Research 2018-2019

Beyond Silicone Elastomer - Ahyeon Koh

The flexible and stretchable electronics and biosensors have sought for intimate integration with the biological system as well as device and package strategies in the form of biocompatible and functional system integration. The recent developments in wearable electronics mainly based on using the elastomeric silicone-based polymer as a substrate of the active and passive components due to its mechanical similarity with the skin. We herein propose to develop the bio-inspired silicone elastomer materials for advancing skin-mountable bioelectronics. Our project involves two primary objectives: (1) The thin, nanomesh elastomeric silicone-based polymer substrates will be investigated for mechanically and biochemically compatible electronics, which may minimize inflammatory response when laminating on the skin. The polydimethylsiloxane (PDMS) will be lectrospun to create an open porous structure. The architecture of nonwoven fibers will support the electronics while it allows not only intimate integrations on the living organisms but also grants free biofluids and gas transport without artificial biochemical accumulation around the devices. (2) We will investigate the collagen composite of the PDMS to capture and store the biofluids in the soft, elastomeric polymer reservoirs for a long time. The gas (vapor) and water permeability of PDMS become an obstacle for long-term operations with aqueous samples. In this task, we hypothesis composite material (e.g., collagen) embedding into PDMS film will decrease vapor and water permeability while maintaining key material superiorities for biomedical applications. Indeed, this project aims to revolutionize the broad range of wearable devices by advancing material platforms and the proposed project, together with analytical assessments and systematic studies, will lead to evolving in advanced materials and manufacturing of bio-integrated electronics.

Computational Study on the Mechanical Properties of Bi alloys for Pb-free Solder" – Manuel Smeu

Lead possesses many desirable mechanical properties for solder joints such as being ductile yet strong. However, it is a heavy metal with high toxicity in humans and also environmentally harmful, so its elimination from use in industry is in demand. One promising alternative element is bismuth, which shares some properties with lead, but is known to be brittle instead of ductile. Addressing this limitation would pave the way for Pb-free solder, which he hope to do by alloying/doping bismuth with various elements. We expect that this will modify the properties of the parent metal, which might render the doped/alloyed bismuth more ductile while maintaining other desired traits. Our approach aims to use computational modeling based on density functional

theory to calculate the mechanical properties of our alloyed Bi1-xMx systems (M = Sb, In, Sn, and Te), where x is increased form 0 to 1 by increments of 0.1. Two figures of merit will be extracted from our calculations, the Pugh ratio as well as the Stacking Fault Energy; both of these quantities are related to the ductility of a materiel, giving us a relation between computation and physical properties. We expect to find that the mechanical properties of bismuth can be modified by alloying this material with other metals, rendering it more suitable for use as a Pb-free solder.

Examination of Electromigration Effects in SnBi eutectic and Sn rich SnAgBi Solders – Eric Cotts and Nikolay Dimitrov

We will study the materials science of Pb free solder joints containing Bi. Pb-free solders containing solid substitutional elements (e.g., Bi) are more stable, and perform better in many applications, than do solders such as SAC305, but little is known about several aspects of applications of these materials. Focus will be on mixed assembly SAC/Bi-Sn solder joints (e.g., SAC305/Bi42Sn, Fig. 1), which promise to provide lower temperature assembly for a wide range of conditions. The eutectic Bi42Sn solder alloy enables process temperatures as low as 170oC (Fig. 2) and is widely available in paste form (the Bi42Sn1Ag is a popular variant intended to improve interfacial toughness in drop shock loading). We propose to examine the properties of mixed solder alloy joints with combinations of SAC305 or SAC405 solder balls, and Bi42Sn or Bi42Sn1Ag solder paste (Fig. 1).

Femtosecond laser micromachining of waveguides and tomographic characterization – Bonggu Shim

In this proposed research, we will first perform laser direct writing (i.e., femtosecond laser micromachining) to fabricate waveguides in rigid and flexible glass materials. Second, to measure the potentially asymmetric index profiles of the fabricated waveguides, we will use computed tomography (CT) which has recently been implemented in our group. Third, to fundamentally understand the dynamics of femtosecond laser micromachining, we will experimentally and theoretically investigate material structural changes during micromachining using femtosecond time-resolved interferometry/holography and sophisticated laser-matter interaction simulations.

Micron-level Thermal and Mechanical Characterization of Thermal Interface Materials to Understand Material Variability to Prevent Hot-Spot – Scott Schiffres

New high thermal conductivity thermal interface materials (TIMs) have been developed to transport the growing heat fluxes from microelectronics, yet spatial variation in the thermal conductance across TIMs leads to localized hot spots that can cause premature device failure. TIMs suffer spatial variation due to many causes, including voiding in TIM, separation of high-thermal conductivity filler from the matrix during curing, poor lid or die mechanical bonding, and mechanical strain from solder joints and CTE (coefficient of thermal expansion) mismatch, yet spatial variations in thermal conductance remains relatively unexplored in the literature.

Properties and Use of Fused Nano-Copper as Copper Pillar Interconnect – Peter Borgesen

Sintered/fused nano-particles of Cu or Ag are finding a growing number of potential applications in electronics manufacturing. It is proposed to continue an ongoing effort to understand and predict the behaviour in a variety of applications. Emphasis will be on the attachment of Cu-pillars to pads and the filling of Through Glass Vias (TGVs), in both cases with Cu nano-particles. However, the overall goal is a general understanding of the behaviour of this class of materials. Focus will be on effects of design, material and process on performance under combinations of aging, thermal and mechanical cycling, and electromigration. Long term goals include the development of accelerated test protocols and means of interpreting results.

Smart Chip Mounting Process Control in PCB Assembly Process via Artificial Intelligence – Daehan Won and Yoon

The 4th industrial revolution (also known as Industry 4.0) is characterized by a fusion of technologies that is blurring the lines between the physical and digital spheres. This revolution will have far-reaching consequences across sectors and, ultimately, it will make life easier on all fronts by streamlining processes, increasing qualities, and leading to advancements. Within electronics manufacturing, production processes are connected and components communicate with each other. Large amounts of data can be collected and it can be used for intelligent systems as a core of 'smart' manufacturing. Surface mount technology (SMT) is the main technology of core components of modern electronics assembly and it has a huge potential to move forward to the next generation of manufacturing along with technological advances. In this project, we aim to build the intelligent control system including automatic control of the chip mounting process via introducing artificial intelligence techniques and providing optimized setting to increase the quality of the product. By use of the state-of-art methodology in machine learning (i.e., Deep learning), we identify the factors related to the quality (via Automated Optical Inspection; AOI) of the chip mounted printed circuit boards (PCBs) based on the inspected results obtained from printed inspection machine (via Solder Paste Inspection; SPI). The determined factors and extracted information will be utilized to formulate the optimization model to determine the best setting for the chip mounting process leading to highly qualified products and support automated decision making. Such two processes will be modularized and embedded into an intelligent manufacturing system to control the process in smart and efficient ways.

Three Phase High Power Density Wide Band Gap (WBG) Devices and commercial off the shelf (COTS) components Based AC to DC Power Converters for More Electric Aircraft Systems – Pritam Das

This project aims to develop SiC MOSFET based single-stage completely interleaved, high efficiency (>96%) and high power density (~40 W/in3) THREE-PHASE bi-directional AC-DC power supply units (PSUs) having input power factor correction (PFC) capability and providing an isolated dc output for medium scale more electric aircrafts (MEA). Further for future commercialization, this project also includes development of improved packaging of SiC MOSFET and GaN based power modules with optimized parasitics dedicated for proposed converter. This module will support fast switching speeds of the order to 12~15 ns from 1 kV OFF state voltage and ON state currents of 75 Amps. The front end AC to DC converter is implemented with SiC MOSFET (900 V) and back end DC to DC converter is implemented with GaN (600 V).