Thermal fatigue life estimation and delamination mechanics studies of multilayered MEMS structures

This paper is concerned with the application of a Physics of Failure (PoF) methodology to assessing the reliability of Micro-Electro-Mechanical-System (MEMS) switches. Numerical simulations, based on the finite element method (FEM) using a sub-domain approach, were performed to examine the damage onset (e.g. yielding) due to temperature variations and to simulated the crack propagation arising after thermal fatigue. In this work remeshing techniques were employed in order to develop a damage tolerance approach based on the assumption that initial flaws exist in the multi-layered structure due, for instance, to manufacturing processes and/or are originates after thermal fatigue, as preliminary experimental tests has shown.     

Intermetallic phase transformations in Au–Al wire bonds

This paper describes the crystallochemical mechanisms that underpin the migration of nano-size alumina, intermetallic growth and phase transformations in Au–Al wire bonds during annealing from 175 °C to 250 °C by utilizing high-resolution transmission electron microscopy (HRTEM). Alumina, encapsulated within the Au–Al intermetallic compounds (IMCs), migrates towards the Au ball during annealing, with Au diffusion into the Al pad. The sequence of Au–Al IMC phase development during annealing was investigated. Initially, Au₈Al₃ appears as a third phase between the Au₄Al and AuAl₂ layers that form during bonding, and gradually becomes the dominant compound. Both AuAl₂ and Au₈Al₃ are transformed into the Au-rich alloyAu₄Al when Al is completely consumed, and this is the terminal product.     

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.     

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.     

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.     

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.     

Large deformation of thermally bonded random fibrous networks: microstructural changes and damage

A mechanical behaviour of random fibrous networks is predominantly governed by their microstructure. This study examines the effect of microstructure on macroscopic deformation and failure behaviour of random fibrous networks and its practical implication for optimisation of its structure by using finite-element simulations. A subroutine-based parametric modelling approach—a tool to develop and characterise random fibrous networks—is also presented. Here, a thermally bonded polypropylene nonwoven fabric is used as a model system. Its microstructure is incorporated into the model by explicit introduction of fibres according to their orientation distribution in the fabric. The model accounts for main deformation and damage mechanisms experimentally observed and provides the meso- and macro-level responses of the fabric. The suggested microstructure-based approach identifies and quantifies the spread of stresses and strains in fibres of the network as well as its structural evolution during deformation and damage. Its simulations also predict a continuous shift in the distribution of stresses due to structural evolution and progressive failure of fibres.     

Finite element simulation of low-density thermally bonded nonwoven materials: Effects of orientation distribution function and arrangement of bond points

A random and discontinuous microstructure is one of the most characteristic features of a low-density thermally bonded nonwoven material, and it affects their mechanical properties significantly. To understand their effect of microstructure on the overall mechanical properties of the nonwoven material, discontinuous models are developed incorporating random discontinuous structures representing microstructures of a real nonwoven material. Experimentally measured elastic material properties of polypropylene fibres are introduced into the models to simulate the tensile behaviour of the material for its both principle directions: machine direction and cross direction. Additionally, varying arrangements of bond points and schemes of fibres’ orientation distribution are implemented in the models to analyse the respective effects.     

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.