An Investigation of Reliability of High Density Electronic Package-to-Board Interconnections from the Perspective of Solder Joint Metallurgy

The integration and miniaturization trend of the electronic packaging leads to much finer pitch of the device and package lead terminations. Several reliability concerns and issues that were previously not encountered are now surfacing. The objective of this thesis work is to investigate the reliability of the package-to-board interconnection from the perspective of solder joint metallurgy. It was carried out with several advanced packages such as CSP, WLCSP and leadless ceramic packages on organic laminate PWBs using tin-silver-copper based interconnection materials. The assemblies were subjected to several loading conditions and levels such as thermal, mechanical, and environmental stresses. As expected, the board level reliability (BLR) of electronic assemblies strongly depended on microstructure and morphology of the solder joints. Dispersion strengthening effect of the intermetallic compounds (IMCs), coarsening of the IMC particles, strain rate hardening, solder fatigue, and recrystallization of Sn grains in the highly stressed areas were observed. These were found to directly impact Pb-free solder joint reliability. Appropriate thermal aging can improve joint reliability up to 50% due to coarsening of the IMC particles. In addition, other factors such as dissolution of metals, interfacial reactions, IMC spalling, and cross interaction of surface materials on the two sides of the joints were also observed and discussed. The effects can be expressed as a series of interactive relationships: materials (pad surface materials and solder alloy composition) and/or soldering process lead to microstructure change in bulk solder and/or at interface, which in turn leads to joint reliability variation

Study of photovoltaic (PV) module interconnections failure analysis and reliability

A thesis submitted in partial fulfilment of the requirements of the University of Wolverhampton for the award of Doctor of Philosophy.Solar Energy is one of the most widely used renewable energy sources, with the solar Photovoltaic (PV) module technologies deployed as one of the primary renewable energy sources to replace fossil fuels. However, the R&D challenge for improving the performance and reliability of PV modules has become an urgent and critical agenda for the energy generation industry sector. The interconnection between the solar PV cells is a very important part of the PV module assembly, and its failure can adversely affect the performance and reliability of the PV module. The interconnection failure has been mostly linked to the crack initiation and propagation in the solder joints used to connect the ribbon interconnection to the cell. This research focuses on the study of the thermal failure of PV module solder joint to determine the optimum ribbon interconnection designs that will give improved thermo-mechanical reliability. It develops a virtual reliability qualification process for the assessment of the life expectancy of PV module interconnections. The FEM simulations in ABAQUS 2019 software are implemented to investigate failure of the solder joints in different ribbon interconnection designs under anticipated life cycle loading conditions and high temperature lamination process. For the first time, the extended finite element method (XFEM) technique is used to determine the crack initiation temperature, crack location, direction and growth rate in solder joint of PV module interconnection under lamination process. Furthermore, the research used the Developed Morrow Energy Density lifetime model to determine the number of cycles to creep-fatigue failure, and then it defined a new generic exponent factor using the Coffin–Manson–Arrhenius model to estimate the lifetime for the designs under different thermal cycling conditions. The research also combines the numerical results of XFEM and creep-fatigue investigation to determine the failure lifetime of PV Module interconnection designs. The results show that the Multi-Busbar interconnection design improves solder joint creep-fatigue life (up to 15%) and consequently provides higher thermo-mechanical reliability for the solar PV modules compared to other studied designs (Conventional and the Light Capturing Ribbon interconnections). The results of this PV module interconnections study can be used for evaluating potential design changes and to facilitate design for reliability validation of different configurations for improving the long-term PV module system reliability.Faculty of Science and Engineering, University of Wolverhampton

The effect of static and dynamic aging on fatigue behavior of Sn3.0Ag0.5Cu solder alloy

In microelectronic assemblies, solder joints serve as interconnection between different packaging levels and are an important cause for the failure of microelectronic products. Sn-Ag-Cu solder alloys became important after lead-based solder alloys were caused to be discarded by regulations in European Union and Japan. However, the constitutive behavior of Sn-Ag-Cu alloys is not as well understood as lead-based solder alloys, and many studies confirm the aging of these alloys with time. The aging of Sn-Ag-Cu alloys and its effect on mechanical behavior challenges the reliability prediction of microelectronic assemblies. In this study, the effect of pretest isothermal aging and in-test aging on the fatigue behavior of Sn3.0Ag0.5Cu alloy are examined using the microstructurally adaptive creep model (MACM) and the maximum entropy fracture model (MEFM). In this thesis, first, the development of microstructurally adaptive creep model is reviewed. Compared to traditional constitutive models, this model considers the effect of thermal history. Two microstructural parameters, the average Ag3Sn particles size and the average primary-Sn cell size are identified as critical parameters and incorporated into a modified Dorn creep form, which can describe both climb-controlled and glide-controlled dislocation motions. Next, the maximum entropy fracture model is discussed and compared to traditional fatigue fracture model. The MEFM utilizes the damage accumulation parameter, which connects the accumulated damage to the accumulated inelastic dissipation. This parameter is independent of sample geometry, test temperature and strain rate. Later, using MACM and MEFM, the extraction of the damage accumulation parameters is presented. The creep models for different aging conditions are constructed first based on microstructural characterization. The damage accumulation parameters of 25 celsius and 100 celsius tests are fit using MEFM. The parameters are presumed different for the two conditions because of the different aging states of the material. The concepts of static aging and dynamic aging are introduced and utilized to describe pretest aging and in-test aging. In 25 celsius test, with longer static aging, the damage accumulation parameter is smaller, indicating a faster fatigue damage accumulation. Through the relationship between damage accumulation parameter and the average primary-Sn cell size, the influence of microstructural evolution introduced by static aging on fatigue behavior is confirmed. In 100 celsius tests, the effect of dynamic aging is captured by the change of damage accumulation parameter in experiments. Comparing the damage accumulation parameters from 25 celsius and 100 celsius tests, during test, further aging of Sn3.0Ag0.5Cu microstructure occurs, degrading fatigue behavior until microstructural evolution is completed. Finally, the thesis is summarized and future work to better characterize the relationship between fatigue behavior and microstructure is put forward. The proposed work includes building a dynamic aging model and microstructural evolution model

Time integration damage model for Sn3.5Ag solder interconnect in power electronic module

In this study, existing damage evolution models in the literature for solder layer in microelectronics have been reviewed. A two dimensional approximate semi-analytic time integration damage indicator model for Sn3.5Ag material solder interconnect in power electronic module has been proposed. The proposed time dependent damage model is dependent on the inelastic strain, the accumulated damage at previous time step and the temperature. The strains were approximated semi-analytically. A numerical modelling methodology combined with the data from public domain for crack initiation and crack propagation of Sn3.5Ag solder layer has been adopted to extract the parameter values of the proposed damage model. The proposed model has advantages over fatigue lifetime models as it instantaneously predicts the damage over time for any loading history. The damage model was compared with Ansys FEA tool based damage prediction using Coffin Manson and Paris law fatigue models. The predicted damage value by the model is slightly higher than those models. Furthermore, this damage model does not need a time consuming numerical simulation evaluating the damage model variables, which is an advantag

Effective Solder for Improved Thermo-Mechanical Reliability of Solder Joints in a Ball Grid Array (BGA) Soldered on Printed Circuit Board (PCB)

Ball grid array (BGA) packages have increasing applications in mobile phones, disk drives, LC displays and automotive engine controllers. However, the thermo-mechanical reliability of the BGA solder joints challenges the device functionality amidst component and system miniaturisation as well as wider adoption of lead-free solders. This investigation determines the effective BGA solders for improved thermo-mechanical reliability of the devices. It utilised a conducted study on creep response of a lead-based eutectic Sn63Pb37 and four lead-free Tin-Silver-Copper (SnAgCu) [SAC305, SAC387, SAC396 and SAC405] solders subjected to thermal cycling loadings and isothermal ageing. The solders form the joints between the BGAs and printed circuit boards (PCBs). ANSYS R19.0 package is used to simulate isothermal ageing of some of the assemblies at -40℃, 25℃, 75℃ and 150℃ temperatures for 45 days and model the thermal cycling history of the other assemblies from 22℃ ambient temperature for six cycles. The response of the solders is simulated using Garofalo-Arrhenius creep model. Under thermal ageing, SAC396 solder joints demonstrate possession of least strain energy density, deformation and von-Mises stress in comparison to the other solders. Under thermal cycle loading conditions, SAC405 acquired the lowest amount of the damage parameters in comparison. Lead-free SAC405 and SAC387 joints accumulated the lowest and highest energy dissipation per cycle, respectively. It is concluded that SAC405 and SAC396 are the most effective solders for BGA in devices experiencing isothermal ageing and temperature cycling during operation, respectively. They are proposed as the suitable replacement of eutectic Sn63Pb37 solder for the various conditions.University of Derb

Thermo-mechanical reliability studies of lead-free solder interconnects

N/ASolder interconnections, also known as solder joints, are the weakest link in electronics packaging. Reliability of these miniature joints is of utmost interest - especially in safety-critical applications in the automotive, medical, aerospace, power grid and oil and drilling sectors. Studies have shown that these joints' critical thermal and mechanical loading culminate in accelerated creep, fatigue, and a combination of these joints' induced failures. The ball grid array (BGA) components being an integral part of many electronic modules functioning in mission-critical systems. This study investigates the response of solder joints in BGA to crucial reliability influencing parameters derived from creep, visco-plastic and fatigue damage of the joints. These are the plastic strain, shear strain, plastic shear strain, creep energy density, strain energy density, deformation, equivalent (Von-Mises) stress etc. The parameters' obtained magnitudes are inputted into established life prediction models – Coffin-Manson, Engelmaier, Solomon (Low cycle fatigue) and Syed (Accumulated creep energy density) – to determine several BGA assemblies' fatigue lives. The joints are subjected to thermal, mechanical and random vibration loadings. The finite element analysis (FEA) is employed in a commercial software package to model and simulate the responses of the solder joints of the representative assemblies' finite element models. As the magnitude and rate of degradation of solder joints in the BGA significantly depend on the composition of the solder alloys used to assembly the BGA on the printed circuit board, this research studies the response of various mainstream lead-free Sn-Ag-Cu (SAC) solders (SAC305, SAC387, SAC396 and SAC405) and benchmarked those with lead-based eutectic solder (Sn63Pb37). In the creep response study, the effects of thermal ageing and temperature cycling on these solder alloys' behaviours are explored. The results show superior creep properties for SAC405 and SAC396 lead-free solder alloys. The lead-free SAC405 solder joint is the most effective solder under thermal cycling condition, and the SAC396 solder joint is the most effective solder under isothermal ageing operation. The finding shows that SAC405 and SAC396 solders accumulated the minimum magnitudes of stress, strain rate, deformation rate and strain energy density than any other solder considered in this study. The hysteresis loops show that lead-free SAC405 has the lowest dissipated energy per cycle. Thus the highest fatigue life, followed by eutectic lead-based Sn63Pb37 solder. The solder with the highest dissipated energy per cycle was lead-free SAC305, SAC387 and SAC396 solder alloys. In the thermal fatigue life prediction research, four different lead-free (SAC305, SAC387, SAC396 and SAC405) and one eutectic lead-based (Sn63Pb37) solder alloys are defined against their thermal fatigue lives (TFLs) to predict their mean-time-to-failure for preventive maintenance advice. Five finite elements (FE) models of the assemblies of the BGAs with the different solder alloy compositions and properties are created with SolidWorks. The models are subjected to standard IEC 60749-25 temperature cycling in ANSYS 19.0 mechanical package environment. SAC405 joints have the highest predicted TFL of circa 13.2 years, while SAC387 joints have the least life of circa 1.4 years. The predicted lives are inversely proportional to the magnitude of the areas of stress-strain hysteresis loops of the solder joints. The prediction models are significantly consistent in predicted magnitudes across the solder joints irrespective of the damage parameters used. Several failure modes drive solder joints and damage mechanics from the research and understand an essential variation in the models' predicted values. This investigation presents a method of managing preventive maintenance time of BGA electronic components in mission-critical systems. It recommends developing a novel life prediction model based on a combination of the damage parameters for enhanced prediction. The FEA random vibration simulation test results showed that different solder alloys have a comparable performance during random vibration testing. The fatigue life result shows that SAC405 and SAC396 have the highest fatigue lives before being prone to failure. As a result of the FEA simulation outcomes with the application of Coffin-Manson's empirical formula, the author can predict the fatigue life of solder joint alloys to a higher degree of accuracy of average ~93% in an actual service environment such as the one experienced under-the-hood of an automobile and aerospace. Therefore, it is concluded that the combination of FEA simulation and empirical formulas employed in this study could be used in the computation and prediction of the fatigue life of solder joint alloys when subjected to random vibration. Based on the thermal and mechanical responses of lead-free SAC405 and SAC396 solder alloys, they are recommended as a suitable replacement of lead-based eutectic Sn63Pb37 solder alloy for improved device thermo-mechanical operations when subjected to random vibration (non-deterministic vibration). The FEA simulation studies' outcomes are validated using experimental and analytical-based reviews in published and peer-reviewed literature.N/

Development of a Rapid Fatigue Life Testing Method for Reliability Assessment of Flip-Chip Solder Interconnects

The underlying physics of failure are critical in assessing the long term reliability of power packages in their intended field applications, yet traditional reliability determination methods are largely inadequate when considering thermomechanical failures. With current reliability determination methods, long test durations, high costs, and a conglomerate of concurrent reliability degrading threat factors make effective understanding of device reliability difficult and expensive. In this work, an alternative reliability testing apparatus and associated protocol was developed to address these concerns; targeting rapid testing times with minimal cost while preserving fatigue life prediction accuracy. Two test stands were fabricated to evaluate device reliability at high frequency (60 cycles/minute) with the first being a single-directional unit capable of exerting large forces (up to 20 N) on solder interconnects in one direction. The second test stand was developed to allow for bi-directional application of stress and the integration of an oven to enable testing at elevated steady-state temperatures. Given the high frequency of testing, elevated temperatures are used to emulate the effects of creep on solder fatigue lifetime. Utilizing the mechanical force of springs to apply shear loads to solder interconnects within the devices, the reliability of a given device to withstand repeated cycling was studied using resistance monitoring techniques to detect the number of cycles-to-failure (CTF). Resistance monitoring was performed using specially designed and fabricated, device analogous test vehicles assembled with the ability to monitor circuit resistance in situ. When a resistance rise of 30 % was recorded, the device was said to have failed. A mathematical method for quantifying the plastic work density (amount of damage) sustained by the solder interconnects prior to failure was developed relying on the relationship between Hooke’s Law for springs and damage deflection to accurately assess the mechanical strength of tested devices

Experimental strain energy density dissipated in SAC305 solder joints during different thermal cycling conditions using strain gages measurements

International audienceDespite being widely investigated for the last two decades, solder joints thermomechanical durability assessment remains a major concern for industries wishing to switch from lead-based (SnPb) to lead-free electronics. Amongst the variety of lead-free solder compositions, 96.5Sn-3.0Ag-0.5Cu (SAC305) solder alloy has become the preferred substitute to classic SnPb solders. However, unlike SnPb assemblies, the return on experience is limited and the microstructure is very different for SAC305 solder joints. The use of SAC305 solder paste requires to understand the mechanical and fatigue behaviors of the soldered interconnects. This paper presents the experimentation based on strain gages measurements, allowing the determination of the shear stress-strain response of SAC305 solder joints subjected to different thermal cycling conditions. The area of the experimental shear strain-stress hysteresis loops gives the values of the strain energy density corresponding to each thermomechanical loading. The finite element modeling of the test assembly showed a good correlation between experimental and numerical strain energy densities. The experimental shear strain-stress curves also provide the necessary data to derive SAC305 solder joints constitutive laws

Creep response of various solders used in soldering ball grid array (BGA) on printed circuit board (PCB)

In electronics packaging, solder joints play a critical role by providing electrical, thermal and mechanical connections between the package and the printed circuit board (PCB). As the joint is both miniature and brittle, it is the weakest part of the assembly and thus susceptible to untimely damage. This paper presents the creep response of solder joints in a ball grid array (BGA) soldered on a PCB subjected to isothermal ageing in one experiment and temperature cycling in another test. The ageing is simulated in an ANSYS package environment at -40, 25, 75 and 150℃ temperatures applied for 45 days. The thermal cycling profile started from 22℃ and cycled between -40℃ and 150℃ with 15 minutes dwell time at the lowest and highest temperatures. The solders used in the investigation are lead-based eutectic solder alloy and lead-free SAC305, SAC387, and SAC396 solders. The research seeks to qualify these solders against strain and strain energy response for improved reliability in operation. The results show that the lead-free SAC387 accumulated the maximum strain and thus strain energy while the lead-based eutectic solder is found to accrue the least amount of the quantities. Further results show the distribution of damage in the BGA solder bump. Based on the results, it is proposed that lead-free SAC396 is the best replacement of the lead-based eutectic solder in the drive for the achievement of comparable thermo-mechanical reliability of assembled BGA on PCB.The authors thankfully acknowledge the funding contributions of the University of Derb