Michael Okereke

Fatigue life of lead-free solder thermal interface materials at varying bond line thickness in microelectronics

Microelectronics failure during operation is commonly attributed to ineffective heat management within the system. Hence, reliability of such devices becomes a challenge area. The use of lead-free solders as thermal interface materials to improve the heat conduction between a chip level device and a heat sink is becoming popular due to their promising thermal and mechanical material properties. Finite element modelling was employed in the analysis of the fatigue life of three lead-free solders (SAC105, SAC305, and SAC405) under commercial thermal cycling load (between −40 °C and 85 °C). This paper presents the results of the simulation work focusing on the effect of varying the solder thermal interface thickness (or bond line thickness) on the reliability of the microelectronic device. The results obtained were based on stress, strain, deformation, and plastic work density. The results showed that the fatigue life of the three solders increases as the solder thermal interface thickness increases. Also, the stresses, strains, and deformation were highest around the edges and vertices of the solder interface. In addition, the optimal solder material of choice based on the criteria of this research is given as SAC405. It has higher operational life span and good reliability capabilities.

HIGH‐RATE COMPRESSION OF POLYPROPYLENE

Three grades of polypropylene were tested in compression at room temperature, across an unusually wide range of strain rate: 10−4 to 104 s−1. The quasi‐static testing was done in a Hounsfield machine fitted with a digital image acquisition kit, while tests at the highest strain rates were carried out using a compression split Hopkinson pressure bar. The strain rate dependence of compressive yield stress was compared with the Eyring prediction, and found to be a nonlinear function of log10(strain‐rate). The nonlinearity is attributed to the presence of two relaxation processes in polypropylene, with differing activation volumes: the α‐ and β‐processes. According to the Bauwens two‐process model this would lead naturally to curved Eyring plots, where the apparent activation volume decreases with increasing strain‐rate. Another prominent feature in the experimental results was the increase in magnitude of post‐yield strain‐softening with increase in strain‐rate. This indicates that the dominant structural re...

The effect of thermal constriction on heat management in a microelectronic application

Thermal contact constriction between a chip and a heat sink assembly of a microelectronic application is investigated in order to access the thermal performance. The finite element model (FEM) of the electronic device developed using ANSYS software was analysed while the micro-contact and micro-gap thermal resistances were numerically analysed by the use of MATLAB. In addition, the effects of four major factors (contact pressure, micro-hardness, root-mean-squared (RMS) surface roughness, and mean absolute surface slope) on thermal contact resistance were investigated. Two lead-free solders (SAC305 and SAC405) were used as thermal interface materials in this study to bridge the interface created between a chip and a heat sink. The results from this research showed that an increase in three of the factors reduces thermal contact resistance while the reverse is the case for RMS surface roughness. In addition, the use of SAC305 and SAC405 resulted in a temperature drop across the microelectronic device. These results might aid engineers to produce products with less RMS surface roughness thereby improving thermal efficiency of the microelectronic application.

Ergonomic Assessment of an Active Orthosis for the Rehabilitation of Flexion and Extension of Wrist

Muscular stiffness and limb rigidity are two main consequences of Parkinson’s disease. These motor symptoms may be present in distinct parts of the body, influencing functional tasks executed by hands. To aid people suffering from these motor symptoms, we developed an active wrist orthosis whose purpose is to enable increase the flexion and extension range of motion of the wrist joint. We identified five relevant ergonomic variables that should be considered when using the orthosis in the clinical practice: (i) device stability, (ii) forearm position; (iii) muscular strength; (iv) amplitude of motion; and (v) mass of the device. These variables were identified based on the observation of movements while users executed the flexion and extension of the wrist with and without the device. In this research, we present a description of the developed orthosis and an evaluation of the ergonomic variables (i), (ii) and (iii). An enhanced support structure has been used with the orthosis and shown to lead to a stability improvement. Electromyographic analysis showed that the use of the orthosis does not introduce undue muscular load on the user at distinct angular positions of the forearm.

Design of Simple Finite Element Modelling Solver

This chapter presents a methodology for designing a simple finite element solver. The implementation is carried out inside MATLAB™ and the simulation engine is driven by the Direct Stiffness Method (DSM) presented in Chap. 3. In order to illustrate this, the authors have presented an in-house finite element solver they created called: MATLAB Finite Element Simulation Engine (MATFESE™). This chapter presents the structure, implementation and execution of MATFESE™. The chapter concludes by illustrating the robustness of MATFESE™ in tackling a range of structural mechanics problems. It is expected that after reading through this chapter, the reader can start creating simple finite element solver by utilizing the principles of the DSM. MATFESE ™ can be downloaded from the textbook website.

Material Response: Constitutive Models and Their Implementation

The material response of a defined virtual domain within FEM is defined using carefully formulated constitutive models. This chapter describes some of the most common in-built material models in commercially available FEM solvers. It is important to gain this theoretical, as well as practical, understanding in order for the user to interpret FEM outputs. The chapter concludes by walking the reader through the principles of developing user-defined material (umat) models. This is very important as not all known material behaviours have been modelled and validated as in-built material models within common FEM solvers. Where new materials are discovered, or even modifications and improvements desired for existing in-built material models, the FEM user usually resorts to describing computationally their versions of the constitutive mathematics to reflect such changes. The UMAT sub-routine within FEM solvers helps the reader to do so. This chapter is a necessary guide for reliable interpretation of FEM results as the knowledge gained here will help the reader query the predictive fidelity of the FEM framework in comparison with the known material response of the test material.

Direct Stiffness Method

At the core of the finite element modelling process are a diverse possible range of solution approaches for any particular problem. Each of these approaches are adapted for the type of problem that one is interested in, for example structural, fluid, thermal or acoustic problems. The commonest type of problems that FEM addresses are the structural and solid mechanics problems and the direct stiffness method is the heart of the solution strategy. This chapter describes the principles of the direct stiffness method. Simple truss elements are introduced as the crudest finite elements for demonstrating the direct stiffness method, although other more advanced discretization finite elements can also be used. The mechanics of the direct stiffness method will be explained. In particular, the discussion highlights the use of nodal properties for the truss elements to determine displacements, velocities, internal and external forces, etc. for a given truss system. The chapter concludes with practical example problems.

The Future of Finite Element Modelling

As powerful as finite element modelling (FEM) is, its future is expected to be both exciting and challenging. This chapter seeks to explore this future in order to identify strands of research that have to be carried out in order to sustain the impact of FEM within academic and industrial communities. This chapter starts by identifying the current challenges that limit the widespread adoption of this strategy of obtaining robust solutions to practical problems. After this, the discussion extends to an exploratory discourse on the future of FEM. In the next two decades, the landscape of FEM is expected to be significantly different from what pertains today. If this vision is to be achieved, the aspects highlighted in this chapter must be addressed. There is so much scope for improving the FEM process and this chapter makes the case for the sustained research around FEM so that this most essential design tool for engineers, and other allied experts, will continue to be sustained and advanced more and more even into the next century.

Computational Mechanics and the Finite Element Method

An essential part of an engineer’s training is the development of necessary skills to analyse and predict the behaviour of engineering systems under different loading conditions . Only a small proportion of real engineering problems can be solved analytically; hence, there arises the need to use numerical methods capable of accurately simulating real phenomena. The finite element method is one such widely used numerical method. This chapter introduces the principles of computational mechanics within the field of engineering mechanics . The finite element method is a key pillar in computational mechanics and this chapter explores the FE method within computational mechanics. Here, the necessity for the FE method and the limitations to its use in solving practical problems are established. The chapter concludes by discussing currently available FE solvers both as open source and proprietary versions.

Material Response: Measures of Stress and Strain

Amongst the many pillars upon which the FEM solutions stand is the pillar of material response. This defines the physical behaviour of the material type under investigation in the FEM problem. It is under this pillar that one distinguishes between rubbery, elastic, nonlinear, fracture and interface failure mechanisms. It is a key component of the FEM problem and must be correctly defined if one is to obtain reliable solutions. A common theme for this pillar of the FEM process is what is described as predictive modelling , which is the use of computational methods to determine the material behaviour of a given material. In this chapter, we have presented the principles of the material response module of an FEM scheme. The focus here is exploiting the principles of continuum mechanics in defining the response of a material body. Specifically, this chapter introduces the kinematics of finite deformation of a material body; measures of strains and stresses; and concludes with the practical formulations of stresses needed by engineers during the design process. Such practical stress formulations include: principal stresses, von Mises stresses, etc. This chapter lays down the theory needed to understand the material model implementations in the finite element solver.