Optimization and finite element analysis for reliable electronic packaging

This paper demonstrates a modeling and design approach that couples computational mechanics techniques with numerical optimisation and statistical models for virtual prototyping and testing in different application areas concerning reliability of eletronic packages. The integrated software modules provide a design engineer in the electronic manufacturing sector with fast design and process solutions by optimizing key parameters and taking into account complexity of certain operational conditions. The integrated modeling framework is obtained by coupling the multi-phsyics finite element framework - PHYSICA - with the numerical optimisation tool - VisualDOC into a fully automated design tool for solutions of electronic packaging problems. Response Surface Modeling Methodolgy and Design of Experiments statistical tools plus numerical optimisaiton techniques are demonstrated as a part of the modeling framework. Two different problems are discussed and solved using the integrated numerical FEM-Optimisation tool. First, an example of thermal management of an electronic package on a board is illustrated. Location of the device is optimized to ensure reduced junction temperature and stress in the die subject to certain cooling air profile and other heat dissipating active components. In the second example thermo-mechanical simulations of solder creep deformations are presented to predict flip-chip reliability and subsequently used to optimise the life-time of solder interconnects under thermal cycling.

Experimental and modeling analysis of the reliability of the anisotropic conductive films

Experiments as well as computer modeling methods have been used to investigate the effect of the solder reflow process on the electrical characteristics and reliability of anisotropic conductive film (ACF) interconnections. In the experiments, the contact resistance of the ACF interconnections was found to increase after a subsequent reflow and the magnitude of this increase was strongly correlated to the peak temperature of the reflow profile. In fact, nearly 40 percent of the joints were opened (i.e. lifted away from the pad) after the reflow with a peak temperature of 260 OC while no openings was observed when the peak temperature was 210 "C. It is believed that the CTE mismatch between the polymer particle and the adhesive matrix is the main cause of this contact degradation. To understand this phenomenon better, a 3-D model of an ACF joint structure was built and Finite Element Analysis was used to predict the stress distrihution in the conductive particles, adhesive matrix and metal pads during the reflow process. The effects of the peak temperature, the CTE of the adhesive matrix and the bump height on the reliability of the ACF interconnections were discussed.

A generic time and space accurate numerical approach to closely coupled fluid-structure interaction problems

Fluid structure interaction, as applied to flexible structures, has wide application in diverse areas such as flutter in aircraft, flow in elastic pipes and blood vessels and extrusion of metals through dies. However a comprehensive computational model of these multi-physics phenomena is a considerable challenge. Until recently work in this area focused on one phenomenon and represented the behaviour of the other more simply even to the extent in metal forming, for example, that the deformation of the die is totally ignored. More recently, strategies for solving the full coupling between the fluid and soild mechanics behaviour have developed. Conventionally, the computational modelling of fluid structure interaction is problematical since computational fluid dynamics (CFD) is solved using finite volume (FV) methods and computational structural mechanics (CSM) is based entirely on finite element (FE) methods. In the past the concurrent, but rather disparate, development paths for the finite element and finite volume methods have resulted in numerical software tools for CFD and CSM that are different in almost every respect. Hence, progress is frustrated in modelling the emerging multi-physics problem of fluid structure interaction in a consistent manner. Unless the fluid-structure coupling is either one way, very weak or both, transferring and filtering data from one mesh and solution procedure to another may lead to significant problems in computational convergence. Using a novel three phase technique the full interaction between the fluid and the dynamic structural response are represented. The procedure is demonstrated on some challenging applications in complex three dimensional geometries involving aircraft flutter, metal forming and blood flow in arteries.

Multi-physics-multi-scale simulation and optimisation: the next generation

FEA and CFD analysis is becoming ever more complex with an emerging demand for simulation software technologies that can address ranges of problems that involve combinations of interactions amongst varying physical phenomena over a variety of time and length scales. Computation modelling of such problems requires software technologies that enable the representation of these complex suites of 'physical' interactions. This functionality requires the structuring of simulation modules for specific physical phemonmena so that the coupling can be effectiely represented. These 'multi-physics' and 'multi-scale' computations are very compute intensive and so the simulation software must operate effectively in parallel if it is to be used in this context. Of course the objective of 'multi-physics' and 'multi-scale' simulation is the optimal design of engineered systems so optimistation is an important feature of such classes of simulation. In this presentation, a multi-disciplinary approach to simulation based optimisation is described with some key examples of application to challenging engineering problems.

Computational modelling of bubbles, droplets and particles in metals reduction and refining

The CFD modelling of metals reduction processes particularly always seems to involve the interaction of liquid metals, a gas (often air) top space, liquid droplets in the top space and injection of both solid particles and gaseous bubbles into the bath. These phases all interact and exhange mass, momentum and energy. Often it is the extent to which these multi-phase phemomena can be effectively captured within the CFD model which determines whether or not a tool of genuine use to the target industry sector can constructed. In this paper we discuss these issues in the context of two problems - one involving the injection of sparging gases into a steel continuous caster and the other based on the development of a novel process for aluminium electrolysis.

A comprehensive copper stockpile leach model: background and model formulation

The first phase in the sign, development and implementation of a comprehensive computational model of a copper stockpile leach process is presented. The model accounts for transport phenomena through the stockpile, reaction kinetics for the important mineral species, oxgen and bacterial effects on the leach reactions, plus heat, energy and acid balances for the overall leach process. The paper describes the formulation of the leach process model and its implementation in PHYSICA+, a computational fluid dynamic (CFD) software environment. The model draws on a number of phenomena to represent the competing physical and chemical features active in the process model. The phenomena are essentially represented by a three-phased (solid liquid gas) multi-component transport system; novel algorithms and procedures are required to solve the model equations, including a methodology for dealing with multiple chemical species with different reaction rates in ore represented by multiple particle size fractions. Some initial validation results and application simulations are shown to illustrate the potential of the model.

Thermal, mechanical and optical modelling of VCSEL packaging

Hybrid OECB (Opto-Electrical Circuit Boards) are expected to make a significant impact in the telecomm switches arena within the next five years, creating optical backplanes with high speed point-to-point optical interconnects. OECB's incorporate short range optical interconnects, and are based on VCSEL (Vertical Cavity Surface Emitting Diode) and PD (Photo Diode) pairs, connected to each other via embedded waveguides in the OECB. The VCSEL device is flip-chip assembled onto an organic substrate with embedded optical waveguides. The performance of the VCSEL device is governed by the thermal, mechanical and optical characteristics of this assembly. During operation, the VCSEL device will heat up and the thermal change together with the CTE mismatch in the materials, will result in potential misalignment between the VCSEL apertures and the waveguide openings in the substrate. Any degree of misalignment will affect the optical performance of the package. This paper will present results from a highly coupled modelling analysis involving thermal, mechanical and optical models. The paper will also present results from an optimisation analysis based on Design of Experiments (DOE).

Polymer waveguide and VCSEL array multi-physics modelling for OECB based optical backplanes [opto-electrical circuit boards]

The deployment of OECBs (opto-electrical circuit boards) is expected to make a significant impact in the telecomm switches arena within the next five years. This will create optical backplanes with high speed point-to-point optical interconnects. The crucial aspect in the manufacturing process of the optical backplane is the successful coupling between VCSEL (vertical cavity surface emitting laser) device and embedded waveguide in the OECB. The results from a thermo-mechanical analysis are being used in a purely optical model, which solves optical energy and attenuation from the VCSEL aperture into, and then through, the waveguide. Results from the modelling are being investigated using DOE analysis to identify packaging parameters that minimise misalignment. This is achieved via a specialist optimisation software package. Results from the thermomechanical and optical models are discussed as are experimental results from the DOE.

A modelling and experimental analysis of the no-flow underfill process for flip-chip assembly

In the flip-chip assembly process, no-flow underfill materials have a particular advantage over traditional underfills as the application and curing of this type of underfill can be undertaken before and during the reflow process - adding high volume throughput. Adopting a no-flow underfill process may result in underfill entrapment between solder and fluid, voiding in the underfill, a possible delamination between underfill and surrounding surfaces. The magnitude of these phenomena may adversely affect the reliability of the assembly in terms of solder joint thermal fatigue. This paper presents both an experimental and mdeling analysis investigating the reliabity of a flip-chip component and how the magnitude of underfill entrapment may affect thermal-mechanical fatigue life.

Constitutive modeling of kinematic-hardening and damage in solder joints

Solder constitutive models are important as they are widely used in FEA simulations to predict the lifetime of soldered assemblies. This paper briefly reviews some common constitutive laws to capture creep in solder and presents work on laws capturing both kinematic hardening and damage. Inverse analysis is used to determine constants for the kinematic hardening law which match experimental creep curves. The mesh dependence of the damage law is overcome by using volume averaging and is applied to predict the crack path in a thermal cycled resistor component

Optoelectronic packaging interfaces for submicron alignment (OPISA) and the dynamics of laser spot weld formation

The work presented in this paper is part of the OPISA project. This is a collaborative research project between the University of Greenwich and Bookham Technology. This report describes some of the initial work undertaken towards the goal of investigating optoelectronic packaging where alignment issues between optical sources and fibers can arise as part of the fabrication process. The focus of this study is on charting the dynamics of laser spot weld formation. This paper introduces some of the initial simulation work that has been undertaken and presents a model describing a transient heat source applied from a laser pulse to weld a stainless steel sleeve and ferrule and the resulting weld formation

What students can tell about lecturers when they are teaching: manifestations of confident and under-confident lecturers

Delivering a lecture requires confidence, a sound knowledge and well developed teaching skills (Cooper and Simonds, 2007, Quinn and Hughes, 2007). However, practitioners who are new to lecturing large groups in higher education may initially lack the confidence to do so which can manifest itself in their verbal and non-verbal cues and the fluency of their teaching skills. This results in the perception that students can identify the confident and non-confident teacher during a lecture (Street, 2007) and so potentially contributing to a lecturer’s level of anxiety prior to, and during, a lecture. Therefore, in the current educational climate of consumerisation, with the increased evaluation of teaching by students, having the ability to deliver high-quality, informed, and interesting lectures assumes greater significance for both lecturers and universities (Carr, 2007; Higher Education Founding Council 2008, Glass et al., 2006). This paper will present both the quantitative and qualitative data from a two-phase mixed method study with 75 nurse lecturers and 62 nursing students in one university in the United Kingdom. The study investigated the notion that lecturing has similarities to acting (Street, 2007). The findings presented here are concerned with how students perceived lecturers’ level of confidence and how lecturers believed they demonstrated confidence. In phase one a specifically designed questionnaire was distributed to both lecturers and students and a response rate of 91% (n=125) was achieved, while in phase two 12 in-depth semi-structured interviews were conducted with lecturers. Results suggested that students in a lecture could identify if the lecturer was confident or not by the way they performed a lecture. Students identified 57 manifestations of non-confidence and lecturers identified 85, while 57 manifestations of confidence were identified by students and 88 by lecturers. Overall, these fell into 12 main converse categories, ranging from body language to the use of space within the room. Both students and lecturers ranked body language, vocal qualities, delivery skills, involving the students and the ability to share knowledge as the most evident manifestations of confidence. Elements like good eye contact, smiling, speaking clearly and being fluent in the use of media recourses where all seen as manifestations confidence, conversely if these were poorly executed then a presentation of under confidence was evident. Furthermore, if the lecturer appeared enthusiastic it was clearly underpinned by the manifestation of a highly confidence lecturer who was secure in their knowledge base and teaching abilities: Some lecturers do appear enthusiastic but others don’t. I think the ones that do know what they are talking about, you can see it in their voice and in their lively body language. I think they are also good at involving the students even. I think the good ones are able to turn boring subjects into lively and interesting ones. (Student 50) Significantly more lecturers than students felt the lecturer should appear confident when lecturing. The lecturers stated it was particularly important to do so when they did not feel confident, because they were concerned with appearing capable. It seems that these students and lecturers perceived that expressive and apparently confident lecturers can make a positive impact on student groups in terms of involvement in lectures; the data also suggested the reverse, for the under confident lecturer. Findings from phase two indicated that these lecturers assumed a persona when lecturing, particularly, but not exclusively, when they were nervous. These lecturers went through a process of assuming and maintaining this persona before and during a lecture as a way of promoting their internal perceptions of confidence but also their outward manifestation of confidence. Although assuming a convincing persona may have a degree of deception about it, providing the knowledge communicated is accurate, the deception may aid rather than hinder learning, because enhances the delivery of a lecture. Therefore, the deception of acting a little more confidently than one feels might be justified when the lecturer knows the knowledge they are communicating is correct, unlike the Dr Fox Effect where the person delivering a lecture is an actor and does not know the subject in any detail or depth and where the deception to be justified (Naftulin, et al., 1973). In conclusion, these students and lecturers perceive that confident and enthusiastic lecturers communicate their passion for the subject in an interesting and meaningful manner through the use of their voice, body, space and interactions in such a way that shows confidence in their knowledge as well as their teaching abilities. If lecturers, therefore, can take a step back to consider how they deliver lectures in apparently confident ways this may increase their ability to engage their students and not only help them being perceived as good lecturers, but also contribute to the genuine act of education.

Design process knowledge reuse challenges and issues

Product knowledge support needs are compared in two companies with different production volumes and product complexity. Knowledge support requirements identified include: function, performance data, requirements data, common parts, regulatory guidelines and layout data. A process based data driven knowledge reuse method is evaluated in light of the identified product knowledge needs. The evaluation takes place through developing a pilot case with each company. It is found that the method provides more benefit to the high complexity design domain, in which a significant amount of work takes place at the conceptual design stages, relying on a conceptual product representation. There is not such a clear value proposition in a design environment whose main challenge is layout design and the application of standard parts and features. The method supports the requirement for conceptual product representation but does not fully support a standard parts library.

The dependence of power consumption of pneumatic conveying systems upon bulk material properties, pipeline bore and routing, and mode of flow

In this paper the dependence of the power consumption of pneumatic conveyors upon conveyed materials, pipeline route and bore, and mode of flow has been examined. The findings are that, with different materials and modes of flow, not only is the amount of power consumed very different but it varies in different ways with pipe bore and routing. Additionally it has been found that, for any given conveying system, the choice of air mover also has a strong influence on the power requirement.