Reliability study of subsea electronic systems subjected to accelerated thermal cycle ageing

Reliability is of increasing importance for electronics systems operating at harsh environments, such as the electronic telecommunication systems used at subsea level. The aim of this research was to investigate the reliability of such electronic systems through a simulated accelerated thermal cycle test. The paper presents a step-by-step process of designing accelerated thermal cycle test using field operating conditions. The Coffin-Mansion equation was used to calculate the accelerated factor for the thermal cycle test. In order to simulate the expected life time of 25 years, the solder assembly samples were subjected to 400 temperature cycles, with every cycle lasting for 40 minutes. Reliability was determined by measuring shear strengths of solder joints of different electronic components at set intervals. Although some of the components showed an initial decrease in shear strength, it was generally concluded that the electronic assemblies are able to maintain their shear strength for up to 25 years. The fracture surfaces of the solder joints, after shear testing, were also analyzed for brittle and ductile fractures, with the use of scanning electron microscopy (SEM)

CFD simulation of solder paste flow and deformation behaviours during stencil printing process.

In 20th century, Electronics elements have become most significant part of the regular life. The main heart of electronic element is PCB which supports and manages mostly machines and equipments these days. Therefore manufacturing of board and assembly of electronic elements is one of the crucial and significant objectives for most of the companies. Better life of PCB’s depends on electronic elements and its assembly with board. Solder paste is used as adhesive material for assembly purpose. It is deposited on board using stencil and electronic elements are mounted on it and heated for strong bond. This study investigates on factors affecting stencil printing process due to variation in squeegee speed and density of solder paste. This study is based on computational fluid dynamics virtual simulation. Prototype is developed for modelling purpose and simulation software is used to simulate the flow behaviour of solder paste during stencil printing process.N/

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

Creep-Fatigue Behaviours of Sn-Ag-Cu Solder Joints in Microelectronics Applications

Electronic manufacturing is one of the dynamic industries in the world in terms of leading in technological advancements. At the heart of electronic assembly lies the 'soldering technology' and the 'solder joints' between electronic components and substrate. During the operation of electronic products, solder joints experience harsh environmental conditions in terms of cyclic change of temperature and vibration and exposure to moisture and chemicals. Due to the cyclic application of loads and higher operational temperature, solder joints fail primarily through creep and fatigue failures. This paper presents the creep-fatigue behaviours of solder joints in a ball grid array (BGA), soldered on a printed circuit board (PCB). Using finite element (FE) simulation, the solder joints were subjected to thermal cycling and isothermal ageing. Accelerated thermal cycling (ATC) was carried out using a temperate range from 40℃ to 150℃, and isothermal ageing was done at -40,25,75 and 150℃ temperatures for 45 days (64,800 mins). The solders studied are lead-based eutectic Sn63Pb37 and lead-free SAC305, SAC387, SAC396 and SAC405. The results were analysed using the failure criterion of equivalent stress, strain rate, deformation rate, and the solders' strain energy density. The SAC405 and SAC396 are found to possess the least stress magnitude, strain rate, deformation rate, and strain energy density damage than the lead-based eutectic Sn63Pb37 solder; they have the highest fatigue lives based on the damage mechanisms. This research provides a technique for determining the preventive maintenance time of BGA components in mission-critical systems. Furthermore, it proposes developing a new life prediction model based on a combination of the damage parameters for improved prediction.N/

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

Printing Morphology and Rheological Characteristics of Lead-Free Sn-3Ag-0.5Cu (SAC) Solder Pastes

Solder paste plays a crucial role as the widely used joining material in surface mount technology (SMT).The understanding of its behaviour and properties is essential to ensure the proper functioning of the electronic assemblies.The composition of the solder paste is known to be directly related to its rheological behaviour. This paper provides a brief overview of the solder paste behaviour of four different solder paste formulations, stencil printing processes, and techniques to characterize solder paste behaviour adequately. The solder pastes are based on the Sn-3.0Ag-0.5Cu alloy,are different in their particle size, metal content and flux system. The solder pastes are characterized in terms of solder particle size and shape as well as the rheological characterizations such as oscillatory sweep tests, viscosity, and creep recovery behaviour of pastes

Study of the time-dependent rheological behaviour of lead-free solder pastes and flux mediums used for flip-chip assembly applications

The two most important trends in electronic industry are "miniaturisation" and "increased functionality". Over the last fifteen years, the electronic manufacturing industries have experienced tremendous pressure to meet the requirements for miniaturised products, particularly, hand-held consumer products. Functionality of these products has also evolved at the same pace through packing in more and more features. As these trends are set to continue, there is an increasing demand for better understanding of soldering technology, particularly in the area of solder pastes used in the reflow soldering of surface mount devices. Successful assembly of electronic devices for ultra-fine pitch and flip-chip applications requires the deposition of small and consistent paste deposits from pad to pad, and from board to board. The paste printing process at this chip-scale geometry depends on conditions such as good paste roll, complete aperture filling and paste release from the apertures onto the substrate pads. This means that the paste flow and deformation behaviour is very important in defining the printing performance of any solder paste. In order to understand rheological phenomena associated with the flow of solder pastes, it is necessary to understand time dependent rheological behaviour. Such behaviour is common to many industrial fluids and consequently has been of interest to rheologists for many years. The time dependent behaviour observed in solder pastes is largely due to the breakdown of the flocculated structure formed during storage or idle period. In general, the breakdown of the structure with shear results in a decrease in apparent viscosity. Recovery after shear is generally a very slow process and depends on the intensity of the breakdown and previous shear history. The work reported in this thesis on the characterisation and modelling of time-dependent rheological behaviour of solder paste and flux mediums used in surface mount applications is made up of four main parts. The first part concerns the characterisation of the time-dependent behaviour of solder pastes and flux mediums. Two types of tests were performed at this stage: hysteresis-loop test and step-shear-rate test. In the second part of the study the time-dependent rheology of solder pastes and flux mediums has been modelled to evaluate the mechanisms for the break-down of the internal structure of the paste materials. A novel technique has been developed which combines the experimental rheological data with a modified structural kinetic model (SKM) to investigate the rate and extent of structural change of solder paste and flux medium. The third part of the study deals with the experimental and modelling studies of the short term build-up of solder paste and flux medium structure using the stretched exponential model. In the final part of the study the printing trials of four different solder paste samples were carried out to investigate the effect of post-print rest period on slumping behaviour of solder paste. From the experimental characterisation it was evident that both the solder paste and flux samples are strongly thixotropic and shear-thinning in nature. The thixotropic breakdown behaviour of solder paste and fluxes has been satisfactorily modelled using a second-order structural kinetic model. The results from this study can be of great help for the solder paste manufacturers and formulators in quantifying and predicting the effect of long term shearing on the solder paste samples. The technique developed can also be utilized for similar materials such as solar pastes and conductive adhesives. The short-term build-up of solder pastes and fluxes has been successfully modelled using the stretched exponential model. The paste manufacturers and formulators can use the technique developed to predict and quantify the slumping behaviour of solder paste. The end-users, for example the electronics assemblers may use the technique to optimize their assembly process by minimising/preventing slumping of solder paste

Rheological characterisation and modelling of electronic materials: time-dependent behaviours of lead-free solder pastes and flux mediums

The two most important trends in electronic industry are "miniaturisation" and "increased functionality". As these trends are set to continue, there is an increasing demand for better understanding of soldering technology, particularly in the area of solder pastes used in the reflow soldering of surface mount devices. This means that the paste flow and deformation behaviour is very important in defining the printing performance of any solder paste. The work reported in this book is focused on experimetal investigation and empirical modelling of time-dependent rheological behaviours of solder paste and flux mediums. The printing trials of four different lead-free solder paste samples were also carried out to investigate the effect of post-print rest period on slumping behaviour of solder paste. The paste manufacturers and formulators can use the techniques developed to predict and quantify the slumping behaviour of solder paste.The end-users, for example the electronics assemblers may use the techniques to optimize their assembly process by minimising/preventing slumping of solder paste

Investigating the effects of area ratio and aspect ratio on stencil printing process used in the electronics assembly line

Stencil printing is the widely used method for depositing solder paste in the surface mount technology (SMT) assembly process. The aim of this study is to investigate the effect two important factors namely area ratio and aspect ratio in the stencil printing process. Taguchi’s orthogonal arrays are used to design the experiments. A total of four factors (with two levels) are considered in the experimental design. The print speed and print pressure are the two other factors considered in the study. The solder paste deposit heights were measured and recorded as response. The results were analysed by performing Analysis of Variance (ANOVA) and creating main-effects plots and response surface curves to check for any interactions and to optimize the response output. The results were also compared to work done by previous researchers and discussed critically. The ‘area ratio’ and ‘print speed’ are identified as the most important factors and there is strong interaction evidenced between ‘print speed’ and ‘aspect ratio’. The response is optimized for maximum performance and to reduce the defects from the stencil printing process