Loading Direction Versus Crack Propagation Path in a Lead-Free Single Solder Joint Sample

In this paper, single solder joints (SSJs) were subjected to moderate speed loading (5mm/sec) in different directions, from pure tensile, mixed mode to pure shear. Fracture surfaces from different loading directions were examined both experimentally and numerically. It is observed that intermetallic compound (IMC) is formed between the solder alloy and the Cu pad, and failure typically occurs at or near the solder/IMC/Cu interfaces on the board side. Pure tensile loading typically leads to interfacial fracture along the IMC/Cu interface. Mixed mode loading usually results in a mixture of interfacial and cohesive failure with crack propagating in a zigzag fashion between the solder/IMC interface and the solder alloy. Loading with higher shear component tends to result in more cohesive failure of the solder alloy near the solder/IMC interface. Under pure shear loading, failure is almost always cohesive within the solder alloy near the solder/IMC interface.

Mechanical and Electrical Properties of Micro-Lines Fabricated by Laser-Assisted Maskless Microdeposition

The present paper is concerned with the analysis of the microstructural properties of silver micro-lines produced by Laser-Assisted Maskless Microdeposition (LAMM). LAMM is a laser based direct write method used in microscale layered manufacturing. In LAMM, liquid-suspended nanoparticles of a variety of materials are deposited in a layer-by-layer fashion and cured by a laser radiation. In this work, conductive micro-lines of silver with widths of 20 μm are fabricated, and their microstructures as well as electrical and mechanical properties are studied. Investigations show that the microstructures are affected by the laser power and the laser scanning velocity. To find the effect of laser processing parameters on the electrical performance of the samples, the conductivity of the samples are expressed in terms of the effective energy absorbed during laser radiation. It is shown that the conductivity of the sintered samples is increased up to 2 × 105 S.m−1 by raising the effective energy density. In addition, mechanical properties, i.e. modulus of elasticity of one of the fabricated samples are obtained using the nanoindentation test.

Optimization of Refrigeration Systems for High-Heat-Flux Microelectronics

Increasingly, military and civilian applications of electronics require extremely high heat fluxes, on the order of 1000 W/cm2. Thermal management solutions for these severe operating conditions are subject to a number of constraints, including energy consumption, controllability, and the volume or size of the package. Calculations indicate that the only possible approach to meeting this heat flux condition, while maintaining the chip temperature below 50 °C, is to utilize refrigeration. Here we report an initial optimization of the refrigeration system design. Because the outlet quality of the fluid leaving the evaporator must be held to approximately less than 20%, in order to avoid reaching critical heat flux, the refrigeration system design is dramatically different from typical configurations for household applications. In short, a simple vapor-compression cycle will require excessive energy consumption, largely because of the superheat required to return the refrigerant to its vapor state before the compressor inlet. A better design is determined to be a “two-loop” cycle, in which the vapor-compression loop is coupled thermally to a primary loop that directly cools the high-heat-flux chip.

An Examination of Flow Regimes Associated With Inkjet-Printed Colloidal Drops

An experimental study of the evaporation dynamics for an inkjet-printed drop on a solid substrate has been attempted. Using fluorescent spherical colloids, the internal flow of an evaporating drop has been observed directly. During the initial stages of the process, a novel inward radial flow carries the particles as a single group towards the center of the drop. Once the particles have converged near the center, a significantly slower outward radial flow proceeds. These flow regimes are qualitatively analyzed and their relationship to a possible thermal Marangoni flow is discussed.

Thermal Stress Measurement of Solder Joints in BGA Packages: Theoretical and Experimental

In this study, an analytical model to obtain a closed-form solution for thermomechanical behaviours of BGA (Ball Grid Array) package was derived and experimentally validated. In the theoretical analysis, the BGA package was represented by a three-layer axisymmetrical model: two layers of dissimilar materials jointed by a graded interlayer. Based on the classical bending theory, the thermal stresses induced by temperature changes were calculated accurately. 2-D FE (Finite Element) meshes of BGA packages subjected to high temperature were used to verify the theoretical solutions. Furthermore, two types of BGA samples, each with eutectic (63wt%Sn/37wt%Pb) and Pb-free SAC387 (95.5wt%Sn/3.8wt%Ag/0.7wt%Cu) solder joints respectively, were experimentally investigated by high resolution Moire´ Interferometry (MI). Thermal cycling tests were performed on each package with temperature variation from 25°C to 125°C. It was found that the thermal deformation obtained from moire´ tests matched well with those from analytical solutions and FE analyses. Based on the shear strain values, the reliability characteristics of BGA assemblies were also assessed.

Investigation of Moisture Sensitivity Related Failure Mechanism of Quad Flat No-Lead (QFN) Packages

Interfacial delamination is a long existing problem in the moisture preconditioning process and reflow. The failure is caused by the competition between interfacial strength and hygrothermal stress. Many simulations based on the finite element model have been applied to study the failure mechanism of this phenomenon. However, the difficulty in obtaining material properties of mini-size packages, the lack of experiment investigation of interfacial adhesion and the less-understood moisture analysis will always bring many challenges to simulations. To avoid the above issues, dummy QFN packages were fabricated as the test vehicle for the investigation of the moisture related failure. The major advantage of using dummy packages is that all material properties could be traced and all geometric parameters could be determined without ambiguities. With everything under control, failure modes could be generated within expectation. This would provide a good experiment comparison for future finite element analysis. In this study, several experiment procedures were implemented to establish the relationship between material selection and moisture sensitivity level (MSL) test performance. They were package fabrication, mechanical tests for interfacial adhesion, C-SAM and cross-section inspections. Based on the experimental results, features of the moisture related failure mechanism are presented in this paper.

Modeling and Experiments of Underfill Flow in Large Dies With Non-Uniform Bump Patterns

This paper presents numerical modeling and experimental results for the problem of underfill flow in a large die with a non-uniform bump pattern in a flip-chip packaging configuration. Two different 2-D flow models coupled with the volume-of-fluid (VOF) method are applied to track the underfill flow front during the simulation of flip-chip encapsulation. The first model employs the modified Washburn model and uses a time-dependent inlet velocity to account for the flow resistance across the gap direction in the presence of bump interconnects. The second model introduces a momentum source term in the Stokes equation to represent the gapwise resistance. Rheological properties, surface tension, and dynamic contact angles for an underfill material are experimentally determined. Simulation results based on the two models are compared with in-situ flow visualization conducted using bumped quartz dies. The comparison demonstrates the applicability of each model for simulating the underfill encapsulation process.

Investigation of Fiber Optic Sensor for Monitoring of Ammonia

Detection and characterization of chemical contaminants in water network is paramount for water quality and water security. The current trend of monitoring the presence of contaminants is the batch sampling technique, where sample of water is collected and analyzed in the laboratory. While this technique is accurate, it fails to provide immediate information. In this work, the authors investigate the effectiveness of utilizing a fiber optics based sensor for detecting ammonia in water. In order for the system to sense ammonia, a small portion of the cladding of the fiber optic cable is stripped and replaced by a porous polymer material. A novel procedure of etching the glass cladding is reported. The modified cladding when interacts with ammonia causes a change in intensity of the electromagnetic wave flowing through the cable. The change in intensity caused by the modified cladding is studied parametrically which will help in forming a correlation between concentration of ammonia and absorbance.

Investigation of Device Interactions Between Two MOSFETs in Si CMOS

Experimental works about the device interactions between nMOS and pMOS in bulk Si CMOS were performed. In the bulk Si CMOS, in the case that the distance between two MOSFETs is not enough, it is important to consider the risk of the device interactions between nMOS and pMOS. In this work, we fabricated bulk Si CMOS, in which the distance between pMOS and nMOS can be variable. And we observed the characteristics of the device operation by using fabricated CMOS under the dc bias condition. In this research, we focused on the leakage current between two MOSFETs in CMOS inverter depending on the distance between two MOSFETs, applied voltage and temperature. Experimental results showed that our fabricated CMOS shows quite small leakage current and the leakage current is less than 1% compared to CMOS on state current even with small distance between two MOSFETs at the high voltage condition and the high temperature condition.

Single Crystal Quartz a Replacement for Diamond Optical Windows

Diamond windows are used extensively in the field of optics due to their high transmittance and durability. However, despite their ability to withstand high pressures, diamond windows are not scratch resistant and need to be replaced when the surface is damaged. Moreover, the high cost of diamond windows necessitates extra care to protect the windows and limits the practical size of the window or lens. Thus, alternatives to the highly expensive diamond windows are needed in the optical sciences. A study of single crystal quartz has been conducted to determine if it will make a suitable replacement material. Since the transmittance of single crystal quartz is well documented and desirable for this application, only strength and surface defect experiments were conducted. Trials were run to determine the modulus of rupture of single crystal quartz samples which were also examined with an interferometer and an atomic force microscope (AFM) to correlate the surface conditions with the modulus of rupture. The results showed that even relatively numerous and large defects on the surface did resulted in single crystal quartz holding to high pressures. In addition, the measured modulus of rupture far exceeded the expected values proving that the single crystal quartz is able to withstand the pressures of vacuum. Single crystal quartz is thus found to be a viable alternative to diamond optical windows.