Narasimalu Srikanth

Grains, deformation substructures, and slip bands observed in thermosonic copper ball bonding

Abstract In integrated circuit manufacturing, bonding a copper wire of diameter 25–75 μm (1–3 mil) on an aluminum-metallized silicon substrate has been accomplished by the application of ultrasonic energy along with compressive force at 200 °C. Prior to the bonding, the wire tip was melted by electric sparking to form a spherical ball, which, when analyzed, displayed columnar grain growth. Microstructural study of the final bonding showed that the application of ultrasonic energy resulted in the subdivision of the grains of a few micrometers in size, giving rise to deformation substructures (or cells). In addition, slip bands were observed inside the cells. Microhardness studies showed that the ball bonds had a higher hardness (112 VHN) than that of the unannealed bare copper wire (102 VHN), suggesting that the applied ultrasonic energy strain hardened the copper ball by creating the deformation substructures and slip bands. Furthermore, the heat-affected zone above the copper ball clearly contained recrystallized grains along with deformation markings.

A Comparative Study of Empirical Mode Decomposition-Based Short-Term Wind Speed Forecasting Methods

Wind speed forecasting is challenging due to its intermittent nature. The wind speed time series (TS) has nonlinear and nonstationary characteristics and not normally distributed, which make it difficult to be predicted by statistical or computational intelligent methods. Empirical mode decomposition (EMD) and its improved versions are powerful tools to decompose a complex TS into a collection of simpler ones. The improved versions discussed in this paper include ensemble EMD (EEMD), complementary EEMD (CEEMD), and complete EEMD with adaptive noise (CEEMDAN). The EMD and its improved versions are hybridized with two computational intelligence-based predictors: support vector regression (SVR) and artificial neural network (ANN). The EMD-based hybrid forecasting methods are evaluated with 12 wind speed TS. The performances of the hybrid methods are compared and discussed. It shows that EMD and its improved versions enhance the performance of SVR significantly but marginally on ANN, and among the EMD-based hybrid methods, the proposed CEEMDAN-SVR is the best method. Possible future works are also recommended for wind speed forecasting.

Development of Sol–Gel Icephobic Coatings: Effect of Surface Roughness and Surface Energy

Sol-gel coatings with different roughness and surface energy were prepared on glass substrates. Methyl triethoxysilane (MTEOS), 3-Glycidyloxypropyl trimethoxysilane (GLYMO) and fluoroalkylsilane (FAS) were used to obtain a mechanically robust icephobic coating. Different amount of hydrophobic silica nano particles was added as fillers to introduce different roughness and surface energy to the coatings. The microstructure, roughness, and surface energy, together with elemental information and surface chemical state, were investigated at room temperature. The contact angle and sliding angle were measured at different temperatures to correlate the wetting behavior at low temperature with the anti-icing performance. The ice adhesion shear strength was measured inside an ice chamber using a self-designed tester. The factors influencing the ice adhesion were discussed, and the optimum anti-icing performance found in the series of coatings. It was found that lower surface energy leads to lower ice adhesion regardless of the roughness, while the roughness plays a more complicated role. The wetting behavior of the droplet on surface changes as temperature decreases. The anti-icing performance is closely related to the antiwetting property of the surfaces at subzero temperatures.

Acid Decapsulation of Epoxy Molded IC Packages With Copper Wire Bonds

Epoxy molded IC packages with copper wire bonds are decapsulated using mixtures of concentrated sulfuric acid (20%) and fuming nitric acid in an automatic decapping unit and, observed with minimal corrosion of copper wires (0.8-6 mil sizes) and bond interfaces. To attain maximum cross-linking of the molded epoxies, the post mold cured packages (175 degC for 4 h) were further, aged at high temperature of 150 degC for 1000 h. These packages are decapsulated using mixtures of higher ratio of concentrated sulfuric acid (40%) along with fuming nitric acid. The shear strength of copper wire bonds with 1 mil (25 mum) diameter of the decapsulated unit is higher than 5.5 gf/mil2. The present study shows copper stitch bonds to Au, Cu, Pd, and Sn alloy plated surfaces are less affected on decapping, with a few grams of breaking load on stitch pull test, while stitch bonds on silver plated surfaces reveal lifting of wire bonds on decapping

Effect of wire diameter on the thermosonic bond reliability

Thermosonic bonding process is a viable method to make reliable interconnections between die bond pads and leads using thin gold and copper wires. This paper investigates interface morphology and metallurgical behavior of the bond formed between wire and bond pad metallization for different design and process conditions such as varying wire size and thermal aging periods. Under thermal aging, the fine pitch gold wire ball bonds (0.6 mil and 0.8 mil diameter wires) shows formation of voids apart from intermetallic compound growth. While, with 1-mil and 2-mil diameter gold wire bonds the void growth is less significant and reveal fine voids. Studies also showed void formation is absent in the case of thicker 3 mil wire bonds. Similar tests on copper ball bonds shows good diffusional bonding without any intermetallic phase formation (or with considerable slow growth) as well as any voids on the microscopic scale and thus exhibits to be a better design alternative for elevated temperature conditions.

Damping characterization of Mg–SiC composites using an integrated suspended beam method and new circle-fit approach

Abstract In the present study, a new methodology of using free–free beam method coupled with circle-fit approach is used to determine damping of Mg–SiC composites. This technique is based on classical vibration theory, by which the geometry and material properties of the metal matrix composites are related to resonant frequency and structural damping of the test specimen. Using the fact that the ratio of the vibration response to the applied force fits to a circle in the Argand plane for each resonant frequency of the test specimen, the damping factor and natural frequency is predicted accurately for the test specimen. An attempt is made to rationalize the increase in damping capability of the composites when compared against the monolithic specimen in terms of increase in dislocation density and presence of plastic zone at the particulate–matrix interface.

Porous cobalt phosphide/graphitic carbon polyhedral hybrid composites for efficient oxygen evolution reactions

We report a novel hybrid composite constructed from ultrasmall CoP nanoparticles embedded within N-doped graphitic carbon polyhedra (CoP@GC) through simultaneous pyrolysis and phosphidation of a preformed Co-based zeolitic imidazolate framework (ZIF-67). The composite structure could be a promising candidate for electrocatalytic oxygen evolution reactions.

Thermal transport in a graphene–MoS2 bilayer heterostructure: a molecular dynamics study

With the availability of various types of two-dimensional materials such as graphene (GE) and MoS2, intensive efforts have been devoted to their van der Waals heterostructures obtained by vertically stacking them together for novel functionalities and applications. The thermal transport behavior of these heterostructures plays a pivotal role in determining their functional performance. This work studies the thermal transport in a GE–MoS2 bilayer heterostructure via molecular dynamics simulation. It is found that the in-plane thermal conductivity λB of the GE–MoS2 bilayer can be approximated by that of an isolated monolayer GE. The λB of an infinitely long GE–MoS2 bilayer is calculated to be 1037 W m−1 K−1, while its out-of-plane interface thermal conductance G is obtained as 5.81 MW m−2 K−1. The increase in the interface coupling strengths can dramatically increase G but has little effect on λB. On the other hand, G also increases with temperature because of the enhanced phonon coupling between GE and MoS2. This study is helpful for understanding the interface thermal transport behaviors of novel van der Waals heterostructures and could provide guidance for optimal design and control of their thermal properties.

A Novel Empirical Mode Decomposition With Support Vector Regression for Wind Speed Forecasting

Wind energy is a clean and an abundant renewable energy source. Accurate wind speed forecasting is essential for power dispatch planning, unit commitment decision, maintenance scheduling, and regulation. However, wind is intermittent and wind speed is difficult to predict. This brief proposes a novel wind speed forecasting method by integrating empirical mode decomposition (EMD) and support vector regression (SVR) methods. The EMD is used to decompose the wind speed time series into several intrinsic mode functions (IMFs) and a residue. Subsequently, a vector combining one historical data from each IMF and the residue is generated to train the SVR. The proposed EMD-SVR model is evaluated with a wind speed data set. The proposed EMD-SVR model outperforms several recently reported methods with respect to accuracy or computational complexity.

An Evaluation of Gold and Copper Wire Bonds on Shear and Pull Testing

In microelectronic packaging technology wire bonding is a common interconnect technique. The quality and reliability of wire bonds are generally evaluated by ball shear and stitch pull testing. From the load versus time and load versus tool tip displacement plots of the shear test, three regions can be observed. Region I primarily exhibits elastic-plastic deformation occur, while crack nucleate in region II which propagates in region III which finally ends in a catastrophic failure. Fractographs reveal in the case of gold ball bonds shows fracture occurs in Al bond pad metallization close to Au-Al intermetallics. In Cu ball bonds of 1, 2, and 4 ml wire sizes also Al bond pad metallization cracks but penetrate deeper into the pad which indirectly shows that the bonding layer is stronger than that of gold ball bonds. Optical microscopic observation of the sheared copper bond surfaces reveal sticking of Al which provides qualitative information of the area of the bond between the ball bond and the bond pad. In thermally aged gold ball bonds, the gold above the intermetallic layer fractures. The energy required to fracture a gold or copper ball bond of 1 ml wire size is around 370 J/m 2 , while an aged gold ball bond consumes about 520 J/m 2 . Void nucleation and coalescence mechanism of ductile fracture takes place in the ball and stitch bonds, however, silicon particles may be the preferential void nucleation sites in bond pad aluminum metallization failures. To understand the second bond strength, a stitch pull test was conducted and the results showed the neck of the stitched wire cracks thus leaving behind a tail bond on the lead finger.