Kikuo Kishimoto

Propagation behavior of love waves in a piezoelectric layered structure with inhomogeneous initial stress

For the piezoelectric layer/substrate structure with slowly varying inhomogeneous initial stress in the layer, the influence of the initial stress on the properties of Love wave propagation is studied. The Wentzel?Kramers?Brillouin?(WKB) approximate approach is adopted for analytical derivations. Numerical results obtained for the BaTiO3 layer/borosilicate glass substrate combination system indicate that, under certain conditions, initial stress in the layer can markedly affect the propagation of the Love wave. The analysis is meaningful for the theoretical analysis and engineering applications of Love waves.

Evaluation of Interfacial Strength by Multi-Stages Peel Test

Multi-layer thin films are widely used in micro-sensor and microelectronics products. These electronic devices contain several metal or polymer thin films and reliability of these systems is strongly dependent on the interfacial adhesion of these thin films. Due to the thermal stress, residual stress or elastic mismatch, the delamination between layers sometimes occurs. Therefore, it is important to evaluate the interfacial strength precisely. Peel tests are simple way to estimate an interfacial strength and, in fact, widely used in industrial field. Recently, a new simple but functional device for peel tests has developed in our group. This test method is called Multi-stages peel test. There are two features in this device. At first, peeling tips can be observed continually and it becomes easy to measure a peeling angle. Second is that the peeling angle can be varied by attaching dead weights on the specimen. This dead weight works as a shear force at the peeling tip and the peeling angle can be changed variously. Therefore, the fracture tests under various phase angles are possible. In this paper, Multi-stages peel test is applied to the evaluation of interface strength of multi-layer thin films that are composed of Cu, Cr, PI and Si layers. By considering the energy balance during the peel test, the interfacial strength independent of the thickness can be obtained.

Experimental Investigation on the Interface Properties Evaluation in Piezoelectric Layered Structures by Love Waves Propagation

The linear ultrasonic technique has been extensively used as a powerful, non-destructive test tool for reliability testing and failure analysis of electronic packaging. This is used most often in the inspection of defects such as delaminations, voids, or cracks through use of a SAM (Scanning Acoustic Microscope). Then, as the reliability level that is required of electronic packaging becomes higher and the thickness of package becomes thinner, the possible defect which needs to be detected becomes smaller. In the conventional SAM, however it is very difficult to detect small defects less than m µ 1 . 0 , such as micro-delaminations. In order to solve such a problem, this paper proposes a nonlinear ultrasonic method, where the nonlinearity caused by the effect of crack-face interactions is considered. The basic concept of this method involves harmonic frequencies that are generated in the transmitted ultrasonic wave due to the partial contact at the interface of micro-delamination. As an evaluation index, the nonlinear parameter dependent on the amplitude of the second order harmonic frequency component is obtained by spectral analysis of the transmitted signal. Experimental results show that the nonlinear parameter has good correlation with the micro-gap and the proposed method can detect the micro-delamination even less than nm 1 .

Influence of Imperfect Interface on the Propagation of Love Waves in Piezoelectric Layered Structures

Layered structures, especially thin film/coating substrate system play important roles in micro-electro-mechanical system (MEMS) and microelectronics packages. Many surface acoustic wave (SAW) devices/sensors adopt the layered structures to achieve high performance that with a piezoelectric layer deposited on the substrate. Recently, much work has been carried out concerning the propagation behavior of surface waves in piezoelectric layered structures, in which the piezoelectric layers are bonded perfectly with the substrate materials. Actually, due to the thermal mismatch, the unavoidable initial stress in the piezoelectric layer and the brittleness nature of piezoelectric ceramics, imperfections exist in the interfaces of these layered structures. Due to the penetration properties of surface waves, the imperfect interfaces may influence the propagation behavior of surface waves. Up to now, little attention has been paid to the propagation behavior of surface waves in layered piezoelectric structures that the imperfect interfaces are taken into account. The propagation behavior of Love waves in a piezoelectric layered structure with imperfect interface is taken into account. Solutions of the mechanical displacement and electrical potential function are obtained for the piezoelectric layer and substrate, respectively, by solving the coupled electromechanical field equations. Effect of imperfect interface on the phase velocity of Love wave propagation is discussed in detail. Results obtained indicate that imperfect interface can greatly influence the propagation of Love waves under some certain conditions. The potential application of these results in the field of mechanical behavior of materials is also shown.

UV-Irradiation Effects on Interfacial Strength between Thin Ceramic Film and Polymer Substrate

This aim of this study is to investigate the effect of UV (Ultra Violet ray) irradiation on the interfacial adhesion strength between thin ceramic films and polymer substrate. Electric conductive films based on polymer substrates have attracted attention for use in flexible optoelectronic devices. It is well known that the mechanical properties of polymeric materials are degraded by UV irradiation. Therefore, it is considered that the UV irradiation also affects the interfacial adhesion strength between ceramic coating and polymer substrate. The interfacial adhesion strength was measured by Multi-stages peel test. The results show that the interfacial strength decreases with UV irradiation. However, if a filter layer is installed between ceramic and polymer substrate, the degradation ratio becomes small.

Evaluation of fatigue strength for solder joints after thermal aging

Recently, preventing environmental pollutions, leadfree(Pb-free) solders areabout toreplace tin-lead(Sn-Pb) eutectic solders. Sn-Ag-Cu alloys areleading candidates for leadfree solders. Inthis study, wecarried outboard level fatigue tests andinvestigated thefatigue life ofseveral lead free solder joints. Also, theeffects ofpadfinish metals and their thickness wereinvestigated.

Coating Interface Fracture Toughness Evaluation by a Combination of Edge Compression and Slinging Load

Previous methods to measure interface fracture toughness between coating and substrate cant easily vary a phase angle as a mixed mode parameter. So that, the new coating interface fracture toughness test method, by which phase angle at interface crack tip can be varied due to applying a combination of compression loading to the coating edge and slinging such as beam bending, is proposed. The simple formula, which connects to complex stress intensity factors and double loading is firstly derived on the basis of the cracked beam model proposed by Suo and Hutchinson [1]. As an application of the method and associated formula, thermal barrier coating/super-alloy interface toughness is evaluated based on numerical analysis.

A New Specimen for Measuring the Interfacial Toughness of Al-0.5%Cu Thin Film on Si Substrate

A new specimen is proposed to measure the interfacial toughness between the Al-0.5%Cu thin film and the Si substrate. The plain and general micro-fabrication processes are sufficient to fabricate the specimen. With the help of the finite element method and the concepts of the linear elastic fracture mechanics, the detailed structure for this specimen is modeled and evaluated. The results obtained from this research show that the proposed specimen provides efficient and convenient method to measure the interfacial toughness between the Al-Cu thin film and the Si substrate.

Fatigue crack growth in lead-free solder joints

This paper investigates the fatigue crack growth behaviors of lead-free solder joints. Metallurgical interactions occur between solders and electrical pad metals in fabrication process and these intermetallic compounds continue to grow during service periods. Due to its brittle nature and lattice mismatch, the solder cracks tend to be generated near the compounds and these cracks affects the mechanical integrity of solder joints. Therefore, it is important to study the effect of intermetallic compounds development on the mechanical properties of the solder joints. In this paper, we focused on the low cycle fatigue of solder joints and investigate the effects of intermetallic compounds development on the fatigue life and the fatigue crack growth behavior of the solder joints.

A Cohesive Zone Model and Interfacial Crack Problems

Based on the internal variable theory of thermodynamics, a continuum interface constitutive model relating the interface traction with interface separation is developed. The interface damage variable is introduced, and the evaluation equation is derived to characterize the degradation of interface rigidity with interface debonding. The cohesive law is formulated by taking into account of the anisotropy of interface debonding. The present constitutive model is applied to the interfacial crack problems.Finite element computation is performed in which the cohesive model is embedded along the line extending ahead of the crack tip. The critical stress intensity factors at the crack initiation and propagation are evaluated for a wide range of bimaterial constant and the fracture boundary curves are obtained. Numerical simulation on peeling test is also presented.