Numerical and experimental analysis of large passivation opening for solder joint reliability improvement of micro SMD packages

In this paper, using a recently developed solder fatigue model for wafer level-chip scale package (WL-CSP), we investigated the improvement on solder joint reliability for a 8-bump micro SMD package by enlarging the passivation layer opening at the solder–die interface. The motivation to enlarge the passivation opening is to reduce the severity of the stress concentration caused by the original design, and also to increase the contact area between the solder bump and aluminum bump pad. It was confirmed in the thermal shock test that with the new design, package fatigue life improved by more than 70%. To numerically predict this improvement represents a unique challenge to the modeling. This is because in order to capture the slightest geometrical difference on the order of a few microns between the two designs, the multiple-layer solder-die interface needs to be modeled using extremely fine mesh, while the overall dimensions of the package and the test board are on the order of millimeters. To bridge this tremendous gap in geometry, a single finite element model that incorporates all necessary geometrical details is deemed computationally prohibitive and impractical. In this paper, we applied a global–local modeling scheme that was also suggested by others [1, 2 and 3]. The global model contains the complete package with much simplified solder–die interface whereas the local model includes only one solder joint, but with detailed solder–die interface. Unlike most global–local models proposed by others, we included time-independent plasticity and temperature-dependent materials in the global model. This greatly improved model correlation accuracy with only moderate increase in run time. Energy-based solder fatigue model was used to correlate the inelastic strain energy with the package fatigue life. In an earlier study [4], we have found that Darveaux’s equations tended to be conservative when applied to the micro SMD, and hence new correlations based on curve-fitting the test data were derived. In this paper, we used the newly derived equation and achieved less than 20% error in N₅₀ life for both designs, which is on par with Darveaux’s equations when used for BGAs. The analysis also revealed two factors that may account for the life improvement. First, a slight decrease in inelastic energy dissipation after enlarging the passivation opening. Second, the shift of the crack initiation location which leads to longer crack growth length for the new design. The second factor was also independently confirmed by the failure analysis.     

Conjugated Polymer Based Nanoparticles as Dual‐Modal Probes for Targeted In Vivo Fluorescence and Magnetic Resonance Imaging

A facile strategy is developed to synthesize dual‐modal fluorescent‐magnetic nanoparticles (NPs) with surface folic acid by co‐encapsulation of a far‐red/near‐infrared (FR/NIR)‐emissive conjugated polymer (PFVBT) and lipid‐coated iron oxides (IOs) into a mixture of poly(lactic‐co‐glycolic‐acid)‐poly(ethylene glycol)‐folate (PLGA‐PEG‐FOL) and PLGA. The obtained NPs exhibit superparamagnetic properties and high fluorescence, which indicates that the lipid coated on IOs is effective at separating the conjugated polymer from IOs to minimize fluorescence quenching. These NPs are spherical in shape with an average diameter of ≈180 nm in water, as determined by laser light scattering. In vitro studies reveal that these dual‐modal NPs can serve as an effective fluorescent probe to achieve targeted imaging of MCF‐7 breast cancer cells without obvious cytotoxicity. In vivo fluorescence and magnetic resonance imaging results suggest that the NPs are able to preferentially accumulate in tumor tissues to allow dual‐modal detection of tumors in a living body. This demonstrates the potential of conjugated polymer based dual‐modal nanoprobes for versatile in vitro and in vivo applications in future.     

Electromigration in Sintered Nanoscale Silver Films at Elevated Temperature

Sintered nanoscale silver is a promising interconnection material for semiconductor devices because it provides improved joint properties compared with solder and wire bonds. It has higher electrical and thermal conductivity and is capable of higher operating temperature. Joints with die shear strength above 20 MPa can be formed at around 250°C even without applied pressure. Sintered silver joints were also found to be an order of magnitude more reliable than solder joints and wire bonds. In this work, the electromigration behavior of sintered nanosilver material under conditions of high applied current density and elevated temperature was investigated. Thin strips of sintered nanosilver formed on ceramic substrates were tested under current densities exceeding 150 kA/cm² at temperatures of 150°C and above. Results based on the percentage change in sample resistance showed that the sintered silver lasted at least ten times longer than aluminum wire bonds. Examination of failed strips revealed that hairline cracks formed during sintering were the main cause of failure. Otherwise, defect-free samples exhibited a 10-fold increase in lifetime over wire bonds under similar conditions.     

Processing of polyurethane/polyester interpenetrating polymer networks by reaction injection molding (RIM). Part I: Design of a high pressure RIM system

This is the first of a three-part paper investigating the feasibility of processing polyurethane/polyester Interpenetrating Polymer Networks (IPN) by Reaction Injection Molding (RIM). The fundamental mixing process is discussed. Various approaches reported to analyze mixing are also critically reviewed. Good part quality in RIM mixing activated systems suggests the need for the creation of a rapid and intense turbulent state within the small impingement chamber. In order to generate the conditions of reactive components intermingling down to submicron dimensions, a high pressure RIM unit was built and tested. The device can inject reactants at high stream Reynolds numbers (around 10,000) and impingement pressures (up to 100,000 psi, 6,895 bar.).     

From cholesterol to oxysterols. Current data

Oxysterols, a class of cholesterol oxidation products exhibit several important biological activities. Some of these natural compounds are potent inhibitors of the enzyme HMG-CoA reductase, a key enzyme in the cholesterol biosynthetic pathway. Many studies have been directed towards to the verification of the hypothesis that some oxysterols are endogenous intracellular regulators of cholesterol homeostasis. In adition to oxysterols derived directly from oxidation of cholesterol, several others are formed from squalene dioxide. It is presently well established that, in addition to the classical cholesterol biosynthetic pathway, there exists an alternate bifurcation from squalene oxide. The cyclisation of squalene dioxide leads to a series of new oxysterols. Thus, several types of oxysterols and several molecular targets are involved in the regulation of steroid biosynthesis. Many oxysterols, particularly those obtained from the oxidations of phytosterols and tetracyclic triterpenes are potent cytotoxic agents. They are selectively cytotoxic against tumorous cells. This cytotoxicity depends markedly on the specific structure of each oxysterol. Some structures are very cytotoxic, while their stereoisomers are inactive. The activity depends on the tumor cells which are used in the assay system: some compounds display inhibitory activity towards hepatoma cells but are inactive against lymphoma cells while others act in the opposite manner. Free oxysterols do not depress tumor growth in living animals. However, several water soluble prodrugs of oxysterols are able to depress different type of tumors in vivo. Clinical trial studies are presently conducted in order to learn the therapeutic values of these oxysterols.     

Computing reachable states for nonlinear biological models

In this paper, we describe reachability computation for continuous and hybrid systems and its potential contribution to the process of building and debugging biological models. We summarize the state-of-the-art for linear systems and then develop a novel algorithm for computing reachable states for nonlinear systems. We report experimental results obtained using a prototype implementation applied to several biological models. We believe these results constitute a promising contribution to the analysis of complex models of biological systems.     

Fourier analysis of energy transfer data obtained by simulating a 14-MeV α-particle in water

Data from Monte Carlo transport codes are used to model radiobiological effects. We previously reported the Fourier analysis of ionization data generated by simulating a 500-keV proton traversing water. Here, we extend Fourier analysis to energy transfer data of another radiation type, a 14-MeV α-particle. A radiobiological model based on this frequency-domain analysis views cell as an information processing system [8]. It lends itself naturally to traditional engineering analyses. One engineering principle—the output response of a linear system to random signal—is applied here to explain the fact that there is measurable difference in the magnitude of the biological effectiveness when a given biological system is irradiated with two different radiation types of the same Linear Energy Transfer (LET).     

Effect of hydrogen environment on the notched tensile properties of T-250 maraging steel annealed by laser treatment

In the present work, slow displacement rate tensile tests were performed to find out the influence of ageing condition and hydrogen-charging on the notched tensile strength and fracture characteristics of T-250 maraging steel aged at various conditions. The influence of embrittling species in the environment on the notched tensile strength was accessed by comparing the measured properties in air, gaseous hydrogen and H₂S-saturated solution. The hydrogen diffusivity, permeation flux and apparent solubility of various specimens determined by electrochemical permeation method, were correlated well with the microstructures and mechanical property. The results indicated that the peak-aged (H900) specimen was highly sensitive to hydrogen embrittlement even in gaseous hydrogen. In contrast, the microstructures of over-aged (H1100) specimen comprising of reverted austenite and incoherent precipitates could trap large amount of hydrogen atoms, resulting in decreased hydrogen permeability and hydrogen embrittlement susceptibility. The solution-annealed specimen had the highest diffusion coefficient and the lowest quantity of trapped hydrogen among the specimens, showing high susceptibility to sulfide stress corrosion cracking. In the presence of notches, hydrogen atoms were prone to segregate and trap at grain boundaries, resulting in the formation of intergranular fracture.     

Organoclay Filled Natural Rubber Nanocomposites: The Effects of Filler Loading

Organoclay filled natural rubber (NR) nanocomposites were prepared using a laboratory two-roll mill. The effect of organoclay loading up to 10 phr was studied. The vulcanized nanocomposites were subjected to mechanical, thermal, and swelling tests. The results indicated that the tensile strength and elongation at break reached optimum at 4 phr of organoclay loading, and the incorporation of organoclay increased the tensile modulus and hardness of NR nanocomposites. The thermal degradation was shifted to a higher temperature and the weight loss decreased with incorporation of organoclay. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were employed to characterize the microstructure of NR nanocomposites. Results from TEM and XRD show the formation of intercalated and exfoliated individual silicate layers of organoclay filled NR nanocomposites particularly at low filler loading (< 4 phr).