ISOTHERMAL MECHANICAL AND THERMO-MECHANICAL DURABILITY CHARACTERIZATION OF SELECTED PB-FREE SOLDERS

Due to the hazards of Pb in the environment and its effect on humans and marketing competition from Japanese electronics manufacturers, the conversion to Pb-free solders in the electronics industry appears imminent. As major mechanical, thermal, and electrical interconnects between the component and the PWB, solder joints are crucial for the reliability of the most electronic packages. There is an urgent need for constitutive properties, mechanical durability and thermo-mechanical durability of Pb-free solders. A partitioned constitutive model consisting of elastic, plastic, primary creep and secondary creep models is obtained for the Sn3.9Ag0.6Cu solder and the baseline Sn37Pb solder from comprehensive monotonic and creep tests conducted on Thermo-Mechanical-Microscale (TMM) setup. The comparison between two solders shows that Sn3.9Ag0.6Cu has much better creep resistance than Sn37Pb at the low and medium stresses. The isothermal mechanical durability of three NEMI recommended Pb-free solders, Sn3.9Ag0.6Cu, Sn3.5Ag, Sn0.7Cu, is tested on the TMM setup under low creep and high creep test conditions. The damage propagation rate is also analyzed from the test data. The generic Energy-Partitioning (E-P) durability model is obtained for three Pb-free solders by using the incremental analytic model developed for TMM tests. The scatter of the test results from the prediction by these E-P durability model constants is small. The thermo-mechanical durability of the Pb-free Sn3.8Ag0.7Cu solder is investigated by a systematic approach combining comprehensive thermal cycling tests and finite element modeling. The effects of mixed solder systems, device types, and underfill are addressed in the tests. Thermal cycling results show that Sn3.8Ag0.7Cu marginally outperforms SnPb for four different components under the studied test condition. The extensive detailed three-dimensional viscoplastic FE stress and damage analysis is conducted for five different thermal cycling tests of both Sn3.8Ag0.7Cu and Sn37Pb solders. Power law thermo-mechanical durability models of both Sn3.8Ag0.7Cu and Sn3Pb are obtained from thermal cycling test data and stress and damage analysis. The energy-partitioning durability models of two solders are also obtained. It is found that the slopes of the plastic and creep curves in the E-P damage model of Pb-free solders for thermal cycling are steeper than those for mechanical cycling and those of Sn37Pb solders

Modelling to predict the reliability of solder joints

The work in this thesis investigates modelling methods to predict the reliability of solder joints under thermo-mechanical cycling. A literature review is presented covering analytical methods, creep laws and fatigue laws, and advanced damage mechanics methods. The use of FEA (Finite Element Analysis) to model creep along with a fatigue law to predict lifetime appears to be the most widely used and validated technique at present. The FEA discretisation of elasticity problems is derived using the principle of minimum potential energy and implemented in the code FATMAN (Finite-element Analysis Tool, Multi-physics And Nonlinear). A novel implicit solution scheme called LENI is proposed to allow modelling of creep in solder. The sinh law for steady-state creep and the Armstrong-Frederick kinematic hardening law to capture primary creep have been implemented in FATMAN using the LENI scheme. The advantage over an explicit discretisation is investigated. An inverse analysis method for determining material properties is used to determine constants for the kinematic hardening law from experimental creep curves. A damage law is presented which allows the prediction of crack propagation through a solder joint. A failure criteria based on the increase in electrical resistance is used, which removes the need for an empirical fatigue law. The steady state creep law, the kinematic hardening law and the damage law are all applied to modelling of tests developed at the NPL (National Physical Laboratory) including novel crack detection tests, an isothermal fatigue test, and accelerated thermal cycling of resistors

Lebensdauervorhersage für (SnAgCu- und SnPb-) aufgelötete Leistungshalbleiter mittels primärem und sekundärem Kriechen

The objective of this thesis was to accurately check and improve the models existing for eutectic solder alloys used in simulation tools. Creep deformation,which is the most important deformation mode of solders, of two solder alloys, the widely used eutectic SnPb and the environmentally friendly alternative solder alloy SnAgCu was tested. It was shown that it is necessary to model two different stages of this high temperature induced mechanism: To improve the current material definition, primary creep must be implemented in the FE-software in addition to the existing secondary creep models. This thesis shows how it is possible to test creep behaviour under cyclic loading conditions with a test specimen of novel design. So, primary creep was observed and reoccurs cyclically under such test conditions. Furthermore, steady state creep is also always observed. A Constitutive equation combining both primary and secondary creep was given and verified. This model was implemented in FE-code Ansys, and after performing different kinds of simulation, the necessity of simulating primary creep was demonstrated. In order to achieve reliability information by FE-simulation of solder die attach, the creep-fatigue behaviour with the mean of crack propagation must be modeled. Various kinds of chips on copper substrate (power-transistors) were thermally tested, and different methods were used to investigate crack propagation. These methods were scanning acoustic microscopy and microstructure analysis by optical microscopy. The influence of damage on thermal behaviour (i.e. the thermal resistance of the device) was also assessed. These results were compared with the simulation results in order to build a lifetime prediction model based on crack propagation analysis.Das Ziel dieser Arbeit war es, die schon vorhandenen Modelle für die Lötstellensimulation zu verbessern. Da das Kriechen für die Verformung von Lötlegierungen der wichtigste Mechanismus ist, wurde das Kriechverhalten für zwei Lötlegierungen untersucht. Es handelt sich dabei um die weltweit bekannten Legierungen eut. SnPb und das umweltverträglichere SnAgCu. Es ist empfehlenswert zwei Bereiche dieses Hoch-Temperatur Mechanismus zu modellieren. Um die bisherigen Werkstoffsmodelle verbessern zu können, ist das primäre Kriechen zusätzlich zu den bestehenden sekundären Kriechmodellen in FE-Programme zu implementieren. Diese Arbeit zeigt, wie das Kriechen unter zyklischer Belastung mit einer neuen Prüfkörpergeometrie untersucht werden kann. Unter solchen Randbedingungen ist erneut primäres Kriechen zu beobachten. Weiterhin wurde immer auch stationäres Kriechen beobachtet. Eine Zustandsgleichung, bestehend aus primärem und sekundärem Kriechen, wurde entwickelt. Dieses Modell wurde in den FE-Code Ansys implementiert, und nach der Durchführung von verschiedenen Packagingsimulationen wurde ebenfalls festgestellt, das das primäre Kriechen unbedingt berücksichtigt werden muss. Um eine Lebensdauerprognose von flächigen Lötstellen zu erreichen, müssen die Kriechermüdung sowie der Rissfortschritt modelliert werden. Einige Testdemonstratoren (Leistungstransistor auf Kupfer) wurden thermo-mechanisch geprüft (Temperaturschock und Temperaturwechsel). Durch zwei verschiedene Methoden (Ultraschallmikroskopie und Gefügeanalyse) wurde die Rissinitiierung und der Rissforschritt untersucht. Der Einfluss der Schädigung auf das thermische Verhalten des Testobjektes (thermischer Widerstand des Bauelements) wurde ebenfalls bewertet. Diese experimentellen Ergebnisse wurden mit den Simulationsergebnissen verglichen, um ein neues Lebensdauermodell basierend auf der Rissfortschrittanalyse zu bauen. Eine sehr gute Übereinstimmung erlaubt nun die Zuverlässigkeitsvorhersage von flächig aufgelöteten Chips im Bereich der Leistungselektronik

System-Layout-Dependent Thermally Induced Solder Stress & Reliability Implications

Electronic flip chip assemblies consist of dissimilar component materials, which exhibit different CTE. Under thermal cyclic operating conditions, this CTE mismatch produces interfacial and interconnect stresses, which are highly dependent on system layout. In this paper, sensitivity analyses are performed using ANSYS FEA to establish how the proximity and arrangement of neighboring devices affect interconnect stress. Flip chip alignment modes ranging from edge-to-edge to corner-to-corner are studied. Results of these FEA studies, demonstrated that closely packing devices together has the effect of making them act as one. This results in a significant increase in the thermomechanical stresses induced on peripheral solder joints, heightening reliability risk. The sensitivity subsides gradually as device spacing increases, and eventually stops being a factor. 6mm is the threshold separation at which this occurs, in both edge-edge and corner-corner placement, for the system under analysis in this paper. Understanding the effect of system layout is instrumental for optimizing system design and improving reliability of power modules to meet the increasing power density needs

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

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/

Mechanical behaviour and reliability of Sn3.8AgO.7Cu solder for a surface mount assembly

The demands for compact, light weight and Iow cost electronic products have resulted in the miniaturisation of solder interconnects to a sub-millimetre scale. With such a reduction in size, the solder joints cannot be assumed to behave in the same way as bulk solder in terms of reliability due to the fact that their material behaviours are influenced by the joint size and microstructure. The complexity of their reliability assessment is furthermore compounded by the demand for the replacement of traditional SnPb solder alloys with lead-free alloys, due to the presence of the toxic and health hazardous element (Pb) in the former alloy. However, these new lead-free alloys have much less history of industrial applications, and their material and reliability data is not as well developed as traditional lead-based alloys. In addition, most previous reliability assessments using finite element analysis have assumed a uniform distribution of temperature within the electronic assembly, which conflicts the actual temperature conditions during circuit operation. Therefore, this research was undertaken to analyse the effect of solder joint size on solder material properties from which material models were developed, and to determine the effect of an actual (nonuniform) temperature distribution in an electronic assembly on the reliability of its solder joints. Following a review of lead-free solders and potential lead-free alloys, lead-free solder microstructures, and the reliability issues and factors affecting the reliability of solder joints, the practical aspects of this research were carried out in two main parts. The first part consisted of substantial work on the experimental determination of the temperature distribution in a typical surface mount chip resistor assembly for power cycling conditions, and the stress-strain and creep behaviour for both Sn3.8AgO.7Cu solder joints and reflowed bulk solder. This also included building material models based on the experimental data for the solder joints tested and comparison with that for bulk solder. Based on the comparison of the material properties, two extreme material models were selected for the reliability study. Size and microstructure effects on the solder material properties were also discussed in this part. The second part comprised of extensive finite element analysis of a surface mount chip resistor assembly and reliability assessment of its solder joints. The simulation began with elasto-plastic analysis for 2D and 3D chip resistor assemblies to decide upon the kind of formulation to be used when the full complexity of both plasticity and creep is considered. The simulation was carried out considering the determined non-uniform temperature distribution and idealized or traditional uniform temperature condition. The solder joint's material properties were modelled using the two material models determined from the experimental results. The effect of temperature distribution during thermal cycling and of the selected material models on the solder joint reliability was demonstrated using finite element analysis and subsequent fatigue life estimation. In summary, this research has concluded that the material behaviour of the solder joint is different from that of bulk solder due to the effect of its size and microstructure. The anisotropic behaviour of the solder joint cannot be ignored in reliability studies, since it has a significant effect on the solder joint's fatigue life. The research also showed the significant effect of an actual (non-uniform) temperature distribution in the electronic assembly on the solder joint fatigue life

Numerical analysis of lead-free solder joints: effects of thermal cycling and electromigration

To meet the requirements of miniaturization and multifunction in microelectronics, understanding of their reliability and performance has become an important research subject in order to characterise electronics served under various loadings. Along with the demands of the increasing miniaturization of electronic devices, various properties and the relevant thermo-mechanical-electrical response of the lead-free solder joints to thermal cycling and electro-migration become the critical factors, which affect the service life of microelectronics in different applications. However, due to the size and structure of solder interconnects in microelectronics, traditional methods based on experiments are not applicable in the evaluation of their reliability under complex joint loadings. This thesis presents an investigation, which is based on finite-element method, into the performance of lead-free solder interconnects under thermal fatigue and electro-migration, specifically in the areas as follows: (1) the investigation of thermal-mechanical performance and fatigue-life prediction of flip-chip package under different sizes to achieve a further understanding of IMC layer and size effects of a flip chip package under thermal cycling; (2) the establishment of a numerical method, simulating void-formation/crack-propagation based on the results of finite-element analysis, to allow the prediction of crack evolution and failure time for electro-migration reliability of solder bumps; (3) the establishment of a flow-based algorithm for combination effects of thermal-mechanical and electro-migration that was subsequent implemented in to an FE model to evaluate the reliability assessment of service lives associated with a flip chip package

Modelling of the reliability of flip chip lead-free solder joints at high-temperature excursions

At high-temperature operations of electronic control devices, Tin-Silver-Copper (SnAgCu) alloy solder joints used to assemble the component of the devices are functioning at homologous temperature above 0.8. In such ambient temperatures, solder alloys have limited mechanical strength and will be sensitive to strain rate. The sensitivity of solder properties to creep/visco-plastic deformation increases the rate of accumulation of plastic damage in the alloy and decreases the number of cycles to failure (Nf) of the joints. Most untimely rupture of solder joints in high-temperature electronics (HTE) system usually culminates in colossal loss of resources and lives. Typical incidences are reported in recent automotive and aircraft crashes as well as the collapse of oil-well logging equipment. To increase the mean time to failure (MTTF) of solder joints in HTE, an in-depth understanding and accurate prediction of the response of solder joints to thermally induced plastic strain damage is crucial. This study concerns the prediction of the reliability of lead-free solder joints in a flip chip (FC) model FC48D6.3C457 which is mounted on a substrate and the assembly subjected to high-temperature excursions. The research investigates the effect of the high-temperature operations on reliability of the joints. In addition, the investigation examines the impact of control factors (component stand-off height (CSH), inter-metallic compound (IMC) thickness, number of thermal cycle and solder volume) on Nf of the joints. A model developed in the course of this investigation was employed to create the assembly solder joints architecture. The development of the model and creation of the bump profile involve a combination of both analytical and construction methods. The assembled package on a printed circuit board (PCB) was subjected to accelerated temperature cycle (ATC) employing IEC standard 60749-25 in parts. The cycled temperature range is between -38 oC and 157 oC. Deformation behaviour of SnAgCu alloy solder in the joints is captured using Anand’s visco-plasticity model and the response of other materials in the assembly were simulated with appropriate model. The results demonstrate that the reliability of solder joints operating at elevated temperatures is dependent on CSH, thickness of IMC and solder volume. It also shows that incorporating the IMC layer in the geometric models significantly improves the level of accuracy of fatigue life prediction to ± 22.5% (from the ± 25% which is currently generally accepted). The findings also illustrate that the magnitude of the predicted damage and fatigue life are functions of the number of ATC employed. The extensive set of results from the modelling study has demonstrated the need for incorporating the IMC layer in the geometric model to ensure greater accuracy in the prediction of solder joint service life. The technique developed for incorporating the IMC layer will be of value to R&D engineers and scientists engaged in high-temperature applications in the automotive, aerospace and oil-well logging sectors. The results have been disseminated through peer reviewed journals and also presentations at international conferences

Solder Interconnect Life Prediction under Complex Temperature Cycling with Varying Mean and Amplitude

Electronic devices are under concurrent loading of the power cycling of the devices and the temperature cycling from the surrounding environment. Temperature histories resultant from these concurrent loading would be a complex temperature cycling with varying cyclic temperature mean and amplitude, as well as spatial thermal gradient. This study developed modeling approaches and quantified accuracies for predicting solder interconnect life under complex temperature cycling. Three modeling approaches were presented in this study: 1) modeling the strain energy under the resultant complex temperature cycling and employing the energy based fatigue life models; 2) segmenting the resultant complex temperature cycle into multiple simple temperature cycles with a single temperature range for each first, then assessing the life expectancy of the solder interconnect under the segmented simple temperature cycles and at last applying Miner's rule to superpose the damage; 3) estimating solder damage under the resultant complex temperature cycling by a standard temperature cycling with a single temperature range. Two case studies were included in this thesis: 1) chamber controlled complex temperature cycling with mini cycles occurring at the upper excursion on ceramic leadless chip carriers assembled by Sn36Pb62Ag2 and SnAg3.0Cu0.5 solder (without spatial thermal gradient); 2) combined temperature and power cycling on plastic ball grid array assembled by Sn63Pb37 and SnAg3.0Cu0.5 solder (with spatial thermal gradient). Physical tests were also conducted to quantify the developed modeling approaches