Micro and Nanosystems - Volume 2, Issue 3, 2010

Microelectronics is one of the most rapidly progressing fields of engineering science. This multidisciplinary field of science which is prevalent in everyday life. Each year completely new innovations in microelectronics appear in the market which immediately become an important and indispensable part of our life. These innovations increasingly demand precise, fast and cheap production technologies. The aim of this special issue is to give a short overview of the latest research in microelectronics applications and technologies which will lead to new innovations in the near future. Most of the corresponding authors of this special issue are researchers in the Department of Electronics Technology at Budapest University of Technology and Economics. The researchers participate in international R&D projects sponsored by the European Union (LeadOut, Felxnolead, μBuilder, EMCI) and collaborate with numerous industrial partners (Bosch, Nokia, Elcoteq, Felxtronics). This hot topic specifically focuses on microelectronics engineering in the following domains: • using contact mode Atomic Force Microscopy (AFM) for qualification of AgPd thick films and detecting very small concentration of surface contaminants; • optimizing the illumination of Automatic Optical Inspection (AOI) appliances for quasi-tombstone detection of chip size components after reflow soldering; • investigating the level differences on the deposited solder paste area by 3D structured white light measurements and FEM simulations; • developing an experimental Vapor Phase Soldering (VPS) station to improve the knowledge about vapor generation, progression and condensation processes; • examining the thickness evolution of Sn-Cu intermetallic compounds (IMCs) in laser reflowed solder joints; • developing a method which is based on the graph theory to describe the heat conduction ability of big Surface Mounted Devices (SMDs) during the reflow soldering process. The problems addressed by the papers reflect on the major trends and results in modern engineering science which are chiefly engaged with: (i) atomic microscopy in quality control (ii) automatic optical inspection; (iii) innovations in stencil printing; (iv) developments in laser and vapor phase soldering; (v) application of new mathematical methods in soldering simulations. These topics may further serve the interest of the readers of Micro and Nanosystems in sensing devices for medical and other applications as well as having broader utility.

This paper introduces a method using contact mode Atomic Force Microscopy (AFM) for qualification of AgPd thick films. The described measurement technique is capable of detecting very small concentration of surface contaminants, which can be crucial if the thick film conductor is directly bonded by ultrasonic welding. During this welding process an aluminum wire is pressed vertically onto the thick film surface; an applied horizontal vibration forms the Alwire-AgPdfilm bonded interface. The introduced method is based on a known artifact of AFM, which is a well-defined image distortion in contact mode height data and AFM deflection signal. In our case, an image distortion carries information about sample-tip interaction, this attractive/repulsive force changes if the tip moves on a contaminated surface. We suggest a short, moderate temperature annealing in order to clean the surface from previously observed contaminations. During the experiments, the optimal parameters of annealing were also determined; these parameters have been validated by pull tests.

In this paper we describe a novel approach to optimizing the illumination of Automatic Optical Inspection (AOI) appliances for quasi-tombstone detection. At present, optical testers use LEDs (Light Emitting Diode) for lighting. The properties of LED make it a perfect choice for fast, accurate lighting. Complex illumination systems have been developed to reliably detect each kind of failure; but even so these illumination devices are not perfect. At this present time they are not able to provide illumination for each kind of failure using its optimal light structure (that gives the maximum difference between good and bad parts). For AOI systems (and in general for optical sensing), illumination has primary importance. It determines what can be seen on the image when taken from the actual scene. So, the efficiency of failure detection is dependent on the appropriate system of illumination. Accordingly, the lighting method determines the capability of AOI systems. By using accurate 3D models of soldered SMDs (Surface Mount Devices), new and novel illumination optimization algorithms are described in this paper. Preparation of 3D models, their verification, the implementation of appropriate reflection properties and method for illumination optimization are also discussed.

The level differences on the Printed Wiring Board (PWB) surface can keep the stencil away from the PWB during stencil printing, which causes large deposited solder paste volume, and can result in solder bridges after reflow soldering. The level differences can be caused by identity decals, or by other artifacts on the PWB. In our research the effect of these level differences on the deposited paste area and height were investigated. Test patterns were designed to have specific thickened surfaces formed by electroplating in continuously growing distance from soldering pads. Thus, there were intentional level differences on the test board between the soldering pads and the thickened surfaces. Three test boards were fabricated with level differences of 25, 55 and 90 μm. If these thickened surfaces were too close to the soldering pads then those surfaces kept away the stencil from the PWB during subsequent printing experiments. This simulates the effect of PWB decals and uneven solder mask thickness on Cu structures. With these test patterns we could investigate the stencil bending during the printing processes. After the printing experiments a Finite Element Method (FEM) model was created to determine the necessary distance which should be kept in order to achieve complete stencil contact to the PWB during the printing process. Different level differences (0-90 μm) were inserted into the FEM model as geometrical parameters, and simulations were executed for different stencil foil thicknesses (75-175 μm). The detailed results are presented in the paper.

Vapor phase soldering (VPS) has been investigated to improve the knowledge about vapor generation, progression and condensation processes. VPS is an emerging reflow method for electronic assemblies, in terms of efficiency and quality. An experimental VPS station was developed for measurements and investigations with applying components and solders of various materials. The flexibility of the system allows custom heat profiling and temperature control. New measuring methods for the investigations have been developed and applied. In the experimental VPS system the following parameters have been studied during the heating up and cooling down processes: the vertical temperature distribution in the tank using Pt500 sensor ladder arrangement combined with data logging; the time and power required to set stationary temperatures inside the tank; the height of the vapor blanket by a custom built fiber optic probe; the height of the vapor blanket by applying a floating polymer pillow as vapor level indicator; the status of the vapor/mist/rain phases by basic optical inspection of light scattering; and the heating efficiency of the vapor blanket as function of height by varying the vertical position of the sample with temperature sensors on it. The experimental investigations show that high proportion of the vapor condensates in the space on the level of the cooling tube in the upper part of the tank, therefore the vapor is rather in mist phase with falling drops of the condensed fluid, which has an effect on the heating efficiency of the medium.

The thickness evolution of Sn-Cu intermetallic compounds (IMCs) in laser reflowed solder joints was investigated. We applied a Q-switched, frequency tripled Nd:YAG laser to precisely control the exposing energy. Sn96.5Ag3.0Cu0.5 lead free solder alloy was heated up by a two-phase, pre-heat and reflow, soldering technique. The microstructure of the solder joint was analyzed with SEM on cross-section samples. We have identified the composing elements by SEM-EDS. The results showed that the mass fraction of the IMC inside the solder bulk is significantly high, which might be a reason of the completely different mechanical properties of the laser reflowed solder joint. At the beginning of the dissolution only small sized particles were observable inside the solder bulk. With the increasing of the soldering time, the growth rate of the IMC decreased.

In this paper we present how the graph theory can be used in the field of thermal study. We have developed a method which can describe the heat conduction ability of big Surface Mounted Devices (SMDs) during the reflow soldering process. Our method established that the heat conduction ability of the materials can be described with time coefficients. These time coefficients are determined by the thermal resistance and the heat capacity of the examined materials. We implemented our method in a Matlab 7.0 program which generates a directed and weighted graph from the discrete thermal model of the investigated SMD. The program uses Dijsktra Algorithm to find the shortest “conduction path” between the different parts of the component. With our method, the following effects can be studied during the reflow soldering process: from which points and which paths can the most effective heating of a chosen contact surface be achieved, the differences between the shortest conduction paths and the effect of heat distraction.

This paper discusses the features and applications of interface tracking techniques for modeling droplet-based microfluidics. The paper reviews the state of the art of methods for tracking and capturing the interface. These methods are categorized as the implicit and the explicit methods. The explicit methods need to reconstruct the interface by reconnecting the markers on the interface. The implicit techniques implicitly describe the interface as a simple function. Thus, complicated topological changes of interface can be handled naturally and automatically. The implicit methods reviewed in this paper are the volume-of-fluid method, the phase-field method, the Lattice-Boltzmann method and the level-set method. The explicit methods mainly include the boundary-integral methods and the tracking methods.

Continuous particle sorting is one of the most important microfluidic applications. Fast and accurate sorting procedures can be performed using unique characteristics of microfluidics. Beside hydrodynamic means, provided by the flow itself, the application of an external field also contributes to sorting efficiency. Particle sorting has been realized in many effective ways. This paper systematically gives an overview on techniques used for particle sorting in microfluidics. The basic theories and fluid mechanics are presented for each approach. The implementation of the various sorting devices is discussed with their working mechanism, design and performance. Typical results from these applications are also presented. Based on the guidance from the existing technology, perspectives for future works are provided.

This paper presents a systematic overview on the recent works on surface acoustic wave (SAW) driven microfluidics. SAW microfluidics is based on acoustic streaming induced by leaky SAW radiation into a liquid. The development of this field attracts attention from microfluidic research community due to its rapid actuation, programmable capability, simple and yet efficient operation. In our paper, SAW microfluidic applications are categorized into droplet-based applications and continuous-flow applications. Droplet is actuated into unique behaviours depending on the applied SAW power. A wide range of droplet based applications have been employed utilizing these behaviours. In continuous-flow system, applications are further categorized based on the interaction of travelling SAW and standing SAW with the bulk liquid. Finally, future perspectives of SAW driven microfluidics are discussed