A shaft-loaded blister test for elastic response and delamination behavior of thin film-substrate system

The elastic response of a thin film of photoresist deposited on a silicon wafer is studied by using a shaft-loaded blister test method developed recently. Experiment data are compared with an analytical solution. Results demonstrated that under shaft loading, the thin film underwent a pure bending mode at small deformation and gradually transformed to a pure stretching mode at larger deformation. The effect of residual stress on elastic response is also studied. The delamination of thin film from substrate can be successfully measured under displacement control mode by the shaft-loaded blister test.     

Strain energy release rate in shaft-loaded blister tests for composite repairs on steel

The design of composite repairs of corroded oil and gas pipelines must take into account the strength of the interface adhesion between composite and metal. A shaft-loaded blister test is a common method to measure interface fracture toughness and energy release rate. The study aimed on evaluating shaft-loaded blister tests as replacements for more complex pressure blister tests. Specimens investigated were thick fibre-reinforced plates bonded on metal disks as substrates containing a circular through-hole defect. This paper presents the influence of different punch head geometries on the resulting energy release rates and compares the results with blister tests using fluid pressure. Test and simulation results are presented and analytical solutions were derived and evaluated to establish best fitting formulations. It was shown, that significant variations between the different means of loading exist.     

鼓膜实验法测试薄膜基体界面结合强度的进展

评述了用鼓膜实验方法测试薄膜与基体界面结合强度的现状和最新进展，分析了该方法针对圆孔、膜内无残余应力柔性膜试样和矩形孔或圆孔、膜内有残余应力试样（薄膜为柔性膜或刚性膜）所用解析解力学模型建立的薄膜力学性能假设前提条件、能量平衡思路和推导过程．介绍了用硅微技术制备Si基体试样的过程，指出了制备方法和解析解力学模型的特点及所存在的问题．给出了该实验方法的应用实例，指出了需要解决非硅基体的制样和薄膜塑性变形阶段力学模型等问题．     

Fracture mechanics analysis of thin coatings under spherical indentation

Spherical indentation of a thin, hard coating bonded to a thick substrate is investigated. The bending of the coating over the softer substrate induces concentrated tensile stresses on the lower and upper coating surfaces, from which transverse cracks may ensue. This work is primarily concerned with ring cracks originating from the top surface of the coating. In-situ indentation tests are carried out on a model glass/polycarbonate bi-layer, with the coating thickness and the indenter radius being the main test variables. As the coating thickness is decreased, the critical load to initiate ring cracks progressively departs from that associated with a critical surface stress, the effect that increases with increasing the indenter radius. A fracture mechanics approach in conjunction with the FEM technique is used to elucidate the onset of cylindrical ring cracks in thin-film bi-layer structures due to spherical indentation. The analysis, conducted as a function of the coating thickness and the indenter radius, reveals the existence of bending-induced compression stress regions ahead of the crack tip, which tend to shield the crack or increase the fracture resistance. The specific behavior is dictated by a complex interplay between the contact radius, a, the coating thickness, d, and the crack length, c. An interesting manifestation of this shielding mechanism is that when the coating surface contains flaws of various sizes, small flaws in this population may be more detrimental than large ones. Incorporation of this aspect into the analysis led to a good correlation with the experimental results. In the limit case of point-load, a closed-form, approximate solution for the stress intensity factors and the critical loads is obtained. This solution constitutes a lower bound for the critical loads, and is furthermore directly applicable to finite size indenters provided da. In the limit c/d/to0, a failure stress criterion may be used irrespective of the ball radius, r. The analysis in this case reveals that decreasing either d/r or the coating/substrate modulus ratio tend to favor ring cracking over radial type cracking. The transition between these two failure modes is identified explicitly as a function of the system parameters.     

Some aspects of the fracture mechanics of thin flexible films

A theory is presented for the determination of a number of physical constants which affect the fracture behaviour of thin plastic and viscoelastic materials. The theory is based on a relatively simple consideration of the energy balance in a tensile-test-strip with an edge-crack.

The total energy needed up to fracture was measured, for each material, for 90 samples approximately, varying both the total length of the test-trips, and the length of the initial cracks. In the first instance, the width of the test-strips was kept constant at a value of 15 mm.

Then, with the help of a special version of a multiple linear regression, values of physical parameters were obtained. Three materials were investigated, viz. a polyethylene, a polyester/polyethylene laminate and a polyamide/polyethylene laminate.

Initial crack lengths between 0.1 and 1 mm were investigated, in two different ways, viz. with a static tensile test and with an impact tensile test.

For each material, considerable differences between static and dynamic behaviour has been measured; a qualitative explanation, however, of these differences can not yet be given.

Secondly, the number of data per material was reduced, in order to get an impression of the reliability of the applied method of approximation. It appeared that even a set of four test-strips for one material produced significant results.

Next, an effort has been made to extrapolate the values obtained for test-strips with a width of 10 and 5 mm, respectively. In general, this extrapolation appeared to be inadmissable, implying that the underlying theory should only be considered as a rough approximation of a very complex reality.

From the experiments and calculations it is concluded that in general the method adopted is suitable to quantify the fracture behaviour of thin films in industrial practice.     

Fracture mechanics analysis of the effect of substrate flexibility on solder joint reliability

Solder joint fatigue failure is a serious reliability concern in area array technologies, such as flip chip and ball grid array packages of integrated-circuit chips. The selection of different substrate materials could affect solder joint thermal fatigue life significantly. The reliability of solder joints in real flip chip assembly with both rigid and compliant substrates was evaluated by the accelerated temperature cycling test and thermal mechanical analysis. The mechanism of substrate flexibility on improving solder joint thermal fatigue lifetime was investigated by fracture mechanics methods. Two different methods (crack tip opening displacement, CTOD and virtual crack closure technique, VCCT) are used to determine the crack tip parameters which are considered as the indices of reliability of solder joints, including the strain energy release rate and phase angle for the different crack lengths and temperatures. It was found that the thermal fatigue lifetime of solder joints in flip chip on flex assembly (FCOF) was much longer than that of flip chip on rigid board assembly (FCOB). The flex substrates could dissipate energy that otherwise would be absorbed by solder joints, that is, substrate flexibility has a great effect on solder joint reliability and the reliability improvement was attributed to flex buckling or bending during thermal cycling.     

Method for debonding of thin glass substrate and carrier for manufacturing thin flexible displays

Modern displays are becoming light, thin, and even curved and flexible. The thickness of glass substrates used in these displays has been reduced from its original value of 1.1 to 0.7 mm and eventually to 0.5 mm to create thinner displays. Glass substrates with a thickness of <?0.3 mm are extremely thin and fragile, can be easily deformed, and are therefore not suitable for direct incorporation into the display-manufacturing process. Glass-substrate suppliers bond thin glass substrates onto carrier glass substrates to form laminated glass substrates with increased overall thickness. The panel makers then manufacture the display on a laminated glass substrate. The carrier substrate is removed from the display to produce a thin panel. Glass-substrate makers debond the carrier from the thin glass substrate by applying mechanical force. However, the substrate may break during this debonding process. In this study, a novel method for bonding and debonding the carrier and glass substrates is proposed in which gas is injected between the carrier and thin glass substrate to separate them. This method effectively prevents the formation of edge defects in the thin glass substrate caused by breaks during the debonding process. We designed a debonding machine and established steps to debond the laminated glass substrate after a passive matrix organic light-emitting diode fabrication process. The results showed that this method can be used to efficiently debond the laminated glass substrate.     

Time resolved synchrotron x-ray strain measurements of gold thin film on flexible substrate

Synchrotron x-ray radiation was used for in situ strain measurements in gold films on polyimide substrate during uniaxial deformation tests. We have used an area detector that allows inspecting multiple directions in the polycrystalline thin film without serial sectioning during straining. We show in this paper the configuration used and the attainable orientations on a pole figure for which the x-ray strains are measured. Moreover, we show the effect on the x-ray strains of the onset on plasticity, which was not detected by optical (macroscopic) strain measurement.     

Fracture toughness measurement of thin films on compliant substrate using controlled buckling test

Thin films and multilayered structures are increasingly used in the industry. One of the important mechanical properties of these thin layers is the fracture toughness, which may be quite different from the known value of the bulk sample due to microstructural difference. In the design towards device flexibility and scratch resistance, for example, fracture toughness is an important parameter of consideration. This work presents a testing scheme using controlled buckling experiment to determine the fracture toughness of brittle thin films prepared on compliant substrates. When the film is under tension, steady-state channelling cracks form in parallel to each other. Critical fracture strain can be calculated by the measuring the displacement of the buckled plate. The fracture toughness can then be obtained with the help of finite element calculation. When the substrate experiences plastic deformation, the energy release rate is increased by the degree of plasticity. Fracture toughness measurement of two types of thin film Cu-Sn intermetallic compounds has been given to illustrate the merits of such a test scheme.     

Effect of adhesive layer elasticity on the fracture mechanics of a blister test specimen

An analytical model of a blister type specimen for evaluation of adhesive bond strength is developed. The model accounts for shear deformation of both the plate and the adhesive layer in order to determine the extent to which a blister specimen may be considered to be a mixed mode fracture specimen. A method for relating the results of numerical stress analysis to energy balance concepts of fracture mechanics is presented. It is shown that the shear deformations have only a small effect on energy balance; however, the magnitude of the shear stress in the adhesive layer may reach a value of 25 percent of normal stress.

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Résumé On développe un modèle analytique d'une éprouvette simulant un cloquage pour l'évaluation de la résistance d'un joint collé. Le modèle tient compte des déformations de cisaillement de la tôle et de l'adhésif afin de déterminer dans quelle mesure l'éprouvette de cloquage peut être considérée comme une éprouvette où se produit un mode de rupture mixte. Une méthode pour relier les résultats de l'analyse numérique des contraintes au concept d'équilibre d'énergie de la mécanique de rupture est présentée. On montre que les déformations de cisaillement n'ont qu'un effet secondaire sur l'équilibre d'énergie; toutefois, la tension de cisaillement dans la couche adhésive peut atteindre une valeur de l'ordre de grandeur de 25% de la tension normale.

     

Nonlinear elastic mechanics of the ball-loaded blister test

The nonlinear elastic mechanics of spherically capped shaft or ball-loaded blister tests is presented. In the test model, a thin film is attached to a substrate with a circular hole running through the thickness of the substrate. A central load is applied to the film through the hole by a spherically capped shaft or a ball with a finite radius. The deformed blister is divided into two parts: a circular region in contact with the sphere of the cap or ball and an outer noncontact annulus. The Reissner’s plate theory is employed to describe the deformation of the contact part and the von Kármán plate theory for the noncontact annulus. A constitutive equation of coupled linear springs is obtained to quantify the effect of the substrate deformation on the blister deflection. For small deflection, the analytical solution of load-deflection is derived. For large deflection, an iteration approach is adopted to predict numerically the load-deflection curve. Finite-element analysis is conducted to verify the analytical and numerical solutions. The influence of the substrate deformation, residual stress, radius of the spherical cap or ball and the friction between the film and ball on the load-deflection relation is investigated.     

Microbolometers on a flexible substrate for infrared detection

Uncooled semiconducting YBaCuO infrared microbolometers have been fabricated on a flexible polyimide substrate formed by spin-coating a silicon wafer with a release layer. The wafer was used as a carrier for the flexible substrate during fabrication. The finished microbolometers on the flexible substrate showed a temperature coefficient of resistance (TCR) TCR =(1/R)(dR/dT) of -3.03% K/sup -1/, at room temperature, which is comparable to the TCR values observed for semiconducting YBaCuO microbolometers fabricated directly on Si. In order to provide protection and better mechanical integrity, some of the devices were encapsulated. The microbolometers attained a responsivity and detectivity as high as 3.5/spl times/10/sup 3/ V/W and 1/spl times/10/sup 7/ cm/spl middot/Hz/sup 1/2//W, respectively, at 2.88 /spl mu/A of current bias. The responsivity and detectivity of the encapsulated microbolometers, on the other hand, were 1.6/spl times/10/sup 3/ V/W and 4.9/spl times/10/sup 6/ cm/spl middot/Hz/sup 1/2//W, respectively at 1 /spl mu/A of current bias. Spin-coated liquid polyimide solved two major problems previously encountered with the solid polyimide sheets when used as a flexible substrate. First, flatness of the flexible substrate was maintained with no air bubbles. Second, the thermal expansion of the flexible substrate during the fabrication process due to thermal cycling was minimal. All measurements reported in this paper, were taken prior to releasing the flexible substrate from the Si wafer containing the finished microbolometers.     

Adhesion of ice to a flexible substrate

The adhesion of ice to a flexible substrate, polyurethane elastomer, has been studied using the Andrews-Stevenson test procedure which involves the pressurization to failure of an enclosed interfacial crack. The temperature, rate of pressurization and substrate layer thickness were varied and the failure energy (critical energy release rate) determined. If energy release from the flexible substrate is ignored an apparent failure energy is obtained which first increases and then decreases as the layer thickness rises from zero to 4 mm. This thickness effect results in a large variation in the pressure needed to produce failure, and is thus important in relation to the ice-release properties of the substrate. It is shown that the thickness effect can be explained quantitatively in terms of the energy release from the flexible substrate, which, in turn, depends on its visco-elastic properties. The true failure energy is derived and is also found to correlate with the visco-elastic response of the rubber. Finally, these ideas are used to explain the effects of rate and temperature on the conditions of failure.     

Wrinkling of thin films on a microstructured substrate

Surface wrinkling of thin films on substrates offers an effective strategy to create controllable surface patterns of wide application. In this article, both theoretical analysis and numerical simulations are performed to study the surface wrinkling of thin films bonded on a microstructured soft substrate under compression. We consider two typical kinds of substrates that have different mechanical properties. One possesses a periodic array of micropillars, and the other has a period of alternating unbonded—bonded regions at the film—substrate interface. The characteristics of surface wrinkling and postbuckling evolution of films are revealed. It is found that the interfacial microstructures modulate the normal mechanical properties of the substrate and, thus, tailor the buckling behavior of the thin film atop it. Under lateral compression, the film on the substrate shows periodic arrays of micropillars exhibiting sinusoidal wrinkling first, and then with further compression, the wrinkles give way to a deep fold accompanied by Euler buckling of the micropillars and the decay of neighboring wrinkles. For the system with periodic unbonded—bonded regions at the film—substrate interface, wrinkling of the film is characterized by a relative shift in the normal direction. This study offers potential applications in the fabrication of tunable surface morphologies.     

Fracture toughness tests of a rigid polyurethane foam

The results of some exploratory tests for determining the fracture toughness of a rigid polyurethane foam are presented. The specimen geometries used included centre- and double-edge-cracked plates, the single-edge-cracked tensile specimen and the double cantilever beam specimen. The validity of applying the concepts of linear elastic fracture mechanics to the fracture of this foam is discussed. Some unique features of the fracture of foam are discussed.     

Pre-cracking technique for fracture mechanics experiments along interface between thin film and substrate

Utilizing the difference in interface strength due to fabrication process, a technique for producing a sharp pre-crack between a thin film and a substrate is proposed. A cracked specimen for examining fracture toughness of interface between a sputtered copper (Cu) thin film and silicon (Si) is made by the method. A vacuum-evaporated Cu thin film, which has poor adhesion to Si, is inserted between the sputtered Cu thin film and the Si substrate as a release layer. The release layer debonds from the Si substrate at very low load, and the sharp pre-crack is successfully introduced along the interface. Using the pre-cracked specimen, the interface fracture toughness test is conducted and the critical J-integral, J_C, is evaluated as about 1 J/m² for the sputtered Cu/Si interface.     

Influence of SHI irradiation on the structure and surface topography of CdTe thin films on flexible substrate

Impact of ion irradiation on thin films is an emerging area for materials modification. CdTe thin films grown by thermal evaporation on flexible molybdenum (Mo) substrate were irradiated with Swift (100 MeV) Ag⁺⁷ ions for various ion fluence in the range 10¹²–10¹³ ions/cm². The modifications in the composition, structure and surface morphology have been studied as a function of ion fluence. The Energy Dispersive X-ray Analysis (EDS) shows slightly Te-rich composition for both as-grown and irradiated films with no significant change after irradiation. X-ray diffraction (XRD) analysis indicates a consistent shift in the (111) peak position towards higher diffraction angle and an increase in the full width at half maximum (FWHM) with increase in ion fluence. The change in the residual stress during irradiation has been evaluated and is related to the corresponding microstructural changes in the films. The initial tensile stress is found to be relaxed after irradiation. Atomic Force Microscopy (AFM) studies revealed significant grain splitting after irradiation and formation of hillocks at higher ion fluence. The surface roughness was significantly increased at higher ion fluence.     

Roll-to-roll printed resistive WORM memory on a flexible substrate

The fabrication process and the operation characteristics of a fully roll-to-roll printed resistive write-once-read-many memory on a flexible substrate are presented. The low-voltage (<10 V) write operation of the memories from a high resistivity '0' state to a low resistivity '1' state is based on the rapid electrical sintering of bits containing silver nanoparticles. The bit ink is formulated by mixing two commercially available silver nanoparticle inks in order to tune the initial square resistance of the bits and to create a self-organized network of percolating paths. The electrical performance of the memories, including read and write characteristics, is described and the long-term stability of the less stable '0' state is studied in different environmental conditions. The memories can find use in low-cost mass printing applications.