Predicting the behaviour of micro-electronic displays using computational mechanics

Micro-electronic displays are indispensible devices used in high performance applications such as aerospace, medical, marine and industrial sectors.These devices provide an interface to real time mission critical devices and therefore require good optical visual performance and high reliability, all this within varied and challenging environments.

Application of modelling for predicting the behaviour of polymers and adhesives in microelectronic and photonic devices

With the growth in computing power, and advances in numerical methods for the solution of partial differential equations, modeling technologies based around computational fluid dynamics, finite element analysis and optimisation are now being widely used by researchers and industry. Polymer and adhesive materials are now being widely used in electronic and photonic devices. This paper will illustrate the use of modeling tools to predict the behaviour of these materials from product assembly to its performance and reliability.

Effect of fluid dynamics and device mechanism on biofluid behaviour in microchannel systems: modelling biofluids in a microchannel biochip separator

Biofluid behaviour in microchannel systems is investigated in this paper through the modelling of a microfluidic biochip developed for the separation of blood plasma. Based on particular assumptions, the effects of some mechanical features of the microchannels on behaviour of the biofluid are explored. These include microchannel, constriction, bending channel, bifurcation as well as channel length ratio between the main and side channels. The key characteristics and effects of the microfluidic dynamics are discussed in terms of separation efficiency of the red blood cells with respect to the rest of the medium. The effects include the Fahraeus and Fahraeus-Lindqvist effects, the Zweifach-Fung bifurcation law, the cell-free layer phenomenon. The characteristics of the microfluid dynamics include the properties of the laminar flow as well as particle lateral or spinning trajectories. In this paper the fluid is modelled as a single-phase flow assuming either Newtonian or Non-Newtonian behaviours to investigate the effect of the viscosity on flow and separation efficiency. It is found that, for a flow rate controlled Newtonian flow system, viscosity and outlet pressure have little effect on velocity distribution. When the fluid is assumed to be Non-Newtonian more fluid is separated than observed in the Newtonian case, leading to reduction of the flow rate ratio between the main and side channels as well as the system pressure as a whole.

Investigation of wall-slip behaviour in lead-free solder pastes and isotropic conductive adhesives

Slippage due to wall depletion effect is well-known in rheological investigation. The aim of this study was to investigate the influence of the paste microstructure on slip formation for the paste materials (lead-free solder paste and isotropic conductive adhesives). The effect of different flowgeometries, gap heights and surface roughness on the paste viscosity was investigated. The utilisation of different measuring geometries has not clearly showed the presence of wall-slip in the paste samples. The existence of wall-slip was found to be pronounced when gap heights were varied using the parallel plate geometry. It was also found that altering the surface roughness of the parallel plate measuring geometry did not significantly eliminate wall-slip as expected. But results indicate that the use of a relatively rough surface helps to increase paste adhesion to the plates and to a certain extent inducing structural breakdown in the paste. Most importantly, the study also demonstrated on how the wall-slip formation in the paste material could be utilised for understanding of the paste microstructure and its flow behaviour.

The effect of wall-slip formation on the rheological behaviour of lead-free solder pastes

Wall-slip plays an important role in the flow behaviour of solder paste materials. The wall-slip arises due to the various attractive and repulsive forces acting between the solder particles and the walls of the measuring geometry. These interactions could lead to the presence of a thin liquid layer adjacent to the wall, which causes slippage. The aim of this study is to investigate the influence of the solder paste formulation on wall-slip formation and its effect on the printability of these pastes material. A wall slip model is utilised to calculate the true viscosity and slip velocity for the lead-free solder pastes samples used in this study. The difference in the measured viscosity and the true viscosity could indicate wall-slip formation between the solder pastes and the parallel plate. Sample P1 showed a higher slip velocity compared to sample P2. The slip velocity calculated for the solder pastes could be used as a performance indicator to understand the paste release characteristics in the stencil printing process.

Modelling the time-dependent behaviour of solder pastes used in the electronics assembly applications

Solder paste is the most widely used bonding material in the assembly of surface mount devices in electronic industries. It generally has a flocculated structure (show aggregation of solder particles), and hence are known to exhibit a thixotropic behavior. This is recognized by the decrease in apparent viscosity of paste material with time when subjected to a constant shear rate. The proper characterisation of this timedependent rheological behaviour of solder pastes is crucial for establishing the relationships between the pastes’ structure and flow behaviour; and for correlating the physical parameters with paste printing performance. In this paper, we present a novel method which has been developed for characterising the timedependent and non-Newtonian rheological behaviour of solder pastes as a function of shear rates. The objective of the study reported in this paper is to investigate the thixotropic build-up behaviour of solder pastes. The stretched exponential model(SEM) has been used to model the structural changes during the build-up process and to correlate model parameters with the paste printing process.

Study of the rheological behaviour of Sn-Ag-Cu solder pastes and their correlation with printing performance

Solder paste plays an important role in the electronic assembly process by providing electrical, mechanical and thermal bonding between the components and the substrate. The rheological characterisation of pastes is an important step in the design and development of new paste formulations. With the ever increasing trend of miniaturisation of electronic products, the study of the rheological properties of solder pastes is becoming an integral part in the R&D of new paste formulations and in the quality monitoring and control during paste manufacture and electronic assembly process. This research work outlines some of the novel techniques which can be successfully used to investigate the rheology of leadfree solder pastes. The report also presents the results of the correlation of rheological properties with solder paste printing performance. Four different solder paste samples (namely paste P1, P2, P3 and P4) with different flux vehicle systems and particle size distributions were investigated in the study. As expected, all the paste samples showed shear thinning behaviour. Although the samples displayed similar flow behaviour at high shear rates, differences were observed at low shear rates. In the stencil printing trials, round deposits showed better results than rectangular deposits in terms of paste heights and aperture filling. Our results demonstrate a good correlation between higher paste viscosity and good printing performance. The results of the oscillatory and thixotropy tests were also successfully correlated to the printing behaviour of solder paste.

Modelling of the time-dependent flow behaviour of lead-free solder pastes used for flip-chip assembly applications

The market for solder paste materials in the electronic manufacturing and assembly sector is very large and consists of material and equipment suppliers and end users. These materials are used to bond electronic components (such as flip-chip, CSP and BGA) to printed circuit boards (PCB's) across a range of dimensions where the solder interconnects can be in the order of 0.05mm to 5mm in size. The non-Newtonian flow properties exhibited by solder pastes during its manufacture and printing/deposition phases have been of practical concern to surface mount engineers and researchers for many years. The printing of paste materials through very small-sized stencil apertures is known to lead to increased stencil clogging and incomplete transfer of paste to the substrate pads. At these very narrow aperture sizes the paste rheology and particle-wall interactions become crucial for consistent paste withdrawal. These non-Newtonian effects must be understood so that the new paste formulations can be optimised for consistent printing. The focus of the study reported in this paper is the characterisation of the rheological properties of solder pastes and flux mediums, and the evaluation of the effect of these properties on the pastes' printing performance at the flip-chip assembly application level. Solder pastes are known to exhibit a thixotropic behaviour, which is recognised by the decrease in apparent viscosity of paste material with time when subjected to a constant shear rate. The proper characterisation of this time-dependent theological behaviour of solder pastes is crucial for establishing the relationships between the pastes' structure and flow behaviour; and for correlating the physical parameters with paste printing performance. In this paper, we present a number of methods which have been developed for characterising the time-dependent and non-Newtonian rheological behaviour of solder pastes and flux mediums as a function of shear rates. We also present results of the study of the rheology of the solder pastes and flux mediums using the structural kinetic modelling approach, which postulates that the network structure of solder pastes breaks down irreversibly under shear, leading to time and shear dependent changes in the flow properties. Our results show that for the solder pastes used in the study, the rate and extent of thixotropy was generally found to increase with increasing shear rate. The technique demonstrated in this study has wide utility for R&D personnel involved in new paste formulation, for implementing quality control procedures used in solder paste manufacture and packaging; and for qualifying new flip-chip assembly lines

Influence of solder paste components on rheological behaviour

Rheological properties of solder pastes are very important for a high quality surface mount technology process. The stencil/screen printing process of solder pastes is one of the most critical steps in the SMT assembly process, as most of the assembly defects can often be shown to originate from paste rheology and associated poor printing performance. This paper concerns an investigation of the effect of solder paste composition on the rheological properties and behaviour of four different solder pastes. We report on the evaluation of three different paste formulations based on the no-clean flux composition, with different alloy composition, metal content and particle size using a range of rheological characterisation techniques - including viscosity measurements, yield stress, oscillatory and creep-recovery tests. Our results show that in the viscosity test, all solder pastes exhibited a shear thinning behaviour in nature with different highest maximum viscosity. In the region of shear thinning behaviour the paste 3 delivered the best results. Viscosity test helps to understand the solid and cohesive behaviour of solder pastes. Good solid and cohesive behaviour indicates a good paste roll and helps to avoid paste bleeding. The yield stress test has been used to study the effect of temperature on the flow behaviour of solder pastes. Yield stress was measured for a range of temperature from 15deg C to 35deg C with an increment of 5degC. The result indicated a decreasing of the yield stress point if the temperature was increased. Paste 4 has shown the minimum dependence on temperature. The oscillatory test has been used to find out the linear visco-elastic range and to study the solid and liquid like behaviours of solder pastes. Paste 1 indicated the biggest linear visco-elastic region (LVR) and the highest value of G' and G" which means solder paste 1 will be needed a higher squeegee pressure in the printing process. In the creep recovery test paste 4 showed the best- - recovery and the lowest values of creep and recovery compliance which indicated a good printing behaviour. The test also has showed the solder paste with smaller particle size exhibit less recovery

Effect of temperature on slumping behaviour of lead-free solder paste and its rheological simulation

Variation in temperature can have a significant impact on the rheological characterisation of solder pastes used in the electronic assembly of surface mount devices. This paper concerns the study of the effect of temperature on slumping characteristics of lead-free solder pastes. The identification of the slumping characteristics can help in the correlation of the pastes characteristics to its printing performance. Further issues, which aid in justifying the undertaking of such a study, include the temperature differences identified both at the squeegee during the print, and during reflow. Due to these temperature variations, it is imperative to understand how slump differs with a temperature gradient

Correlation Between the Electrical and Optical Properties of an Atmospheric Pressure Plasma During Siloxane Coating Deposition

The effect of varying process parameters on atmospheric plasma characteristics and properties of nanometre thick siloxane coatings is investigated in a reel-to-reel deposition process. Varying plasma operation modes were observed with increasing applied power for helium and helium/oxygen plasmas. The electrical and optical behaviour of the dielectric barrier discharge were determined from current/voltage, emission spectroscopy and time resolved light emission measurements. As applied power increased, multiple discharge events occurred, producing a uniform multi-peak pseudoglow discharge, resulting in an increase in the discharge gas temperature. The effects of different operating modes on coating oxidation and growth rates were examined by injecting hexamethyldisiloxane liquid precursor into the chamber under varying operating conditions. A quenching effect on the plasma was observed, causing a decrease in plasma input power and emission intensity. Siloxane coatings deposited in helium plasmas had a higher organic component and higher growth rates than those deposited in helium/oxygen plasmas.

Detection of aberrant behaviour in home environments from video sequence

We introduce an application for the detection of aberrant behaviour within home based environments, with a focus on repetitive actions, which may be present in instance of persons suffering from dementia. Video based analysis has been used to detect the motion of a person within a given scene in addition to tracking them over the time. Detection of repetitive actions has been based on the analysis of a person's trajectory using the principles of signal correlation. Along with the ability to detect repetitive motion the developed approach also has the ability to measure the amount of activity/inactivity within the scene during a given period of time. Our results showed that the developed approach had the ability to detect all patterns in the data set examined with an average accuracy of 96.67%. This work has therefore validated the proposed concept of video based analysis for the detection of repetitive activities.

Electrical estimulation of retinal pigment epithelial cells

We investigated and characterized the effect of externally applied electric fields (EF) on retinal pigment epithelial (RPE) cells by exposing primary cultures of human RPE cells (hRPE) and those from the ARPE19 immortalized cell line to various strengths of EF (EF-treated cells) or to no EF (control cells) under different conditions including presence or absence of serum and gelatin and following wounding. We evaluated changes in RPE cell behavior in response to EF by using a computer based image capture and analysis system (Metamorph). We found that RPE cells responded to externally applied EFs by preferential orientation perpendicular to the EF vector, directed migration towards the anode, and faster translocation rate than control, untreated cells. These responses were voltage-dependent. Responses were observed even at low voltages, of 50-300 mV. Furthermore, the migration of hRPE cell sheets generated by wounding of confluent monolayers of cells at early and late confluence could be manipulated by the application of EF, with directed migration towards the anode observed at both sides of the wounded hRPE. In conclusion, RPE cell behaviour can be controlled by an externally applied EF. The potential for externally applied EF to be used as a therapeutic strategy in the management of selected retinal diseases warrants further investigation. © 2010 Elsevier Ltd.

The spark of life:The role of electric fields in regulating cell behaviour using the eye as a model system

Endogenous electric fields (EF) have long been known to influence cell behaviour during development, neural cell tropism, wound healing and cell behaviour generally. The effect is based on short circuiting of electrical potential differences across cell and tissue boundaries generated by ionic segregation. Recent in vitro and in vivo studies have shown that EF regulate not only cell movement but orientation of cells during mitosis, an effect which may underlie shaping of tissues and organs. The molecular basis of this effect is founded on receptor-mediated cell signalling events and alterations in cytoskeletal function as revealed in studies of gene deficient cells. Remarkably, not all cells respond directionally to EF in the same way and this has consequences, for instance, for lens development and vascular remodelling. The physical basis of EF effect may be related to changes induced in 'bound water' at the cell surface, whose organisation in association with trans-membrane proteins (e.g. receptors) is disrupted when EF are generated. Copyright © 2007 S. Karger AG.