations on microstructure.It was found that the IF and TMF performance of Sn-3.8Ag-0.7Cu alloy was better than Sn-37Pb alloy when expressed as stress-life curves. Additionally, the Sn-3.8Ag-0.7Cu was less susceptible to the changes in temperature. This study provides a comprehensive insight into the reliability of solder alloys under a wide range of loading conditions.

Fatigue Crack Networks in the Die-Attach Joint of a Power Semiconductor Module During Power Cycling Testing and Effects of Test Parameters on the Joint Fatigue Life

The fracture mechanisms of a Sn-Ag-Cu lead-free solder joint and the effects of test parameters on power cycling life were investigated. In this study, three levels of average temperature (348, 373 and 398 K), two levels of each of current on-and-off time (2 s/10 s and 5 s/22 s), and three levels of ΔTj (75, 100 and 125 K) were used. Because the thermal expansion of the die-attach material was restrained by Si, equibiaxial tensile and compressive creep deformation occurred during power cycling. This deformation generated fatigue cracks in the central part of the die-attach joint, and the cracks connected to each other, resulting in a fatigue crack network. The fracture occupied 70% or more of the die-attach area and was the dominant factor affecting the joint’s fatigue life. As ΔTj increased, the strain energy density in the central part increased, resulting in a decrease in fatigue life. Although the strain energy density at an average temperature of 398 K decreased by about 10% compared with that at an average temperature of 348 K, the fatigue life decreased to about 40% when the average temperature increased. Continuous dynamic recrystallization occurred during power cycling, leading to fatigue failure in high-energy grain boundaries. Since continuous dynamic recrystallization tended to occur more readily at higher temperatures, the fatigue life decreased with increasing average temperature, even for the same junction temperature range ΔTj. Moreover, the area of equibiaxial tensile and compressive creep deformation in the die-attach material became larger with increasing current on-and-off times. As a result, the fracture area under long current on-and-off time conditions was increased by about 25% in comparison with that under short current on-and-off time conditions.

Book Review: Assembly and reliability of lead-free solder joints. Springer. (2020)

Book Review: Assembly and reliability of lead-free solder joints. Springer. (2020)