Dynamic fracture process of solder/intermetallic interface in lead-free solder interconnects using cohesive zone model

Solder joint reliability (SJR) is an important requirement in electronics packaging. Most of the failures in a package are found in solder joints and interconnections. Brittle solder/intermetallic (IMC) interface fracture is the dominant failure mode in cases of impact loading and fast mechanical fatigue loading. In this study, the response of a single solder specimen subjected to cyclic shear deformation and a typical ball grid array (BGA) package undergoing board-level drop test is investigated. The finite element (FE) analysis of the single reflowed solder specimen and the BGA package is employed to understand the mechanics of the solder joints and the brittle solder/ IMC fracture process. Inelastic behavior of the solder joints is described using unified inelastic strain model (Anand model) with optimized model parameters. The brittle solder/IMC interface fracture is demonstrated using cohesive zone model (CZM). The accuracy of interface fracture description depends on the CZM model prescribed in the analysis. The CZM model is modified further to ensure better predictive capability especially in cyclic loading. FE results for single solder specimen under shear fatigue test simulation shows that the CZM parameters degraded as the number of cycles is increased. Rapid damage progression occurs at the beginning of cycle and propagated slowly for subsequent cycles. For a boardlevel drop test simulation, the critical solder joint is located the farthest away from the center of the board. The highest stress and inelastic strain are confined to a small edge region at solder/IMC interfaces. Damage initiated from the outer peripheral solder and propagated into the inner peripheral solder joint

The durability of solder joints under thermo-mechanical loading; application to Sn-37Pb and Sn-3.8Ag-0.7Cu lead-free replacement alloy

Solder joints in electronic packages provide mechanical, electrical and thermal connections. Hence, their reliability is also a major concern to the electronic packaging industry. Ball Grid Arrays (BGAs) are a very common type of surface mount technology for electronic packaging. This work primarily addresses the thermo-mechanical durability of BGAs and is applied to the exemplar alloys; traditional leaded solder and a popular lead-free solder. Isothermal mechanical fatigue tests were carried out on 4-ball test specimens of the lead-free (Sn-3.8Ag-0.7Cu) and leaded (Sn-37Pb) solder under load control at room temperature, 35°C and 75°C. As well as this, a set of combined thermal and mechanical cycling tests were carried out, again under load control with the thermal cycles either at a different frequency from the mechanical cycles (not-in-phase) or at the same frequency (both in phase and out-of-phase). The microstructural evaluation of both alloys was investigated by carrying out a series of simulated ageing tests, coupled with detailed metallurgical analysis and hardness testing. The results were treated to produce stress-life, cyclic behaviour and creep curves for each of the test conditions. Careful calibration allowed the effects of substrate and grips to be accounted for and so a set of strain-life curves to be produced. These results were compared with other results from the literature taking into account the observations on microstructure made in the ageing tests. It is generally concluded that the TMF performance is better for the Sn-Ag-Cu alloy than for the Sn-Pb alloy, when expressed as stress-life curves. There is also a significant effect on temperature and phase for each of the alloys, the Sn-Ag-Cu being less susceptible to these effects. When expressed as strain life, the effects of temperature, phase and alloy type are much diminished. Many of these conclusions coincided with only parts of the literature and reasons for the remaining differences are advanced

Effect of Dynamic Flexural Loading on the Durability and Failure Site of Solder Interconnects in Printed Wiring Assemblies

This dissertation investigates the durability of solder interconnects of area array packages mounted on Printed Wiring Assemblies (PWAs) subjected to dynamic flexural loads, using a combination of testing, empirical curve fitting and mechanistic modeling. Dynamic 4-point bend tests are conducted on a drop tower and with an impact pendulum. Failure data is collected and an empirical rate-dependent durability model, based on mechanistic considerations, is developed to estimate the fatigue failure envelopes of the solder, as a function of solder strain and strain-rate. The solder plastic strain histories are obtained from the PWA flexural strain and strain rate, using transfer functions developed from 3D transient Finite Element Analysis (FEA) with rate-dependent solder material properties. The test data also shows the existence of multiple competing failure sites: solder, copper trace, PWB under solder pads, and layers of intermetallic compound (IMC) between the solder and solder pads. The failures in the IMC layers are found to be either in the bulk of the IMC layers or at the interface between different species of IMC layers. The dominant failure site is found to be strongly dependent on the loading conditions. The empirical model is demonstrated for solder failures as well as Cu trace failures, and the transition between their competing failure envelopes is identified. This dissertation then focuses in detail on two of these competing failure sites: (i) the solder and (ii) the interface between two IMC layers. A strain-range fatigue damage model, based on strain-rate hardening and exhaustion of ductility, is used to quantify the durability and estimate the fatigue constants of the solder for high strain rates of loading. Interfacial fracture mechanics is used to estimate the damage accumulation rates at the IMC interface. The IMC failure model and the solder failure model provide a mechanistic perspective on the failure site transitions. Durability metrics, based on the mechanics of these two failure mechanisms, are used to quantify the competing damage accumulation rates at the two failure sites for a given loading condition. The results not only identify which failure site dominates but also provide estimate of the durability of the solder interconnect. The test data shows good correlation with the model predictions. The test vehicles used in this study consist of PWAs with Sn37Pb solder interconnects. But the proposed test methodologies and mechanistic models are generic enough to be easily extended to other emerging lead free solder materials. Wherever possible, suggestions are provided for the development of test techniques or phenomenological models which can be used for engineering applications. A methodology is proposed in the appendix to implement the findings of this thesis in real-world applications. Damage in the solder interconnect is quantified in terms of generic empirical metrics, PWA flexural strain and strain rate. It is shown that the proposed metrics (PWA strain and strain rate) can quantify the durability of the solder interconnect, irrespective of the loading orientation or the PWA boundary conditions

Dynamic Mechanical and Failure Properties of Solder Joints

Ph.DDOCTOR OF PHILOSOPH

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/

Numerical modelling of additive manufacturing process for stainless steel tension testing samples

Nowadays additive manufacturing (AM) technologies including 3D printing grow rapidly and they are expected to replace conventional subtractive manufacturing technologies to some extents. During a selective laser melting (SLM) process as one of popular AM technologies for metals, large amount of heats is required to melt metal powders, and this leads to distortions and/or shrinkages of additively manufactured parts. It is useful to predict the 3D printed parts to control unwanted distortions and shrinkages before their 3D printing. This study develops a two-phase numerical modelling and simulation process of AM process for 17-4PH stainless steel and it considers the importance of post-processing and the need for calibration to achieve a high-quality printing at the end. By using this proposed AM modelling and simulation process, optimal process parameters, material properties, and topology can be obtained to ensure a part 3D printed successfully

NEW AND IMPROVED TECHNIQUES FOR CHARACTERIZING BRITTLE ROCK RESPONSE

To improve the production from shale, stimulation technique such as hydraulic fracturing with proppants is essential. To maximize the effectiveness of hydraulic fracturing, brittle intervals with the minimum creep deformation are preferable as the target zone. A literature review on rock brittleness evaluation is first conducted to analyze the pros and cons of each assessment method. To capture the Class II behavior of brittle shale, damage-controlled compression test is improved using inelastic strain as the control parameter. Indentation test as a simple and fast brittleness evaluation method is used to indent on four lithologies, the indentation displacement (depth) is considered as the brittleness index; Hydraulic fractures are thought to initiate from tensile fractures, however, currently no brittleness indices are derived from tensile failure. This mismatch of failure mechanism renders existing brittleness indices not representative for application in hydraulic fracturing. To mitigate the mismatch, a new brittleness evaluation method is proposed, that is damage-controlled Brazilian test, brittleness of different lithologies have been measured and compared, Brittle-Ductile Transition and strength envelope are obtained, and fracture angle inclination is observed in different confining pressure. To assess the contribution of creep to closure rate and conductivity loss of hydraulic fractures in gas shale, the viscoelastic characteristics of shale have been investigated. A series of creep tests were conducted on reservoir shale core samples. First, a few uniaxial creep tests were performed on several selected samples, and then multistage triaxial creep tests were carried out at room temperature. Samples used in the tests come from three different gas shale reservoirs. Creep strain can be described by a power-law function of time. The clay and carbonate contents of these shale samples vary noticeably. Results indicate that rocks with more quartz and less clay have higher elastic moduli. Pseudo-steady creep rate increases linearly with deviatoric stress and higher confining pressures increase the amount of creep strain under the same deviatoric stress. Creep tests at elevated temperatures have also been carried out to show that temperature increase creep rate. The key findings and contribution of this dissertation include: Indentation test and damage controlled Brazilian test are effective and fast methods to evaluate the brittleness of rocks, the Crack Opening Displacement (COD) can be used as a brittleness index (reverse relation). The indentation depth (displacement) is the brittleness index of indentation test. An alternative damage (inelastic strain) controlled compression test is developed based on the original method (linear combination of load and displacement). A brittleness formulation ε_p/(ε_r l) is derived based on material characteristic length. The Brittle-Ductile Transition of tensile failure is first obtained from confined Brazilian test. Fracture angles in confined Brazilian test progressively increase with confining pressure. Brazilian discs no longer fail in tensile fracturing under high confining pressure when the minimum principal stress in the middle of the disc is compressive; this provides convincing laboratory evidence for the existence of hybrid fractures that constitute transition from extension to shear fractures. Clay content and TOC in shale determine their brittleness and creep properties. For the joint test, multistage compression tests, multistage shear tests and joint stiffness test have been combined to maximize the dataset from one single core plug