Actual Solder Research Articles

Reliability and thermal fatigue life prediction of solder joints using nanoindentation

The construction of accurate constitutive equations is critical for the reliability analysis of the solder joints in high-density interconnect. However, there are some distinct differences between the actual solder joints and conventional bulk solder in determining the parameters for constitutive equations. In this work, the Young's modulus and hardness of various grain regions are experimentally investigated using electron backscatter diffraction and nanoindentation for the Sn-3.0Ag-0.5Cu polycrystalline and single-grain solder joints. The disparities in the Young's modulus and hardness of different grain regions in the polycrystalline solder joint are up to 25.11% and 28.34%, respectively, while those of the single-grain solder joint exhibit stability. It indicates the anisotropy of β-Sn grains and the size effects between actual solder joints and bulk solder materials. Furthermore, a method is proposed to determine the parameters of the Anand constitutive equations using high-temperature nanoindentation creep data. A flip-chip assembly in thermal shock conditions is numerically investigated. The Darveaux model is employed to predict the thermal fatigue life of the critical solder joint. Compared to experimental results, the life prediction of the solder joint using nanoindentation is found to be accurate. A concept of life correction factor n has been proposed to improve the accuracy of the predicted life.

Size-on-demand preparation of SAC305 solder balls based on pulsated orifice ejection method

Drop-on-demand ejection of solder droplets has significant potential applications in field such as ball grid array packaging and microcircuit printing. To apply this technique successfully, it is important to ensure stability during the ejection of droplets and control the size of the droplets accurately as required. In this study, the pulsated orifice ejection method (POEM) is used to investigate the relationship between key parameters and the stability and size of SAC305 droplet ejection. Additionally, the microstructural morphology changes in solder balls with different sizes are investigated. Reducing the duration of the tup, increasing of voltage, and decreasing rod distance all lead to bigger solder balls. To achieve stable and size-on-demand droplet ejection, key parameters should be coordinated to ensure the deviation between the actual solder ball size and the target solder ball size remains within a range of ±10 μm. Observation and analysis of the microstructure reveal that as the size of solder ball increase, the cooling rate decreases rapidly, resulting in significant differences in the dendritic growth within the solder balls. This study presents an effective method and parameter selection for the drop-on-demand droplet ejection process, providing a solid foundation for direct solder bump deposition and microcircuit printing.

Challenge of Establishing Materials Testing Standards with Miniature Specimen for Solders

Abstract Solder is an essential material for connecting parts inside electrical devices. Although there is an international standard for the chemical composition of solder, there is no testing standard for evaluating the mechanical properties and characteristics of solder as a structural material. The Committee on High Temperature Strength of Materials, the Society of Materials Science, Japan, proposed some testing standards for solders in the early 2000s based on the results from research activities of the Solder Strength Evaluation Method Working Group. The primary objective of the standardization at that time was to acquire stable testing data and to eliminate institutional differences. The recommended specimen diameter is 10 mm. To verify the size difference between the 10-mm-diameter specimen and an actual solder joint inside an electrical device, the working group has been investigating materials testing techniques using a specimen with a reduced diameter of 3 mm. The research results were published in 2020 as Standard for Miniature Testing of Solders. We have also conducted a series of systematic experiments based on the testing standards for Sn-3.0Ag-0.5Cu. The results obtained so far are as follows: In the tensile tests, the tensile strength of the 10-mm- and 3-mm-diameter specimens are almost identical, independent of the specimen size, although the casting method and heat treatment conditions are different. The effect of specimen size on the creep rupture life is insignificant. In the low-cycle fatigue tests, stable fatigue data can be obtained even at high temperatures using the 3-mm-diameter specimen.

Fatigue Properties and Microstructure of SnAgCu Bi-Based Solder Joint

Abstract The reliability of solder joints plays a critical role in electronic assemblies. SnAgCu solder alloys with doped elements such as Bi and Sb is one of the candidates for high reliability applications. However, the mechanical and fatigue properties of the actual solder joint structure have not been studied for these new alloys. In this paper, a cyclic fatigue test was conducted on individual real solder joints of different alloys, including SnAgCu, SnCu–Bi, SnAgCu–Bi, and SnAgCu–BiSb. The fatigue property of those solder joints was analyzed based on the characteristic fatigue life and stress–strain, hysteresis, loops. The results show that solder joints with both Ag and Bi content have a better fatigue resistance than the solder joints with Ag or Bi content only. The results of SnAgCu and SnCu–Bi solder alloys show similar fatigue performance. Also, the fatigue performance of SnAgCu–Bi is close to SnAgCu–BiSb in the accelerated test. But the SnAgCu–Bi alloy is estimated to have a longer characteristic life under low-stress amplitude cycling. The microstructure analysis shows a bismuth-rich phase formed around the Ag3Sn precipitates. Adding bismuth in the solder alloy can significantly improve the fatigue properties through solid solution hardenings. On another hand, the plastic strain range and work dissipation were measured from the hysteresis loops for all tests. The Morrow Energy and the Coffin–Manson models were developed from the fitted data to predict the fatigue life as a function of work dissipation and plastic strain range.

Reliability modeling for aged SAC305 solder joints cycled in accelerated shear fatigue test

Solder joints reliability is a determinant factor for the life of the electronic assemblies. In this study, the reliability of actual SAC305 solder joints is investigated using accelerated shear fatigue tests. The effect of aging time and stress amplitude are studied. Five levels of aging time at 100 °C and three levels of stress amplitudes are applied in the experiments. Two-parameters Weibull distribution is used to describe the fatigue life of the solder joints at each test condition. A numerical model for the solder joints reliability is developed as a function of stress amplitude and aging time. The hysteresis loops at different aging times and stress amplitudes are demonstrated. The relationships between the inelastic work, plastic strain, and fatigue life are identified by exploiting the Coffin-Manson and Morrow Energy models. The results indicate a reduction in fatigue life and an increase in inelastic work and plastic strain when the cyclic stress amplitude is increased. Increasing the aging time has a negative impact on the fatigue life of the solder joint. The Morrow Energy model significantly outperforms the Coffin-Manson model in describing the fatigue behavior of the solder joints regardless of the aging conditions. Finally, a general reliability model as a function of the inelastic work per cycle is introduced.

Improving the Solder Joint Reliability of a Power Leadframe Package Using Thermomechanical Simulation

Leadframe-based packages are commonly used for semiconductor power devices. With these packages, heat dissipation is much better compared with laminate substrated-based packages. However, the solder joint reliability requirement under thermal cycling condition is also higher and this is what makes the development of a power package challenging. One of the usual requirements from customers is that there should be no solder joint failure up to 2,000 thermal cycles. This paper presents the thermomechanical simulation of a power leadframe package that was conducted to improve its solder joint reliability. Board level solder joint cycle life was predicted using finite element analysis and the result was validated with actual solder life result from board level reliability evaluation. Since available solder prediction equation was for the characteristic life (63.2% accumulative failure), using the normalized characteristic life was implemented for predicting the number of cycles to first failure of the solder joint connection and the approach showed good agreement with the actual result. Results also indicated that the choice of epoxy mold material and the type of PCB (printed circuit board) have a significant contribution to the solder joint reliability performance.

Estimation of Solder Ball Collapse Height in Semiconductor Packaging Using Theoretical and Solid Modeling Techniques

Semiconductor packages using solder balls as interconnect to the printed circuit board (PCB) are very popular especially in mobile products like smart phones. Recent requirement to make the package much thinner is very challenging. The solder ball collapse height after the solder ball is reflowed on the package substrate metal pad would need to be tightly controlled and aligned with the required height to meet the target overall package thickness. Another challenge is that the package has to be developed in a short period of time. In this study, theoretical and solid modeling techniques were developed to estimate the solder ball collapse height and compared with actual evaluation results. With these, the solder ball collapse height could be quickly estimated to make the package design and development faster avoiding several trial evaluations on different combinations of solder ball size, substrate pad solder mask opening diameter and solder mask thickness. Based on the estimation results, using these techniques showed good agreement with actual solder ball height measurements and have now been successfully used in coming up with final package designs in a fast and cost-effective way.

Realistic Creep Characterization for Sn3.0Ag0.5Cu Solder Joints in Flip Chip BGA Package

Creep tests were performed using a fixture composed of a spring, a micrometer and a heating pad to apply both heat and constant compressive load and elevated temperature to the actual solder joint. A microscopic digital image correlation technique was used to measure creep strains. A full-field deformation map of the cross-sectioned solder joint was generated as different constant loads were applied under different isothermal conditions on Sn3.0Ag0.5Cu flip chip ball grid array solder joints. Nonlinear regression was used to generate constitutive properties using the Garofalo hyperbolic sine model. The obtained constitutive properties were used to perform a finite element analysis simulation to compare the model with experimental results, which showed good agreement.

Recurrent Neural Network-Based Stencil Cleaning Cycle Predictive Modeling

This paper presents a real-time predictive approach to improve solder paste stencil printing cycle decision making process in surface mount assembly lines. Stencil cleaning is a critical process that influences the quality and efficiency of printing circuit board. Stencil cleaning operation depends on various process variables, such as printing speed, printing pressure, and aperture shape. The objective of this research is to help efficiently decide stencil printing cleaning cycle by applying data-driven predictive methods. To predict the printed circuit board quality level, a recurrent neural network (RNN) is applied to obtain the printing performance for the different cleaning aging. In the prediction model, not only the previous printing performance statuses are included, but also the printing settings are used to enhance the RNN learning. The model is tested using data collected from an actual solder paste stencil printing line. Based on the predicted printing performance level, the model can help automatically identify the possible cleaning cycle in practice. The results indicate that the proposed model architecture can predictively provide accurate solder paste printing process information to decision makers and increase the quality of the stencil printing process.

Effects of high temperature aging on the microstructural evolution and mechanical behavior of SAC305 solder joints using synchrotron X-ray microdiffraction and nanoindentation

Isothermal aging causes degradation in the mechanical behavior of lead-free solder materials used in microelectronic packaging, leading to significant reductions in product reliability. In this work, we have characterized the aging-induced changes that occur in the mechanical properties and creep behavior of actual solder joints. These changes occur due to microstructure evolution and residual strains/stresses in the solder material, and measurement of these evolutions are critical to developing a fundamental understanding of solder aging phenomena. The enhanced high-energy X-ray microdiffraction beamline present at the synchrotron at the Advanced Light Source at Lawrence Berkeley National Laboratory was employed to characterize the solder joints after various aging times at 125 °C. Aging-induced changes in the Sn grain orientation, Sn grain distribution, and residual strain distribution were measured. The observed microstructural evolution was then correlated with changes in the mechanical response of the solder joints measured using nanoindentation.

Degradation behaviors of micro ball grid array (μBGA) solder joints under the coupled effects of electromigration and thermal stress

Due to the complicated configuration of real micro ball grid array (μBGA) package, kinds of physical failure mechanisms occurs and mixed together in high current density applications. By considering electromigration and thermal stress-related effects, the degradation behaviors of actual μBGA solder joints were studied under the current density of 1 × 104 A cm−2 at 120 °C. Experimental results indicated that severe current crowding and open failure occurred at the narrow on-chip metal trace. At the current crowding region, the Ni under bump metallization was dissolved under the electromigration coupled with the thermal stress-induced stress migration, while the narrow Cu trace were consumed by the dominated electromigration. Meanwhile, the Sn and Pb atoms backfilled the vacancies formed by the migration of Cu and Ni. With the joule heat-induced temperature increasing, the backstress released, and those backfilled Sn and Pb atoms were migrated away, resulting in the final open failure. Additionally, due to polarity effect, the excessive growth of dendritic (Cu, Ni)6Sn5 compounds at anode side was also a potential source of failure in microelectronic devices.

Dominant effects of Sn orientation on serrated cathode dissolution and resulting failure in actual solder joints under electromigration

Excessive metal dissolution is one of the major electromigration-induced degradation mechanisms in interconnects, and it often produces a distinctive serrated cathode interface with most of the serrated teeth inclined toward a specific direction. In this study, actual flip-chip solder joints were systematically analyzed to understand this highly interesting morphology. It was unequivocally established that the Sn grain orientation is a dominant factor that controls the direction of the serrated teeth. When the c-axis of a Sn grain was nearly parallel to the electron flow direction, serrated dissolution occurred, with the serrated teeth inclined toward the c-axis. These observations were rationalized based on the diffusion anisotropy of Cu in Sn.

Intermetallic growth study on SnAgCu/Cu solder joint interface during thermal aging

The intermetallic compound (IMC) growth behavior at SnAgCu/Cu solder joint interface under different thermal aging conditions was investigated, in order to develop a framework for correlating IMC layer growth behavior between isothermal and thermomechanical cycling (TMC) effects. Based upon an analysis of displacements for actual flip-chip solder joint during temperature cycling, a special bimetallic loading frame with single joint-shear sample as well as TMC tests were designed and used to research the interfacial IMC growth behavior in SnAgCu/Cu solder joint, with a focus on the influence of stress–strain cycling on the growth kinetics. An equivalent model for IMC growth was derived to describe the interfacial Cu-Sn IMC growth behavior subjected to TMC aging as well as isothermal aging based on the proposed “equivalent aging time” and “effective aging time”. Isothermal aging, thermal cycling (TC) and TMC tests were conducted for parameter determination of the IMC growth model as well as the growth kinetic analysis. The SnAgCu/Cu solder joints were isothermally aged at 125, 150 and 175 °C, while the TC and TMC tests were performed within the temperature range from −40 to 125 °C. The statistical results of IMC layer thickness showed that the IMC growth for TMC was accelerated compared to that of isothermal aging based on the same “effective aging time”. The IMC growth model proposed here is fit for predicting the IMC layer thickness for SnAgCu/Cu solder joint after any isothermal aging time or thermomechanical cycles. In addition, the results of microstructure evolution observation of SnAgCu/Cu solder joint subjected to TMC revealed that the interfacial zone was the weak link of the solder joint, and the interfacial IMC growth had important influence on the thermomechanical fatigue fracture of the solder joint.

GS27 改良型ラップジョイント試験片によるはんだ接合部のせん断疲労試験方法

This paper presents the shear fatigue test method of the solder joint by the advanced lap joint specimen. The advanced lap joint specimen was developed for purpose of stiffness improvement and manufacturing of solder joint thickness equivalent to an actual solder joint of an electric board. The usefulness of the advanced lap joint specimen was checked by FEM analysis and the tensile test, and the test method was considered.

Effects of Package Level Structure and Material Properties on Solder Joint Reliability Under Impact Loading

In this paper, the effects of package level structures and material properties on solder joint reliability subjected to impact loading are investigated by the integrated experimental testing, failure analysis, and finite element modeling. Three different package structures: ball on I/O wafer level package (WLP), copper post WLP, and chip-scale (CS) ball grid array (BGA) package, are studied. Experimental testing based on JESD22-B111 is conducted to obtain the components' failure mode, rate, location, and the corresponding board strains and accelerations. Finite element models are developed and validated against the experimental results. For a CS BGA package, the compliance of the plastic substrate/mold compound provides a “stress buffer mechanism” at corner joints in BGA to relieve stresses. For a copper post (or pillar) WLP, wafer level epoxy, which encapsulates copper pillars, serves as a compliant layer for solder joint stress reduction under dynamic loading. Comprehensive data from simulation and experimental results show that package structure and material properties play a significant role on the dynamic responses of solder joints. The actual solder joint reliability performance of a CS BGA or WLP package depends on the resultant effects of package structure, material properties, package body size, and the component locations.

Liquid-Phase Separation in the Interdendritic Region After Growth of Primary β-Sn in Undercooled Sn-2.8Ag-0.3Cu Melt

An unusual microstructure consisting of both Sn-Ag3Sn and Sn-Cu6Sn5 binary eutectic structures is observed in actual solder balls. In this study, the solidification process of the Sn-Ag3Sn binary eutectic structure after the growth of primary b-Sn in an undercooled Sn-2.8Ag-0.3Cu alloy was investigated by using thermal analysis and interruption tests to understand the formation of the unusual microstructure. First, fine Ag-enriched liquid zones formed around b-Sn after the growth of primary b-Sn. The Ag-enriched zones then gradually enlarged with the accumulation of Ag from the remnant liquid with a decrease in temperature. This indicated that the liquid-phase separation occurred in the remnant liquid after the nucleation of b-Sn. Eventually, when the temperature of the specimen decreased to approximately the binary eutectic temperature, eutectic Ag3Sn nucleated in the Ag-enriched zones. From interruption tests, we determined the precursor of the Sn-Ag3Sn binary eutectic structure before the beginning of Sn-Ag3Sn binary eutectic solidification. This finding corresponds to the precursor of the Sn-Cu6Sn5 binary eutectic structure observed in the Sn-1.0Ag-0.5Cu alloy.

The Modeling and Prediction Study of BLP Device Solder Joint Three-Dimensional Shape

Solder joint shape refers to geometry that molten solder can be achieved with spreading and wetting along the metal surface in the junction between components solder pins and printed circuit board (PCB) pad, to the metal surface contact angle and solder fillet shape[1]. BLP (Bottom Leaded Plastic) [2] which is not a type of thin out-line surface mount device(SMD) without the side lead is widely used in manufacturing of new generation memory such as SDRAM \ RDRAM \ DDR. For this non-lead SMD, the shape and reliability of solder joint is the focus of the study. In this paper, we select the C-BLP of 28-pin device as the research object, complete the forming model of solder joint shape based on minimum energy principle and the solder joint shape theory by using Surface Evolver software, and analyze two critical process parameters (solder volume and solder pad width ) impacting on the solder joint shape. The study results show that the solder volume and pad width have a significant impact on the three-dimensional shape of BLP solder joint. The law of which is that: in the case of the other fixed parameters, as the increase of the solder volume, solder joint surface gradually changes from the relatively flat to the convex, solder joint gradually changes from the completely soldered to the partly soldered in the pad connections, and solder joint heights are gradually decreased; in the case of the other fixed parameters, as the increase of the pad in width, the solder joint surface gradually changes from the convex to the flat, solder joint gradually changes from the completely soldered to the partly soldered in the pad connections, and solder joint heights are gradually decreased. The actual solder joint shape designs of BLP devices can be instructed based on predicted results.

Application of Synchrotron Radiation X-Ray Microtomography to Nondestructive Evaluation of Thermal Fatigue Process in Flip Chip Interconnects

New nondestructive inspection methods with high spatial resolution are expected to support the evaluation and enhancement of the reliability of microjoints on printed circuit boards. An X-ray microtomography system, the SP-μCT has been developed at the Super Photon ring-8 GeV (SPring-8), the largest synchrotron radiation facility in Japan. In this work, the SP-μCT was first applied to the nondestructive evaluation of thermal fatigue phenomena, namely microstructure evolution (i.e., phase growth) and microcrack propagation, appearing in actual solder microbumps of flip chip interconnects due to thermal cyclic loading. In addition, a refraction-contrast imaging technique was simultaneously applied to visualize the fatigue cracks with an actual opening of less than 100 nm. The observed specimen has a flip chip structure joined by Sn-37wt%Pb eutectic solder bumps 150 μm in diameter. Consequently, the process of phase growth and crack propagation was determined via observation of consecutive computed tomography (CT) images obtained in the same plane of the same specimen. As the thermal cycle proceeded, remarkable phase growth was clearly observed, followed by the appearance of fatigue cracks in the corners of the interfaces between the solder bump and Cu pad. Moreover, the CT images also enabled us to evaluate the fatigue lifetime of the bumps, as follows. The lifetime to fatigue crack initiation was estimated by quantifying the increase in the phase growth. The crack propagation lifetime to failure was then determined by measuring the average crack propagation rate. Such results have not been obtainable at all by X-ray CT systems for industrial use and demonstrate the possibility of nondestructive inspection by a synchrotron radiation X-ray microtomography system.

Characterization of elasto-plastic behavior of actual SAC solder joints for drop test modeling

This paper focuses on the characterization of the elasto-plastic properties of actual Sn–1.0 wt.%Ag–0.5 wt.%Cu (SAC105), Sn–3.0 wt.%Ag–0.5 wt.%Cu (SAC305) and Sn–4.0 wt.%Ag–0.5 wt.%Cu (SAC405) solder joints for drop test modeling. Several actual ChipArray® BGA (CABGA) packages were cross-sectioned, polished and used as the test vehicles. The drop tests were performed with various impact amplitudes using a specially designed mini drop table along with a conventional drop table to generate plastic deformations in the solder joints. The plastic deformations in the solder joints were measured after each drop using microscope imaging in conjunction with the digital image correlation (DIC) technique. The coefficients of the Ramberg–Osgood elasto-plastic model for the solder alloys were extracted from the experimental results using a finite element method (FEM) modeling-based iteration process. An application and validation of the developed model were carried out using pendulum tests.

A study of SnAgCu solder paste transfer efficiency and effects of optimal reflow profile on solder deposits

The reliability of solder joints in electronic products are greatly enhanced by good stencil printing and quality reflow soldering. The stencil printing process is widely used in Surface Mount Technology (SMT) to deposit solder paste on the substrate and is a critical step in SMT assembly as it has been widely claimed that up to 50% of the defects found in the assembly of printed circuit boards (PCBs) are attributed to stencil printing. Solder paste release from stencil during printing is a key factor which affects the quality of solder prints. Thus, the efficient transfer of paste from stencil through aperture to pad is a fundamental concern in SMT production process. The recent trends on further miniaturisation of electronic products have introduced more process challenges in the SMT assembly. The assembly of surface mount packages such as flip chips, chip scale packages and fine pitch ball grid arrays are challenging the current stencil printing and reflow profiling capabilities. The effective mounting of these miniaturised packages requires good transfer efficiency of solder paste through small stencil aperture. As further electronic product miniaturisation culminates in progressive use of decreasing stencil aperture sizes, the in-depth understanding of the dependency of transfer efficiency of solder paste on diminishing stencil aperture areas has become vital to improving solder joint reliability. This study investigates the transfer efficiency of type 3, 96.5Sn3.0Ag0.5Cu solder paste through linearly decreasing rectangular stencil aperture sizes typically used in PCB assembly. In addition, the research examines the effects of optimal reflow profile (ORP) on the solder deposit volumes. The results from the study show a power law relationship between actual solder deposit volume (SDV) and aperture cavity. The observed effect of ORP on actual SDVs was a 46% volume change which was fairly constant across the pad geometries. Our study shows that the transfer efficiency of solder paste decreases with decreasing stencil aperture size. A novel mathematical model η T = 100 e - | V A - V C | ∗ | V A - V C + D m + e 10 - 5 D m - 1 | for V C ⩾ 0 is proposed for computing the transfer efficiency of solder paste through rectangular stencil aperture sizes.