Initial bond formation in thermosonic gold ball bonding on aluminium metallization pads

The morphological features of lift-off footprints on the aluminium metallization pads were investigated to gain an understanding of the effects of bonding parameters on formation of initial bonds during thermosonic gold ball bonding. The obtained results showed that metallurgical bonding initiated at the peripheral areas of the contact area situated along the direction of ultrasonic vibration. Those areas extended inwards with an increase in ultrasonic power. Both the external bonded area and central non-bonded area increased with increasing bonding force. Based on the evolution of footprints, the bonding models were proposed, and the effects of the bonding parameters on the formation of initial bonds were discussed.     

Microstructural evolution of 96.5Sn–3Ag–0.5Cu lead free solder reinforced with nickel-coated graphene reinforcements under large temperature gradient

In this study, 96.5Sn–3Ag–0.5Cu (SAC305) lead-free composite solder containing graphene nanosheets (GNS) decorated with Ni nanoparticles (Ni-GNS) was prepared using a powder metallurgy method. A lab-made set-up and a corresponding Cu/solder/Cu sample design for assessing thermo-migration (TM) was established. The feasibility of this setup for TM stressing using an infrared thermal imaging method was verified; a temperature gradient in a solder joint was observed at 1240 K/cm. Microstructural evolution and diffusion of Cu in both plain and composite solder joints were then studied under TM stressing conditions. Compared to unreinforced SAC305 solder, the process of diffusion of Cu atoms in the composite solder joint was significantly reduced. The interfacial intermetallic compounds (IMCs) present in the composite solder joint also provide a more stable morphology after the TM test for 600 h. Furthermore, during the TM test, the Ni-GNS reinforcement affects the formation, migration and distribution of Ni–Cu–Sn and Cu–Sn IMCs by influencing the dissolution rate of Cu atoms.     

Simulations of delamination in CFRP laminates: Effect of microstructural randomness

Due to their high specific strength and stiffness, fibre-reinforced composite materials are being increasingly used in structural applications where a high level of performance is important (e.g. aerospace, automotive, offshore structures, etc.). Performance in service of these composites is affected by multi-mechanism damage evolution under loading and environmental conditions. For instance, carbon fibre-reinforced laminates demonstrate a wide spectrum of failure mechanisms such as matrix cracking and delamination. These damage mechanisms can result in significant deterioration of the residual stiffness and load-bearing capacity of composite components and should be thoroughly investigated. The delamination failure mechanism is studied in this paper for a double cantilever beam (DCB) loaded in mode I. Several sensitivity studies are performed to analyse the effects of mesh density and of parameters of the cohesive law on the character of damage propagation in laminates. The microstructural randomness of laminates that is responsible for non-uniform distributions of stresses in them even under uniform loading conditions is accounted for in the model. The random properties are introduced with the use of Weibull’s two-parameter probability density function. Several statistical realisations are carried out which show that the effect of microstructure could significantly affect the macroscopic response emphasizing the need to account for microstructural randomness for accurate predictions of load-carrying capacity of laminate composite structures.     

SPH-BEM simulation of underwater explosion and bubble dynamics near rigid wall

A process of underwater explosion of a charge near a rigid wall includes three main stages: charge detonation, bubble pulsation and jet formation. A smoothed particle hydrodynamics(SPH) method has natural advantages in solving problems with large deformations and is suitable for simulation of processes of charge detonation and jet formation. On the other hand, a boundary element method(BEM) is highly efficient for modelling of the bubble pulsation process. In this paper, a hybrid algorithm, fully utilizing advantages of both SPH and BEM, was applied to simulate the entire process of free and near-field underwater explosions. First, a numerical model of the free-field underwater explosion was developed, and the entire explosion process–from the charge detonation to the jet formation–was analysed. Second, the obtained numerical results were compared with the original experimental data in order to verify the validity of the presented method. Third, a SPH model of underwater explosion for a column charge near a rigid wall was developed to simulate the detonation process. The results for propagation of a shock wave are in good accordance with the physical observations. After that, the SPH results were employed as initial conditions for the BEM to simulate the bubble pulsation. The obtained numerical results show that the bubble expanded at first and then shrunk due to a differences of pressure levels inside and outside it. Here, a good agreement between the numerical and experimental results for the shapes, the maximum radius and the movement of the bubble proved the effectiveness of the developed numerical model. Finally, the BEM results for a stage when an initial jet was formed were used as initial conditions for the SPH method to simulate the process of jet formation and its impact on the rigid wall. The numerical results agreed well with the experimental data, verifying the feasibility and suitability of the hybrid algorithm. Besides, the results show that, due to the effect of the Bjerknes force, a jet with a high speed was formed that may cause local damage to underwater structures.     

Growth of Intermetallic Compounds in Thermosonic Copper Wire Bonding on Aluminum Metallization

Interface evolution caused by thermal aging under different temperatures and durations was investigated by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that approximately 30-nm-thick and discontinuous Cu-Al intermetallic compounds (IMCs) were present in the initial bonds before aging. Cu-Al IMCs grew under thermal aging with increasing aging time. The growth kinetics of the Cu-Al IMCs was correlated to the diffusion process during aging; their combined activation energy was estimated to be 1.01 eV. Initially, Al-rich Cu-Al IMCs formed in the as-bonded state and early stage of aging treatment. Cu₉Al₄ was identified by selected-area electron diffraction (SAD) as the only type of Cu-Al IMC present after thermal aging at 250°C for 100 h; this is attributed to the relatively short supply of aluminum to the interfacial reaction.     

Quantifying the mechanical properties of polymeric tubing and scaffold using atomic force microscopy and nanoindentation

Measurement of mechanical parameters of polymeric scaffolds presents a significant challenge due to their intricate shape and small characteristics dimensions of their elements—around 100 μm. In this study, mechanical properties of polymeric tubing and scaffold, made of biodegradable poly(l ‐lactic) acid (PLLA), were characterized using atomic force microscopy (AFM) and nanoindentation, complemented with tensile testing. AFM was employed to assess the properties of the tube and scaffold locally, while nanoindentation produced results with a dependency on the depth of indentation. As a result, the AFM‐measured elastic modulus differs from the nanoindentation data due to a substantial difference in indentation depth between the two methods. With AFM, a modulus between 2 and 2.5 GPa was measured, while a wide range was obtained from nanoindentation on both the tube and scaffold, depending on the indentation scale. Changes in the elastic modulus with in‐vitro degradation and aging were observed over the 1‐year period. To complement the indentation measurements, tensile testing was used to study the structural behavior of the tube, demonstrating the yielding, hardening and fracture properties of the material. POLYM. ENG. SCI., 59:1084–1091, 2019. © 2019 Society of Plastics Engineers     

Effect of micro-randomness on macroscopic properties and fracture of laminates

Composite materials demonstrate a considerable extent of heterogeneity. A non-uniform spatial distribution of reinforcement results in variations of local properties of fibrous laminates. This non-uniformity not only affects effective properties of composite materials but is also a crucial factor in initiation and development of damage and fracture processes that are also spatially non-uniform. Such randomness in microstructure and in failure evolution is responsible for non-uniform distributions of stresses in composite specimens even under externally uniform loading, resulting, for instance, in a random distribution of matrix cracks in cross-ply laminates. The paper deals with statistical features of a distribution of carbon fibres in a transversal cross-sectional area in a unidirectional composite with epoxy matrix, based on various approaches used to quantify its microscopic randomness. A random character of the fibres’ distribution results in fluctuations of local elastic moduli in composites, the bounds of which depend on the characteristic length scale. A lattice model to study damage and fracture evolution in laminates, linking randomness of microstructure with macroscopic properties, is discussed. An example of simulations of matrix cracking in a carbon fibre/epoxy cross-ply laminate is given.     

Full-scale natural fire tests on gypsum lined structural insulated panel (SIP) and engineered floor joist assemblies

SIPS are formed from the lamination of two oriented strand board (OSB) facing plates and a highly insulating polymer-based foam such as expanded polystyrene (EPS) or polyurethane (PUR). The resulting lightweight panels are typically used as primary load-bearing compression elements for buildings such as domestic dwellings, apartment blocks, schools and hotels.The regulatory fire performance of SIPS, like many systems, is assessed via a standard fire test. However, it is widely accepted that this is merely a comparative method for determining the relative performance of one product when compared to another; hence, it gives little indication of a component's likely behaviour in a real fire. With this in mind BRE Global, with support from the UK Department for Communities and Local Government (CLG), have undertaken a research programme to determine the performance of SIPS subject to realistic fire conditions.The research programme exposed four two storey SIP buildings to natural fire scenarios using timber cribs. Two buildings were constructed with EPS core SIPS. The other two were constructed with PUR core SIPS. Each material set was subdivided by passive fire protection specification (PFP). These were specified on the basis of 30 and 60-min fire resistance.The experiments highlighted a number of weaknesses in the system performance of SIP structures with engineered floors. Firstly, where PFP is under specified or poorly installed, collapses of the engineered floor plate are very likely. Mechanisms for fire spread were also identified where fitting details were not appropriately sealed. In addition, there appeared to be little appreciable difference in the behaviour of buildings formed with EPS or PUR core SIPS. Finally, a number of system redundancies and alternative load paths were identified, which prevented total collapse of any of the test buildings.     

Long-term effect of norepinephrine on iodide uptake in FRTL-5 cells

The sympathetic nervous system plays a role in the regulation of thyroid function. In FRTL-5 rat thyroid cells, norepinephrine (NE) acutely depresses intracellular I- by increasing I- efflux. The present study was undertaken to determine the effect of NE on iodide transport after a longer time period. NE inhibited the ability of thyrotropin (TSH) to induce iodide uptake by FRTL-5 cells after 48 or 72 hours, but not after 24 hours. The effect of NE was more evident with increasing concentrations of TSH. NE did not modify the rate of I- efflux. Inhibition was associated with a decrease in the Vmax and no change in the Km for iodide influx. To determine if this was a generalized effect of NE on thyroid cell membrane, the uptake of alpha-aminoisobutyric acid (a nonmetabolizable aminoacid) and of 2-deoxyglucose was measured. NE did not inhibit TSH stimulation of the uptake of the two compounds. NE inhibited the action of dibutyryl cAMP (dbcAMP) on iodide uptake in a similar manner to TSH, but did not alter the cyclic adenosine monophosphate (cAMP) levels increased by TSH. The effects of different adrenoreceptor agonists and antagonists demonstrated that norepinephrine acts through an alpha1-adrenergic receptor.