1-1. Background
As technology advances, use of adhesives is becoming ever more widespread because adhesive can also help make it easier to manufacture products and be used extensively to bond metallic, ceramic, plastic and composite components in many fields where structures are subject to high levels of service. Therefore, nowadays, product designers and manufacturing engineers rely on adhesives more than ever for greater design flexibility, more efficient production, and improved performance. In addition, adhesives applied to joints have been used for many years in aircraft structures and in many other applications including particular aircraft repair, civil engineering, automotive engineering, medical field, and the electronics industry.
Adhesive bonding has many advantages over the conventional fastening techniques such as welding, riveting and bolting because its application does not require high temperatures in welding and hole in structure component like the cases of riveting and bolting. Thereby, stress concentration in the adhesive joints is less than that caused by high temperature of welding as well as hole of rivets and bolts; stress distributions of adhesive joints are more uniform.
Additionally, using adhesive bonding has the substantial benefit of weight reduction that is an important advantage, especially for lightweight structures. Therefore, the use of adhesive materials as a means for assembly of structure is being accepted as an alternative means to conventional joining processes. Except weight reduction, the advantages of structural adhesive bonding over other joining techniques include cost savings (including lower labor costs), elimination of stress point concentrations by even more uniform distribution of stress over the entire bonded joint described above, bonding of dissimilar materials, and resistance to shock as well as vibration et al.. In Addition, adhesive bonding applied to composites is
being increasingly used in structural applications, which is also justified by its well-known advantages over mechanically fastened joints: fewer sources of stress concentrations, more uniform distribution of load, and better fatigue properties. Hence, the use of adhesives is more widespread than ever in technically demanding applications.
Though adhesive bonding has many aforementioned advances and great potential, it, however, results in some inconveniences. For instance, adhesive bonding is almost always irreversible; in other words, to disassemble the bonding without damaging the structural components is not easy.
1-1-1. Adhesive
As for adhesively bonded joints applied to structure component, adhesive selection is very important. The selection criteria are based upon material information, joint type and loading condition et al.. Material information involves the characteristics of adhesives and adherends as well as the boned strength of adhesive. Understanding these characteristics is very important for selecting suitable adhesive employed in bonded joints. Because there is a great variety of adhesives over 18 different generic types of adhesives as well as numerous sub-types and hybrids of the adhesive, the selection of the most suitable adhesive for the adhesive joint probably is one of the most daunting areas in the design process.
In this research, only radiation-cured adhesives are introduced because they are often applied to the IC chip pick-up process. They become active and cured when exposed to radiation, usually ultra-violet (UV) light. The mechanism depends upon special modifications to the monomer's structure and the inclusion of light-sensitive compounds that start the reaction. Also, they are also widely used for bonding glass, ceramics and transparent plastics.
However, some tapes of radiation-cured adhesives are applied to the IC pick-up process.
These adhesives make it possible to achieve a higher tack before the exposure of UV light while ensuring the easy removal of the adhesives and the reduction of boned strength after exposure. That is to say, as exposure to UV light source breaks adhesive bond, the tack of the adhesive can be reduced. Specially, the adhesives offer worse resistance to peel force.
1-1-2. Joint Type
There are many sub-types of single lap joints as shown in. Generally speaking, single lap joints are the simplest joint geometry and usually used in structural joints but its shortcoming is that peel stresses still arise in this joint. Other joint geometries may be considered to reduce peel stresses; for example, butt joint, double lap joint and scarf joint as shown in Figs 1.2, 1.3 and 1.4 are better selection to reduce peel stresses. Especially, as scarf joints allow a large adhesive contact area, the joint is an ideal joint for eliminating peel stresses, but parts joined in this way must maintain a close fit; however, in practice these joints are harder to create and are not suitable for use with thin sheet adherends.
Joint design requires selection of the correct joint types depending on loading condition, geometrical shape and assembly procedures. Joint design should minimize stress concentrations by ensuring that the load is distributed over the entire bonded area. Some stresses, such as peel, cleavage, and shear stresses, should be minimized (see Fig. 1.5) because these stresses will cause the failure of adhesive joint. Most adhesives applied to structure components withstand tensile stress well, so maximizing the tensile stress and minimizing others is one of the vital targets in designing the structure components but the tensile stress is less than the critical stress of adhesives. Joint type should serve to improve bond strength. It may be important to choose the most suitable joint type of geometrical structures because this joint type minimizes the peeling or shear stresses at the edges of the
overlap in the well-bonded situation. Nevertheless, in the IC chip pick-up process, it is expected that at the edges of joint, the peeling and shear stresses can be maximized and their values also can exceed the critical stresses of adhesive; and IC chips whose stress distribution can be minimized do not fail during the process.
Fig. 1.1 Singe lap joints
Fig. 1.2 Butt joints
Fig. 1.3 Double lap joints
Fig. 1.4 Scarf lap joints
Fig. 1.5 Stress catalogy
Fig. 1.6 depicts the IC manufacture procedure [1], to elucidate the causes of failure. A wafer is stuck in tape, and polished thinner and flatter. Then, it is removed from the tape and
stuck to the blue tape. The wafer can be fixed to the blue tape by radiation-cured adhesive, and cut into pieces (called IC chips) by a diamond cutter. Sequentially, in the IC chip pick-up process, the IC chips must be pierced and broken off by using the piercer, before being separated from the blue tape without any cracks. However, as IC chips are getting shorter and thinner, IC chips easily fail during IC manufacture. As shown in Fig. 1.7, adhesively bonded joint is applied to IC chip pick-up process. In the adhesively bonded joint, both adherends subjected to a concentrated force, are bonded by an adhesive under the pin-pin boundary conditions. Because its joint type is different from the aforementioned joint types but this joint is a three-laminar structure which is similar as that of a single lap joint, this dissertation mainly investigates the joint. Specially, joint’s adherends are consisted of different materials.
To sum up, the use of adhesively bonded joint keeps increasing but there are still some important issues, such as stress distributions of the joints, to be explored. In this study, the stress distributions of the joints are affected by the key factors which involve the consideration of a variety of geometries, material properties of adhesive and blue tape, and loading conditions.
To perform stress analysis requires reliable and efficient closed-form solutions (analytical solutions) to obtain stress distribution of bonded joints. A large variety of models have been developed to analyze the adhesively bonded joint. Some of these techniques yield closed-form solutions, which generally involve some simplified assumptions. Many of them are limited to a certain range of geometries or loading conditions. Therefore, symbolic manipulation was employed to derive the reliable, efficient and complicated closed-form solutions which can be linked by the C++ program of genetic algorithm. Though finite element methods have provide a general tool to analyze arbitrary geometries and loading conditions, and have been extensively used with success, however, this kind of method requires much finer meshing in the issue and a large set of nodes in order to obtain reasonably accurate results. This needs a
large investment in engineering time and computer resources.
Fig. 1.6 Processes in the IC manufacturing procedure. [1]
Adhesive layer Upper adherend
Lower adherend
Fig. 1.7 Two adherends of the adhesively bonded joint bonded by an adhesive layer.
1-2. Objectives
The main objective of this study is to derive closed-form solutions (analytic solutions) linked with the C++ program of genetic algorithm to predict the behavior of the adhesively bonded joint in the IC chip pick-up process. To achieve the main objective, the following objectives are indispensable:
1. To obtain closed-form solutions (analytic solutions) to peeling and shear stresses for the adhesive layer, and to normal stress due to bending moment and longitudinal force of adherends as well as to displacement and slope of adherends.
2. Close-form solutions applied to cantilever beam strengthened by adhesively bonding and a single lap joint.
3. The effect of geometric shapes, action point of concentrated force and material properties of adhesive and adherends on peeling and shear stresses.
4. Examining whether adhesive joints are in the well-bonded situation or not.
5. To apply Genetic algorithm linked with analysis of adhesive joint in the IC chip pick-up process, and to discuss the analytical results and experimental results.
6. Investigating how to improve success rate in the IC chip pick-up process.
1-3. Significance and Limitations
In this study, a concentrated force is applied to both adherends bonded by an adhesive under the pin-pin boundary conditions. The aim of the proposed research is to attain closed-form solutions that will include the most relevant factors of the adhesively bonded joint.
These solutions are able to be linked with the C++ program of genetic algorithm since it is still somewhat difficult to converge and directly solve the differential equations by using the numerical method. However, Cornell [29], who claimed that obtaining complete theoretical solutions (closed-form solutions) to this problem would be very difficult, only considered a cantilever beam consisting of the same adherends. Only if the characteristic solutions of these equations have considerable values can his method produce classical solutions for the differential equations. As obtaining analytical solutions (closed-form solutions) is even more difficult here than in the work of Cornell [29], the model uses symbolic manipulation to solve the coupled differential equations in the Mathematica package, thereby enabling to find complete and complicated solutions that are not limited to solving only the characteristic solutions having large values (i.e. the characteristic solutions had to have large values [29]).
Estimating the peel and shear stresses of the adhesive between the IC chip and the blue tape is very important for the adhesive joint in the IC chip pick-up process. The results found in the experiments [45] are that as the thickness 0.1 mm of IC chips subjected to 4.8N, IC chips are easy to fail while as thickness, 0.34 mm of IC chips subjected to 3.5N, IC chips without crack can completely be separated from the blue tape. These closed-form solutions may be applied to analyze the behavior of adhesive in IC chip pick-up process. Therefore, genetic algorithm associated with these closed-form solutions aims to seek the suitable characteristic of adhesive material and blue tape to be able to reduce the failure of IC chips in the IC chip pick-up process. This research shows that conclusions drawn can increase the
success rate of IC chips which without crack, can be successfully separated from blue tape during IC chip pick-up process.
The closed-form solutions are only applied to a single lap joint. Adhesively bonded joint must be based on linear and elastic theory as well as small-deflection (Euler) beam theory under small deflection assumption.
1-4. Dissertation Outlines
In order to carry out the objective described before, the following chapters further illustrate how to accomplish the targets in more details. Here these chapters are briefly introduced in this section. Chapter 2 is devoted to discussing the related literatures regarding adhesively bonded joints and genetic algorithm. In regard to adhesively bonded joints, some published papers, basing on thermal or external load, material properties, plastic behavior, crack analysis, and strengthening structure, are introduced in order. As for genetic algorithm, here discuss some articles including penalty function, adaptive search techniques and its application to adhesively bonded joint. Next, some methods employed to solve the adhesively bonded joint are investigated.
Chapter 3 is dedicated to analysis of adhesive joint and mainly develops theoretical model of the adhesively bonded joint applied to the IC chip pick-up process. The use of symbolic manipulation is employed to solve the closed-form solutions (analytical solutions) to the adhesively bonded joint. These closed-form solutions involve the expressions of the transverse and longitudinal displacements, longitudinal and shear forces, moment in the adherends as well as peel and shear stress of the adhesive. These expressions are also shown to be correct by re-substituting them into coupled differential equations and by comparing the
results of the examples in references [29-30] with those obtained by the application of the expressions to solving those examples.
Sequentially, the IC chip pick-up problem is solved by using these closed-form solutions on which boundary and constraint conditions are imposed. Then, under some conditions, examine whether adhesively bonded joints are in the well-bonded situation or not.
Chapter 4 focuses on comparing the results of the experiments [45] with those of the analysis of adhesive joint and drawing conclusions which can increase the success rate of IC chips in the process. In the experiments, because of the different thicknesses, 0.1mm, and 0.34mm of IC chips, the success rate of the IC chip pick-up process has a great difference.
The 0.1mm IC chips nearly fail in slower speed but the 0.34mm IC chips without breakage can almost be completely separated from blue tape. These phenomena are discussed by theoretical model. However, because the characteristics of the adhesive layer are not easily found, genetic algorithm with penalty function, associates with analysis of adhesive joint method to solve the thickness and mechanical properties of the adhesive layer. The program of the genetic algorithm is written by the C++ language. Some conditions are proposed to improve the success rate in the pick-up IC process.
Chapter 5 draws conclusions and discussions about further works of this study in the future.