Breaking symmetry in liquid bridges: the effect of pinning and aspect ratio on capillary forces

Capillary bridges between solid substrates are critical to a myriad of natural and industrial processes, such as oil recovery from porous rocks, or the packaging of micro-circuitry components. Generally, the surfaces of these solids are not uniform, and contain physical or chemical heterogeneities that result in asymmetric bridge morphologies (due to the partial pinning of the solid-liquid-vapor interface). While such pinning can greatly affect the forces and morphologies of the liquid, many studies that investigate capillary bridges assume the solid surfaces to be ideal, and the subsequent bridge profiles to be highly symmetric. This thesis details our investigations of how breaking symmetry (through changing of the pinning condition or the shape of the substrates) results in quantitative changes to the properties of the capillary bridges such as the morphology, forces and torques. First, we explored the importance of the pinning condition by studying capillary bridges in a narrow rectangular slit pore, which is one of the simplest ways to break symmetry. We employed experiments, numerical simulations, and theory to highlight the importance of pinning on the bridge morphology and associated capillary forces. Experiments showed that as the height of the slit pore is increased past the width of the strip the mean curvature of the capillary bridge changes sign from negative to positive (concave to convex). This counterintuitive observation was confirmed by using Surface Evolver simulations. Interestingly, the force the capillary bridge exerts on the pore itself always remains attractive due to the dominance of the vertical projection of the surface tension force at the pinning boundary. It was also found that the mean curvature was independent of the liquid volume in the pore, as long as the bridge did not extend to the end of the strip. We developed a simple theory to show that the change in mean curvature can be described as a competition between the confinement of the liquid bridge and the wetting of the strip. Next, we studied the role of substrate shape on the restoring forces and torques of capillary based self-alignment systems, such as “flip-chip” micro-circuit packaging. To do this we varied systematically the aspect ratio of rectangular substrates under conditions where the fluid was pinned on all sides. We found that increasing the aspect ratio of the substrates (even when holding the substrate area, and liquid volume constant) resulted in higher total restoring forces and torques under both lateral and rotational perturbations. It is also shown that the rotational restoring force is of order the shift restoring force, and should generally be considered in alignment analysis. Finally, parameters from experimental flip-chip devices were used in our simulations to show how current capillary self-alignment schemes can benefit from using rectangular substrate shapes with aspect ratio greater than one

Experimental and Modeling Studies on Solder Self-alignment for Optoelectronic Packaging

Solder self-aligning technology is important to the manufacturing of cost-effective optoelectronic modules requiring accurate alignments. This thesis is to understand major effects on self-alignment accuracies in order to establish a model to guide the design for precision solder self-alignments. A solder self-alignment model based on force optimization with six degrees of freedom in a static configuration has been developed to predict an alignment accuracy with respect to different manufacturing parameters and variations. The model was used to design a VCSEL (vertical cavity surface emitting laser) array soldered on a substrate. It was proven to be a powerful tool for the design of optoelectronic modules. For example, when using Ø80 µm solder spheres with 2 µm diameter variation to attach a VCSEL chip (3200 µm × 500 µm × 650 µm) on a substrate, the model shows that the chip's standoff height variation could be reduced from 5.6 to 2.0 µm by adding extra alignment pads. Solder insufficient wetting on the bonding pads was identified to be the most undesirable factor affecting self-alignment accuracy. It could result in a planar misalignment from several to tens of µms depending on wetting quality and design parameters. Solder void was another undesirable factor that could increase the average standoff height of the assembled unit by 4 to 10 µms in the cases studied. Other factors, e.g. manufacturing variations in pad position and diameter, chip/substrate warpage, small tilt of the reflow stage, could only account for less than ±1 µm misalignments. The accuracy of the solder self-alignment model was verified by experimental characterizations using 3 mm × 3 mm glass-on-silicon flip-chip test vehicles comprising 25 solder joints. In addition to static cases, solder self-alignments in a dynamic condition was studied. The vibration of the substrate near the resonant frequencies could cause large chip-to-substrate misalignments. The resonant motion could be "frozen in" during the solidification of the reflow process and resulted in large misalignments. For a 25 mm × 25 mm ball grid array test vehicle reflowed under a horizontal vibration at 12 Hz and less than 2 µm amplitude, the chip-to-substrate lateral misalignments could reach beyond 100 µm due to the resonance effect. For any real applications, it is important to characterize the frequency range of the manufacturing environment and make sure the resonant frequencies of the assembly are far from the range

Enabling Capillary Self-Assembly for Microsystem Integration

Efficient and precise assembly of very-large quantities of sub-millimeter-sized devices onto pre-processed substrates is presently a key frontier for microelectronics, in its aspiration to large-scale mass production of devices with new functionalities and applications (e.g. thin dies embedded into flexible substrates, 3D microsystem integration). In this perspective, on the one hand established pick&place assembly techniques may be unsuitable, due to a trade-off between throughput and placement accuracy and to difficulties in predictably handling very-small devices. On the other hand, self-assembly processes are massively parallel, may run unsupervised and allow contactless manipulation of objects. The convergence between robotic assembly and self-assembly, epitomized by capillarity-enhanced flip-chip assembly, can therefore enable an ideal technology meeting short-to-medium-term electronic packaging and assembly needs. The objective of this thesis is bridging the gap between academic proofs-of- concept of capillary self-assembly and its industrial application. Our work solves several issues relevant to capillary self-assembly of thin dies onto preprocessed substrates. Very-different phenomena and aspects of both scientific and technological interest coexist in such a broad context. They were tackled both experimentally and theoretically. After a critical review of the state-of-the-art in microsystem integration, a complete quasi-static study of lateral capillary meniscus forces is presented. Our experimental setup enables also a novel method to measure the contact angle of liquids. Recessed binding sites are introduced to obtain perfectly-conformal fluid dip-coating of patterned surfaces, which enables the effective and robust coding of geometrical information into binding sites to direct the assembly of parts. A general procedure to establish solder-mediated electro-mechanical interconnections between parts and substrate is validated. Smart surface chemistries are invoked to solve the issue of mutual adhesion between parts during the capillary self-assembly process. Two chemical kinetic-inspired analytic models of fluidic self-assembly are presented and criticized to introduce a novel agent-based model of the process. The latter approach allows realistic simulations by taking into account spatial factors and collision dynamics. Concluding speculations propose envisioned solutions to residual open issues and further perspectives for this field of rapidly-growing importance

Characterisation of lead-free solder pastes and their correlation with the stencil printing process performance

Solder pastes are complex materials whose properties are governed by many factors. Variations exhibited in solder paste characteristics make it increasingly difficult to understand the correlations between solder paste properties and their printing process performance. The recent EU directives on RoHS (Restriction of Hazardous Substances – enacted by UK regulations) and WEEE (Waste from Electrical and Electronic Equipment) has led to the use of lead-free soldering in the SMA (surface mount assembly) process, and an urgent need for better understanding of the characteristics and printing performance of new solder paste formulations. Equally, as the miniaturisation of hand-held and consumer electronic products continues apace, the solder paste printing process remains a real challenge to the electronics assembly industry. This is because the successful assembly of electronic devices at the ultra-fine pitch and flip-chip geometry requires the deposition of small and consistent paste deposits from pad to pad and from board to board. The paste printing process at this chip-scale geometry depends on conditions such as good paste roll, complete aperture filling and paste release from the apertures onto the substrate pads. This means that the paste flow and deformation behaviour, i.e. the paste rheology, is very important in defining the printing performance of any solder paste. Rheological measurements can be used as a tool to study the deformation or flow experienced by the pastes during the stencil printing process. In addition, the rheological measurements can also be used as a quality control tool in the paste production process for identifying batch-to-batch variation, and to reduce the associated printing defects in the paste printing process. The work reported here on the characterisation of lead-free solder pastes and their correlation with the stencil printing process is divided into five main parts. The first part concerns the study of the effect of variations in flux and particle size distribution (PSD) on the creep recovery performance of lead-free solder pastes used for flip-chip assembly. For this study, a novel technique was calculating the extent of paste recovery and hence characterising the slumping tendency in solder pastes. The second part of the study concerns the influence of long-term ageing on the rheology and print quality of lead-free solder pastes used for flip-chip assembly, and the main focus of the work was to develop methodologies for benchmarking new formulations in terms of shelf life, rheological deterioration and print performance. The third part of the work deals with a rheological simulation study of the effect of variation in applied temperature on the slumping behaviour of lead-free solder pastes, and the fourth part considers the rheological correlation between print performance and abandon time for lead-free solder paste used for flip-chip assembly. The final part of the study concerns the influence of applied stress, application time and recurrence on the rheological creep recovery behaviour of lead-free solder pastes. The research work was funded through the PRIME Faraday EPSRC CASE Studentship grant, and was carried out in collaboration with Henkel Technologies, Hemel Hempstead, UK. The extensive set of results from the experimental programme, in particular relating to the aspect of key paste performance indicators, has been adapted by the industrial partner for implementation as part of a quality assurance (QA) tool in its production plant, and the results have also been disseminated widely through journal publications and presentations at international conferences

Workshop on "Robotic assembly of 3D MEMS".

Proceedings of a workshop proposed in IEEE IROS'2007.The increase of MEMS' functionalities often requires the integration of various technologies used for mechanical, optical and electronic subsystems in order to achieve a unique system. These different technologies have usually process incompatibilities and the whole microsystem can not be obtained monolithically and then requires microassembly steps. Microassembly of MEMS based on micrometric components is one of the most promising approaches to achieve high-performance MEMS. Moreover, microassembly also permits to develop suitable MEMS packaging as well as 3D components although microfabrication technologies are usually able to create 2D and "2.5D" components. The study of microassembly methods is consequently a high stake for MEMS technologies growth. Two approaches are currently developped for microassembly: self-assembly and robotic microassembly. In the first one, the assembly is highly parallel but the efficiency and the flexibility still stay low. The robotic approach has the potential to reach precise and reliable assembly with high flexibility. The proposed workshop focuses on this second approach and will take a bearing of the corresponding microrobotic issues. Beyond the microfabrication technologies, performing MEMS microassembly requires, micromanipulation strategies, microworld dynamics and attachment technologies. The design and the fabrication of the microrobot end-effectors as well as the assembled micro-parts require the use of microfabrication technologies. Moreover new micromanipulation strategies are necessary to handle and position micro-parts with sufficiently high accuracy during assembly. The dynamic behaviour of micrometric objects has also to be studied and controlled. Finally, after positioning the micro-part, attachment technologies are necessary

Mechanical and electrical characterisation of anisotropic conductive adhesive particles

This thesis presents research into the mechanical and electrical characterisation of Anisotropic Conductive Adhesive (ACA) particles and their behaviour within typical joints. A new technique has been developed for study of individual ACA particle mechanical and electrical performance when undergoing deformation. A study of the effects of planarity variations on individual electrical joints in real ACA assemblies is presented firstly, followed by the research on the mechanical deformation and electrical tests of individual ACA particles undergoing deformation. In the co-planarity research, experiments introducing deliberate rotation between a chip and substrate were designed and carried out to simulate planarity variations in ACA assemblies. There are two outputs from this part of the research. One is the planarity variation effects on individual electrical joints in ACA assemblies, and the other is the effect of bond thickness on the resistance of a real joint. [Continues.

Conceptual Study of Rotary-Wing Microrobotics

This thesis presents a novel rotary-wing micro-electro-mechanical systems (MEMS) robot design. Two MEMS wing designs were designed, fabricated and tested including one that possesses features conducive to insect level aerodynamics. Two methods for fabricating an angled wing were also attempted with photoresist and CrystalBond™ to create an angle of attack. One particular design consisted of the wing designs mounted on a gear which are driven by MEMS actuators. MEMS comb drive actuators were analyzed, simulated and tested as a feasible drive system. The comb drive resonators were also designed orthogonally which successfully rotated a gear without wings. With wings attached to the gear, orthogonal MEMS thermal actuators demonstrated wing rotation with limited success. Multi-disciplinary theoretical expressions were formulated to account for necessary mechanical force, allowable mass for lift, and electrical power requirements. The robot design did not achieve flight, but the small pieces presented in this research with minor modifications are promising for a potential complete robot design under 1 cm2 wingspan. The complete robot design would work best in a symmetrical quad-rotor configuration for simpler maneuverability and control. The military’s method to gather surveillance, reconnaissance and intelligence could be transformed given a MEMS rotary-wing robot’s diminutive size and multi-role capabilities

Micro/Nano Structures and Systems

Micro/Nano Structures and Systems: Analysis, Design, Manufacturing, and Reliability is a comprehensive guide that explores the various aspects of micro- and nanostructures and systems. From analysis and design to manufacturing and reliability, this reprint provides a thorough understanding of the latest methods and techniques used in the field. With an emphasis on modern computational and analytical methods and their integration with experimental techniques, this reprint is an invaluable resource for researchers and engineers working in the field of micro- and nanosystems, including micromachines, additive manufacturing at the microscale, micro/nano-electromechanical systems, and more. Written by leading experts in the field, this reprint offers a complete understanding of the physical and mechanical behavior of micro- and nanostructures, making it an essential reference for professionals in this field

Microscale Infrared Technologies for Spectral Filtering and Wireless Neural Dust

Pivotal technologies, such as optical computing, autonomous vehicles, and biomedical implantables, motivate microscale infrared (IR) components. Hyperspectral imagers (HSI), for example, require compact and narrowband filters to obtain high-spatial and -spectral resolution images. HSIs acquire continuous spectra at each pixel, enabling non-destructive analyses by resolving IR scattering/absorption signatures. Toward this end, dielectric subwavelength gratings (SWG) are intriguing filter candidates since they are low-loss, have no moving parts, and exhibit narrow spectral features. Wireless neural implantables are another apropos microscale IR technology. Wireless IR data and power transfer disposes of infection-prone percutaneous wires by leveraging the IR transparency window in biological tissue. This dissertation contains two related topics. The first details SWG IR filters, and the second studies progress toward wireless neural motes. This work extends the capabilities of SWG IR filters. Following a theoretical overview, mid-wave infrared (MWIR, 3-7 um) transmittance filters are experimentally demonstrated using the zero-contrast grating scheme. Via a facile silicon fabrication process, we realize narrowband polarization-dependent and polarization-independent MWIR transmittance filters with some of the highest Q observed in MWIR SWGs. An empirical study confirms the relationship between filter performance and grating size, an important trade-off for HSIs. We then demonstrate GaAs SWG filters for monolithic integration with active optoelectronic devices. The GaAs SWGs perform comparably to their silicon counterparts. To enable narrowband filtering at normal incidence, we investigate symmetry-breaking in geometrically asymmetric gratings. The presented SWG geometries access quasi-bound states in the continuum (BIC). Studies in Fano resonance and diffraction efficiency symmetry provide physical insight. Asymmetric 1D and 2D SWGs furnish polarization-dependent and -independent filtering, respectively. We experimentally demonstrate normal incidence long-wave IR (LWIR, 7-12 um) transmittance filtering in asymmetric SWGs and confirm symmetry-breaking implications. A reduced-symmetry hexagonal pattern presents an early design for truly polarization-independent quasi-BIC coupling in SWGs. Advancements in implantable neural devices promise great leaps in brain mapping and therapeutic intervention. To meet this challenge, we investigated a wireless neural mote system using near-infrared (NIR, 800 nm – 3 um) photovoltaics and LEDs to wirelessly harvest power and transmit data. The neural recorders consist of three subsystems: an epitaxial GaAs-based optoelectronic chip, a Si CMOS IC, and a carbon fiber probe. Though this work encompasses the efforts of many, this dissertation outlines contributions in a few critical areas. To overcome low-flux LED emission, we devise an optical setup with ≈0.1% photon detection efficiency. Monte Carlo techniques model NIR scattering in biological tissue. Another steep challenge is the heterogeneous integration of the three material systems in a compact (200x170x150 um^3) package. To relay data and power between the GaAs and CMOS chips, through-wafer vias are critical. Using a novel selective copper plating technique, we demonstrate through-wafer GaAs vias with <2 Ohm series resistance. Additionally, conductive blind vias are presented for carbon fiber probe insertion. A self-aligned parylene etch mask permits sub-kOhm connection to a buried metal contact while maintaining GOhm substrate isolation. Both via structures meet the requirements of being low-resistance, insulated from the substrate, and amendable to thinned wafer processing. Finally, we demonstrate extensive processing on thinned chips and advances toward full heterogeneous integration via flip-chip alignment and solder bump bonding.PHDElectrical and Computer EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/169986/1/barrowm_1.pd