Papers by Keyword: Void Formation

Abstract: This paper discusses creep void formation and rupture lifetimes in multiaxial stress states for a SUS 304 stainless steel at elevated temperatures. Biaxial and triaxial tension creep tests were performed using a cruciform and a cubic specimen, respectively. These two types of the specimens were designed to achieve uniform equi-biaxial and equi-triaxial stress distributions by a finite element analysis in the gage parts. Void formation at grain boundaries was observed by intermitting biaxial creep tests and by interrupting triaxial creep tests. Creep rupture lifetimes were also obtained in biaxial and triaxial creep tests. Biaxial stresses increase the void formation but give a little influence on a creep rupture lifetime in the correlation with von Mises equivalent stress. Triaxial stresses also increase the void formation and drastically reduce a creep rupture lifetime in the correlation with von Mises equivalent stress. Evident void formation in an equi-triaxial stress condition demonstrates that von Mises equivalent stress is not a suitable measure to evaluate creep damage development in multiaxial stress states. A new equivalent stress is proposed to evaluate creep rupture lifetimes in biaxial and triaxial stress states.

Abstract: 3-D packaging is a technology that offers high density packaging and high performance. It enables chips to be stack in a single package and widely adopted in multi-media products. Thermosetting material is used for their encapsulation, is flowed through their thin space and wide filling area during package encapsulation process has become vital concern i.e. void formation. In this paper, such issue has been numerical studied due to the effect of transfer speed of the plunger head during encapsulation process. There are five transfer speeds of 1.6, 2.0, 3.4, 6.8, 9.04 mm/s have been chosen in order to investigate the quality of air entrapped (hereafter namely as void), where it degraded the package’s reliability. It is found that the longest transfer speed delivered to the best encapsulation process, which had lowest volume of air trap or void formation in the package.Keywords: Encapsulation, Transfer speed, Vent arrangement, Void Formation

Abstract: Thermosonic Copper (Cu) wire interconnection has been under an extensive research and development to replace expensive Gold (Au) wire material in the semiconductor industry. However, a reliability concern is raised due to void formation at the bonding interface of Copper wire-Aluminum bond pad (Cu-Al) after High Temperature Storage (HTS) annealing condition. It is believed that the Intermetallic Compound (IMC) layer growth and evolution lead to a volumetric shrinkage which in turn results in the void formation. Annealing conditions influence the development of the IMC at the bonding interface which is related to the bonding reliability. In this work, the effects of annealing toward the micro-structure and IMC growth at the bonding interface were evaluated using Scanning Transmission Electron Microscope equipped with Energy Dispersive X-ray analysis. In the as-synthesized sample bonded at 100°C, an inhomogeneous IMC formation dominated by grain boundary diffusion was observed. After High Temperature Storage of 1000 hours, the consumption of the Al bond pad resulted in the formation of irregular IMC layers. The variation of phases existed in a localized region was believed due to simultaneous growth of IMC by both grain boundary and volume diffusions. Moreover, the diffusion of Cu into Si was observed. This resulted in the formation of the mixture of Si + η phases in the affected sea region.

Abstract: PVC/BaSO₄ composites were prepared by melt blending method. The mechanical properties, microstructure and thermal stability of the composites were investigated. The results indicated that BaSO₄ decreased the tensile yield stress and improved the elongation at break of PVC composites. Ductile fracture characteristics such were observed in the tensile fracture surface of PVC/BaSO₄ composites. The toughening mechanism was cavitations toughening mechanism and shear zone toughening mechanism. The reaction of dehydrochlorination was limited by the addition of BaSO₄.

Abstract: Void formation in a duplex scale formed on Fe-5Cr alloy at 773 K has been elucidated by oxygen chemical potential distribution, the flux of oxide ion and its divergence. The calculation predicts that voids preferentially form at the interface between inner and outer scales in the low oxygen partial pressure in which the predominant defect of iron is interstitial ion. The flux of oxide ion changes discontinuously at this interface and the divergence of the flux gives voids. Calculated volume fraction of voids at this interface is in good agreement with that has been measured.

Abstract: Results of kinetic Monte Carlo simulation of the formation of a hollow nanosphere by interdiffusion from a core-shell binary system are presented for the first time. The faster diffusing species is located in the core whilst the slower diffusing species form the shell. With its self-generated vacancy composition all stages of the hollow sphere formation process are observed in our model: interdiffusion, the supersaturation of the core of the nanosphere by vacancies, precipitation of pores and eventual void formation. Results of this simulation confirm the experimental conclusions that interdiffusion accompanied by the Kirkendall effect and Kirkendall porosity is one of the mechanisms responsible for the formation of hollow nano-objects.

Abstract: Voids are frequently generated and dispersed in oxide scales formed in high temperature oxidation of metals. The divergence of ionic flux may play an important role in the void formation in a growing scale. Kinetic equations were derived for describing chemical potential distribution, ionic fluxes and their divergence in the scale. The divergence was found to be the measure of void formation. Defect chemistry in scales is directly related to the sign of divergence and gives an indication of the void formation behavior. The quantitative estimation on the void formation was successfully applied to a growing magnetite scale in high temperature oxidation of iron at 823 K.

Abstract: A quantitative elucidation of the void formation in a growing scale with Schottky defects and p-type conduction during high temperature oxidation of metals. The evaluation of the divergence of ionic fluxes indicates that (1) Voids form in the scale preferentially in the vicinity of the metal/scale interface, (2) The volume of voids increases in a parabolic manner, (3) The volume fraction of voids and the scale is independent of time. The comparison between the calculation and the experimentally observed scale microstructure of NiO and CoO confirmed well the validity of the prediction.

Abstract: Estimation of void formation in oxide scale is important for predicting exfoliation of the oxide scale. Void formation in magnetite scale formed on iron at 823 K has been elucidated by chemical potential distribution, flux of oxide ion and its divergence. This calculation also estimates a effective diffusion coefficient, which includes both lattice diffusion and grain boundary diffusion in magnetite scale. The resulting effective diffusion coefficients give the quantitative elucidation of the morphology of the magnetite scale. The divergence of oxide ion explains well a position and an amount of void in magnetite scale.

Abstract: A tin-lead solder is generally used to mount an electronic package on to a printed circuit board (PCB) of an electronic devices. In this study, we investigated the microstructure of the solder joint from a four years old used mobile phone. Microstructure and morphology of the joint was obtained using optical and scanning electron microscopy (OM and SEM) respectively. Cracks and voids can clearly be seen in the solder area for the most of the sample observed. Crack up to 10 μm wide appeared to be propagating along the solder line between Cu lead and Cu substrate. Energy dispersive X-ray (EDX) analysis revealed that cracks occurred at the richer tin content. It was not clear the exact mechanism that leads to the existence of the crack. However we believe that thermomechanical cause such as thermal fatigue, void formation and the thicker layer of intermetallic compound contributed to the failure of the solder joint.