Key Engineering Materials Vols. 306-308

Abstract: The delamination problem in plastic ball grid array electronic package was investigated analytically and experimentally. The ANSYS code was used in the analysis to find the deformation and the stress distribution in electronic package due to the thermal mismatch between different materials at reflow temperature with saturated moisture pressure at the delaminated region. The stress intensity factor and the strain energy release rate at the tip of interfacial delamination were calculated using the fracture mechanics approach. The results show that the delaminaton occurs at the corner of the die pad and at the middle region of the die attach/die pad interface. The delaminaton grew from the middle region of the die attach/die pad interface, along the epoxy molding compoend/die pad interface, the epoxy molding compound/substrate interface to the exterior surface. Possible growth directions of the interfacial delamination in plastic ball grid array electronic package were identified and observed in the experimental results. In addition, the reflow temperature affected the stress distribution and the strain energy release rate at the tip of interfacial delaminaton.

Abstract: Multi-layer structures are common in electronic package especially for the micro devices manufactured via the semi-conductor processes or MEMS processes. Interfacial crack due to the delamination significantly weakens the multi-layer structure. It is desired to understand the interfacial fracture properties of the electronic packaging materials. In this research, three specimens named Doubled Cantilever Beam (DCB), End-Notched Flexure (ENF), and Four-Point-Bending are proposed to investigate the fracture toughness associated with mode I, mode II and mixed mode. Basing on the Euler-Bernoulli beam theory, the strain energy in a bi-layer beam is derived. The strain energy before and after the propagation of the interfacial crack are calculated, lead to the determination of the strain energy release rate. The analytical results of strain energy release rate derived in this investigation are compared with the numerical results obtained from finite element method. The effects of material properties and thickness between the adjacent layers of interfacial crack are examined through the parametric study.

Abstract: A dynamic analysis method has been developed to investigate and characterize the effect due to the presence of discrete single and multiple delaminations of composite laminated structures. The Fermi-Dirac distribution function is combined with an improved layerwise laminate theory to model a smooth transition in the displacement and the strain fields of the delaminated interfaces. In modeling piezoelectric composite plates, a coupled piezoelectric-mechanical formulation is used in the development of the constitutive equations. Based on the developed model, the effects of discrete delaminations are quantified by comparing transient responses of composite plates and piezoelectric sensor outputs.

Abstract: Compression tests are conducted on composite laminates consisting of 16 unidirectional carbon/epoxy layers with two through-width delaminations. Two types of delamination length and location are considered. One is that a short delamination is located at the middle surface along the thickness direction and a long delamination is positioned between the second layer and the third layer. The other is that a long delamination is located at the middle surface and the position of the short elamination is between the second layer and the third layer. The results indicate that if the long delamination is close to the surface of the laminate, the inner, short delamination has no effect on the critical buckling stresses. However, the presence of inner, short delamination may significantly change the critical delamination growth stresses. If the short delamination is above the long delamination that is located on the middle surface, the presence of the short delamination may significantly reduce the critical buckling stresses. But its effects on the critical delamination growth stresses are minor.

Abstract: Thermal barrier coating (TBC) on the element for high temperature service, such as a gas turbine blade, has become an indispensable technology. In this study, adhesive strengths of plasmasprayed CoNiCrAlY coatings were examined using an indentation method. In order to examine the effects of the spraying process on the adhesive strength of a sprayed coating, coatings were deposited by both atmospheric plasma spraying (APS) and low pressure plasma spraying (LPPS). The half number of CoNiCrAlY(LPPS) specimens were thermally aged for diffusion treatment. The load-displacement curves during the indentation were measured, and the delamination energy per unit area of delamination was estimated. The delamination load was high when the distance from an edge of a specimen was long, however the energy per unit delamination area was almost independent of the distance from the edge and was uniquely determined. The delamination energy of CoNiCrAlY(APS) and CoNiCrAlY(LPPS) coatings were found to be approximately the same. The delamination energy of CoNiCrAlY(LPPS) with diffusion thermal treatment was widely scattered as compared with foregoing two coatings, however the energy of the CoNiCrAlY(LPPS) coating with diffusion thermal treatment was found to be about three times higher than those of both CoNiCrAlY(APS) and CoNiCrAlY(LPPS) coatings. It was concluded that diffusion thermal treatment was effective in improving the delamination strength. The CoNiCrAlY(LPPS) coating with diffusion thermal treatment was also found to be effective in improving the fatigue fracture life of a thermal-barrier-coated material.

Abstract: When a sandwich structure is loaded in uniaxial in-plane compression the skin over the debonded region may buckle allowing further propagation of the debond, leading to structural collapse. Hence how to detect the shape and size of debonded defect, and to trace propagation trend of the defect are key factor in the process of damage evaluation of subsurface defect. In the paper, a nondestructive testing system is developed based on liquid crystal cell and Wollaston shearing prism. Nematic liquid crystal has the property of modulating the intensity and phase of light, and the intensity will not change during the phase modulating. According to this property, the phase-shifting of digital shearography are performed successfully. The experimental results prove that we can not only perform detection of the defect size and shape in subsurface of sandwich structure, but also trace the further propagation of debonded defect because of local buckle. This indicates that the method of digital shearogrephy will offer an effective measurement tool to research the buckle behavior of debond defect in subsurface of sandwich under uniaxial in-plane compression.

Abstract: By using the mathematical uniform design, a new agglomerated flux for high speed and multi-arc submerged arc welding (SAW) has been developed in this paper, which performs excellently in welding process, and the effects of flux composition on the welding characteristics is analyzed as well. The mechanical tests show that the weld metal obtained in SAW with this flux exhibits high toughness at low temperature, and the other mechanical properties of the weld metal also satisfy the technical specifications for the West to East Pipeline Project in China. The microstructures of the weld metal are also examined. Both optical and TEM microphotoes demonstrate that the weld consists mainly of small-sized and uniformly distributed acicular ferrite. These microstructures can effectively prevent the initiation and propagation of micro cracks, resulting in high notch toughness of the weld metal at low temperature.

Abstract: Pipelines widely used for the transportation of varying fluids from one place to another should be maintained in good condition to avoid, if possible, the occurrence of corrosion in pipelines to keep its reliability in terms of fracture and damage. The reliability of buried pipelines with corrosion defects is estimated using the failure probability. The FORM (first order reliability method) is utilized to estimate the failure probability of buried pipeline with various formulas for external stress in pipe and three different corrosion models. In this paper, it is recognized that the failure probability increases not only with increasing exposure time, operating pressure and diameter of pipe but also with decreasing wall thickness and yield stress of pipe material in three different corrosion models. And the effects of the scattering of random variables regarding reliability of pipelines on failure probability are investigated, systematically. Furthermore, the target safety level is used to determine the level of safe of corroded pipeline and the effects of varying boundary conditions on target safety level are also estimated.