Long Bin Tan

Rate-dependent properties of Sn–Ag–Cu based lead-free solder joints for WLCSP

The increasing demand for portable electronics has led to the shrinking in size of electronic components and solder joint dimensions. The industry also made a transition towards the adoption of lead-free solder alloys, commonly based around the Sn-Ag-Cu alloys. As knowledge of the processes and operational reliability of these lead-free solder joints (used especially in advanced packages) is limited, it has become a major concern to characterise the mechanical performance of these interconnects amid the greater push for greener electronics by the European Union. In this study, solder joint array shear and tensile tests were conducted on wafer-level chip scale package (WLCSP) specimens of different solder alloy materials, SAC 105 (Sn-1%wt Ag-0.5%wt Cu) and SAC 405 (Sn-4%wt Ag-0.5%wt Cu) under two test rates of 0.5 mm/s (2.27 s−1) and 5 mm/s (22.73 s−1). These WLCSP packages have an array of 12×12 solder bumps (300μm in diameter); and double redistribution layers with a Ti/Cu/Ni/Au under-bump metallurgy (UBM) as their silicon-based interface structure. Good mechanical performance of package pull-tests at high strain rates is often correlated to a higher percentage of bulk solder failures than interface failures in solder joints. The solder joint array tests show that for higher test rates and Ag content, there are less bulk solder failures and more interface failures. Correspondingly, the average solder joint strength and peak load also decrease under higher test rate and Ag content. The solder joint results relate closely to the higher rate-sensitivity of SAC 405 in gaining material strength which might prove detrimental to solder joint interfaces that are less rate sensitive. In addition, specimens under shear yielded more bulk solder failures, higher average solder joint strength and ductility than specimens under tension.

Board level solder joint failures by static and dynamic loads

Quasi-static bend tests and drop tests of surface mounted MCMs on printed circuit boards (PCBs) are conducted to compare the failure patterns from the two types of tests. The boards are 120/spl times/90 mm in dimension. The modules are 40/spl times/40 mm with a brass heat spreader of the same dimension on top. Five-point bend tests are conducted with different ramp-rates from 0.1 to 10.0 mm/min to determine the effect of different PCB strain rates on the reliability of the solder interconnections. Drop tests are conducted for the same batch of PCBs at 1.0 m and 1.5 m drop heights and PCB strains from drop tests are compared with those taken from the quasi-static tests. A failure analysis is also conducted to determine the possible mechanisms of failure. Findings show that solder joint failure in five-point bend tests is dependent on the ramp-rate. The tests also show that mass inertia of the heat-spreader may cause solder joints to fail at lower PCB strain levels than expected especially for packages with heavier heat-spreaders which are needed for higher heat dissipation.

Development and validation of two subject-specific finite element models of human head against three cadaveric experiments

Head injury, being one of the main causes of death or permanent disability, continues to remain a major health problem with significant socioeconomic costs. Numerical simulations using the FEM offer a cost-effective method and alternative to experimental methods in the biomechanical studies of head injury. The present study aimed to develop two realistic subject-specific FEMs of the human head with detailed anatomical features from medical images (Model 1: without soft tissue and Model 2: with soft tissue and differentiation of white and gray matters) and to validate them against the intracranial pressure (ICP) and relative intracranial motion data of the three cadaver experimental tests. In general, both the simulated results were in reasonably good agreement with the experimental measured ICP and relative displacements, despite slight discrepancy in a few neutral density targets markers. Sensitivity analysis showed some variations in the brains relative motion to the material properties or markers location. The addition of soft tissue in Model 2 helped to damp out the oscillations of the model response. It was also found that, despite the fundamental anatomical differences between the two models, there existed little evident differences in the predicted ICP and relative displacements of the two models. This indicated that the advancements on the details of the extracranial features would not improve the models predicting capabilities of brain injury.

Impact of complex blast waves on the human head: a computational study

Head injuries due to complex blasts are not well examined because of limited published articles on the subject. Previous studies have analyzed head injuries due to impact from a single planar blast wave. Complex or concomitant blasts refer to impacts usually caused by more than a single blast source, whereby the blast waves may impact the head simultaneously or consecutively, depending on the locations and distances of the blast sources from the subject, their blast intensities, the sequence of detonations, as well as the effect of blast wave reflections from rigid walls. It is expected that such scenarios will result in more serious head injuries as compared to impact from a single blast wave due to the larger effective duration of the blast. In this paper, the utilization of a head-helmet model for blast impact analyses in Abaqus(TM) (Dassault Systemes, Singapore) is demonstrated. The model is validated against studies published in the literature. Results show that the skull is capable of transmitting the blast impact to cause high intracranial pressures (ICPs). In addition, the pressure wave from a frontal blast may enter through the sides of the helmet and wrap around the head to result in a second impact at the rear. This study recommended better protection at the sides and rear of the helmet through the use of foam pads so as to reduce wave entry into the helmet. The consecutive frontal blasts scenario resulted in higher ICPs compared with impact from a single frontal blast. This implied that blast impingement from an immediate subsequent pressure wave would increase severity of brain injury. For the unhelmeted head case, a peak ICP of 330 kPa is registered at the parietal lobe which exceeds the 235 kPa threshold for serious head injuries. The concurrent front and side blasts scenario yielded lower ICPs and skull stresses than the consecutive frontal blasts case. It is also revealed that the additional side blast would only significantly affect ICPs at the temporal and parietal lobes when compared with results from the single frontal blast case. By analyzing the pressure wave flow surrounding the head and correlating them with the consequential evolution of ICP and skull stress, the paper provides insights into the interaction mechanics between the concomitant blast waves and the biological head model.

Conventional and complex modal analyses of a finite element model of human head and neck

This study employs both the traditional and the complex modal analyses of a detailed finite element model of human head–neck system to determine modal responses in terms of resonant frequencies and mode shapes. It compares both modal responses without ignoring mode shapes, and these results are reasonably in agreement with the literature. Increasing displacement contour loops within the brain in higher frequency modes probably exhibits the shearing and twisting modes of the brain. Additional and rarely reported modal responses such as ‘mastication’ mode of the mandible and flipping mode of nasal lateral cartilages are identified. This suggests a need for detailed modelling to identify all the additional frequencies of each individual part. Moreover, it is found that a damping factor of above 0.2 has amplifying effect in reducing higher frequency modes, while a diminishing effect in lowering peak biomechanical responses, indicating the importance of identifying the appropriate optimised damping factor.

Global-Local Analysis of a Full-scale Composite Riser during Vortex-Induced Vibration

This paper presents a global-local analysis procedure to demonstrate the feasibility of a composite riser and its advantages over the traditional steel counterpart. This procedure starts from the local design of the sandwich tubular structure of riser section. The equivalent material properties of the sandwich tube are obtained using classic composite theory and they are used to parameterize the full-scale riser model in global analysis. The global analysis mainly focuses on the vortex-induced vibration (VIV). The methodology is first verified by comparison with experimental data and results produced by SHEAR 7. Four representative cases are then studied and the results show that the critical loads experienced by the composite riser are much lower than that of the steel one due to its lightweight. The lightweight composite riser requires lower top tension and fewer buoyancy cans, which is economically beneficial. The failure envelopes of both composite and steel riser sections are obtained by performing damage modelling techniques. The results show that composite riser yields larger safety margin. Overall, this paper demonstrates that composite riser is technically feasible and its high performance/weight ratio would make it a promising design for deepwater environment, where self-weight is a big challenge that is hindering the development of traditional steel riser.Copyright

Effect of helmet liner systems and impact directions on severity of head injuries sustained in ballistic impacts: a finite element (FE) study.

The current study aims to investigate the effectiveness of two different designs of helmet interior cushion, (Helmet 1: strap-netting; Helmet 2: Oregon Aero foam-padding), and the effect of the impact directions on the helmeted head during ballistic impact. Series of ballistic impact simulations (frontal, lateral, rear, and top) of a full-metal-jacketed bullet were performed on a validated finite element head model equipped with the two helmets, to assess the severity of head injuries sustained in ballistic impacts using both head kinematics and biomechanical metrics. Benchmarking with experimental ventricular and intracranial pressures showed that there is good agreement between the simulations and experiments. In terms of extracranial injuries, top impact had the highest skull stress, still without fracturing the skull. In regard to intracranial injuries, both the lateral and rear impacts generally gave the highest principal strains as well as highest shear strains, which exceed the injury thresholds. Off-cushion impacts were found to be at higher risk of intracranial injuries. The study also showed that the Oregon Aero foam pads helped to reduce impact forces. It also suggested that more padding inserts of smaller size may offer better protection. This provides some insights on future’s helmet design against ballistic threats.

Mapping the failure envelope of board-level solder joints.

Abstract Single solder interconnects were subjected to a series of combined tension–shear and compression–shear tests to determine their failure load. The failure envelope of these interconnects was obtained by plotting the normal component against the shear component of the failure load. The interconnect failure force map was found to be elliptical like the failure envelopes of many materials. The failure map can be described by a simple mathematical expression to give a simple force-based criterion for combine loading of solder joints. Post mortem analyses were conducted on the solder joint specimens to identify the failure mechanisms associated with various segments of the failure map. Computational simulations of actual board tests show that the failure map obtained for joint tests provides good predictions of board-level interconnect failures and hence suggest that such failure maps are useful in the design and analysis of board assemblies subjected to mechanical loads. The industry could adopt the methodology to obtain failure envelopes for solder joints of different alloys, bump size and reflow profiles which they could later use to aid in board-level and system-level designs of their products for mechanical reliability.

Semi-Empirical VIV Analysis of Full-Scale Deepwater Composite Risers

A global-local analysis methodology based on fluid-structure coupling is used to investigate the mechanical responses of both composite and steel risers. Since the design of the riser system can be a daunting task, involving hundreds of load cases for global analysis, semi-empirical fluid load models are considered for the reduced order computations of full-scale riser models. The structural performance of composite risers under real sea current conditions is investigated systematically and discussed with regard to the practical concerns in full-scale settings. The failure envelops of internal liners are found to be within that of the composite layers, which reveals that the liner is the weakest link for composite riser design. Results show that the composite risers can be more prone to vortex-induced-vibration (VIV) due to their lower structural frequencies. In the present study, the composite riser yields 25.5% higher RMS strains than the steel riser. Placement of buoyancy modules along the riser may be critical for the design against VIV, and our results show that the modules are not recommended at the top region of the riser, especially if a top-sheared current is expected. Instead, it is preferable to implement them at the bottom-half portion of the riser and as a continuously buoyed region rather than short discrete buoys separated with gap spaces.© 2014 ASME

Rate-dependent properties of Sn-Ag-Cu based lead free solder joints

The increasing demand for portable electronics has led to the shrinking in size of electronic components and solder joint dimensions. The industry also made a transition towards the adoption of lead-free solder alloys, commonly based around the Sn-Ag-Cu alloys. As knowledge of the processes and operational reliability of these lead-free solder joints (used especially in advanced packages) is limited, it has become a major concern to characterise the mechanical performance of these interconnects amid the greater push for greener electronics by the European Union. In this study, solder joint array shear and tensile tests were conducted on wafer-level chip scale package (WLCSP) specimens of different solder alloy materials, SAC 105 (Sn-1%wt Ag-0.5%wt Cu) and SAC 405 (Sn-4%wt Ag-0.5%wt Cu) under two test rates of 0.5 mm/s (2.27 s−1) and 5 mm/s (22.73 s−1). These WLCSP packages have an array of 12×12 solder bumps (300μm in diameter); and double redistribution layers with a Ti/Cu/Ni/Au under-bump metallurgy (UBM) as their silicon-based interface structure. Good mechanical performance of package pull-tests at high strain rates is often correlated to a higher percentage of bulk solder failures than interface failures in solder joints. The solder joint array tests show that for higher test rates and Ag content, there are less bulk solder failures and more interface failures. Correspondingly, the average solder joint strength and peak load also decrease under higher test rate and Ag content. The solder joint results relate closely to the higher rate-sensitivity of SAC 405 in gaining material strength which might prove detrimental to solder joint interfaces that are less rate sensitive. In addition, specimens under shear yielded more bulk solder failures, higher average solder joint strength and ductility than specimens under tension.