Key Engineering Materials Vols. 306-308

Abstract: A nondestructive inspection is required to check for defects inside of the actually used components and structures and to confirm their generation and growth rate. Using the backward radiated ultrasonic inspection system, we performed nondestructive inspection of cracks and micro cavities resulting from creep. From the ultrasound test results, the generation and growth of cracks and micro-cavities were confirmed, and the fracture life of the components and structures could be predicted. We confirmed degraded region by crack size and cavity based on the amplitude of the backward radiated ultrasound. The size of degraded region at 600oC was about 10mm, while that at 650oC was about 15mm. The size of crack and the cavity area fraction confirmed by ultrasound were very close to the actual size and cavity area fraction, indicating the validity of the predicted creep crack growth rate and creep.

Abstract: The creep experiments of 2Cr11NiMoVNbNB steel were carried out at 550oC and 600oC. The creep damage laws of this material are discussed. An increase of the stress exponent with decreasing temperature was found experimentally and a modified secondary creep rate equation including in creep threshold stress was derived. Creep damage analyses are made under constant stress and under relaxed stress with creep threshold stress. The experimental results show that the damage value calculated with creep time is almost equal to that calculated with the creep rate, and an iteration method of calculating is proposed.

Abstract: In this paper, the creep deformation of PBX was measured using the moiré interferometry. The experimental results show a different creep process compared with pure high polymer and this phenomenon is preliminary analyzed from damage mechanics.

Abstract: This research studied the thermal fatigue life for eutectic solder balls of thermally enhanced flip-chip plastic ball grid array (FC-PBGA) packages with different lid materials under thermal cycling tests. Three FC-PBGA packages with different lid materials, i.e., Al, AlSiC, and Cu, were utilized to examine the lid material effect on solder ball reliability. The cyclic stress/strain behavior for the packages was estimated by using the nonlinear finite element method. The eutectic solder was assumed to be elastic-plastic-creep. The stable stress/strain results obtained from FEM analysis were utilized to predict the thermal fatigue life of solder balls by using the Coffin-Manson prediction model. Simulation results showed that the fatigue life of the FC-PBGA package with a Cu lid was much shorter than FC-PBGA packages with other lid materials. The relatively shorter fatigue life for the FC-PBGA package with a Cu lid was due to the complex constrained behavior caused by the thermal mismatch between the lid, substrate and the printed circuit board. The difference was insignificant in the fatigue lives between the package with an Al lid and the conventional package.

Abstract: The use of flip-chip technology has many advantages over other approaches for high-density electronic packaging. ACF (anisotropic conductive film) is one of the major flip-chip technologies, which has short chip-to-chip interconnection length, high productivity, and miniaturization of package. In this study, thermal fatigue life of ACF bonding flip-chip package has been predicted. Elastic and thermal properties of ACF were measured by using DMA and TMA. Temperature dependent nonlinear bi-thermal analysis was conducted and the result was compared with Moire interferometer result to verify FEM results. Calculated displacement field was well matched with experimental result. Thermal fatigue analysis was also conducted. The maximum shear strain occurs at the outmost located bump. Shear stress-strain curve was obtained to calculate fatigue life. Fatigue model for electronic adhesives was used to predict thermal fatigue life of ACF bonding flip-chip packaging. Approximately, 700 cycles have been expected under current conditions (JEDEC standard No. 22-A104)

Abstract: As the larger capacity and smaller packaging size are required in the design of a semiconductor, areas and pitches of solder joints have been miniaturized. Therefore the importance of bondability and reliability in soldering technique for the printed circuit board design has been increased since the fatigue failure has been observed at the place between the lead frame and solder joints. To evaluate thermal reliability of SMD type electronic packaging such as SOIC, PLCC and BGA, an exclusive module for modeling a lead frame structure is developed using JavaScript and the thermal stress analysis is performed using ANSYS. A modeling program with GUI can define geometric dimensions of an electronic packaging, assign material properties of a lead frame and a solder and apply boundary conditions. A seamless integration between modeling and analysis is achieved by implementing an interface program to generate an analysis model of a lead frame structure directly from the model information. Heat transfer analysis and stress analysis considering thermal loading are carried out in ANSYS and the results are exploited to estimate the fatigue life of a lead frame based on the S-N curve. The effect of different lead frame materials is examined to identify the mechanical characteristics and the different shapes of the lead frame with same SMD type are investigated to distinguish the reliability.

Abstract: The backend structures of advanced VLSI (Very-Large Scale Integration) devices have become increasingly complex because of the need to combine several types of dielectric and metal layers in order to enhance device performance and/or reliability. For example, the inter-metal dielectric (IMD) stack of an aluminum-metallization device may consist of high density plasma (HDP) fluorine-doped silicate glass (FSG) for gap fill and RC-delay minimization, plus a silicon-rich oxide (SRO) cap to prevent aluminum attack from fluorine out-diffusion. Depending on the interplay between the tensile stress of the aluminum, and the typically compressive stress of the dielectric stack, the wafer may develop a large bow of either positive or negative sign. A large positive bow may negatively impact wafer handling during processing steps that use vacuum chucking, and can also lead to excessive edge polishing during tungsten and oxide chemical–mechanical planarization (CMP) steps. This paper presents a methodology to reduce final wafer bow from ~+40 to ~-10 micron by a careful selection of SRO deposition conditions to achieve a stress value that results in flatter wafers.

Abstract: Graphite nanofibers (GNFs) reinforced aluminum matrix composites have been fabricated successfully by powder metallurgy methods. The GNF-Al mixtures were prepared through ultrasonication and ball milling. The GNF-Al composites were consolidated by hot isostatic pressing (HIP) and then a high density of compacts could be obtained. The microstructure and the distribution of nanofibers in matrix material were investigated by microscopy observations. A uniform distribution of nanofibers in aluminum matrix was obtained. The mechanical properties were measured by microhardness and compression tests. The optimal contents of nanofibers were determined in view of the mechanical properties. The results of thermal tests indicate that the addition of nanofibers were enhanced the thermal conductivity but, the dimensional stability such as thermal expansion was not improved significantly.

Abstract: Two different types of multi-walled carbon nanotube (MWNT), the dispersed and the network MWNTs, were used to reinforce the phenolic resin. The MWNTs/phenolic nanocomposites were tested by a dynamic mechanical analyzer (DMA) to characterize their dynamic mechanical properties. The results showed that increasing the MWNT content can increase the storage modulus, the loss modulus and the glassy transition temperature of the MWNTs/phenolic nanocomposites. A subambient loss transition is seen in the nanocomposites with network MWNTs which results in a better impact resistance property in the nanocomposites. The glassy transition temperature of the nanocomposites with network MWNTs is higher than that of nanocomposites with dispersed MWNTs. The MWNT additive in phenolic resin can be used to improve the dynamic mechanical properties of the MWNTs/phenolic nanocomposites. The tensile failure morphologies of MWNTs/phenolic nanocomposites were also examined using field emission scanning electron microscope (FESEM) to explain the difference between the two types of nanocomposites.

Abstract: In this article, based on the method of molecular dynamics, the mechanics response of nano-component is simulated. Some details of MD technique have been included in this paper, dealing with time step, algorithm and other problems. According to the work of predecessors, systemically a general form of atomic stress under atomic scale, as well as its specific form using EAM method, is derived. Unlike the macro stress, the atomic stress stands for the atom’s potential ability of movement, and should be defined again. The mechanical properties of copper nano-rod are studied, including relaxation state, tensile state and compress state. Because of the high energy atoms on the surface, the surface effect is generated. So when in relaxation state, the nano-rod will be shortened, and the more atoms the rod has, the less is shortened. When the strain is between –0.1 and 0.1, the nano-rod performs elastic situation. The nano-rod has a highly tensile strength. We find that dislocation can reduced the flow stress of the nano-rod. Finally the effect of nano-hole effect is discussed.