Rao Tummala

Bio: Rao Tummala is an academic researcher from Georgia Institute of Technology. The author has contributed to research in topics: Interposer & Capacitor. The author has an hindex of 43, co-authored 623 publications receiving 11663 citations. Previous affiliations of Rao Tummala include Qualcomm & IBM.

3D IPAC — A new passives and actives concept for ultra-miniaturized electronic and bioelectronic functional modules

Abstract: This paper describes an innovative 3D IPAC (Integrated Passive and Active Components) concept for ultra-miniaturized and highly-functional sub-systems, going beyond discrete passives and Integrated Passive Devices (IPDs). The 3D IPAC concept consists of an ultra-thin 3D structure made of low loss and ultra-thin glass substrates with small-diameter through-vias, and ultra-thin active devices and thin or thick passive films or thin discrete passives on both sides. The first part of the paper describes the benefits of the 3D IPAC concept. The second part of the paper describes a wireless power telemetry module for a bioelectronic system application, and provides proof-of-concept demonstrations for the thinfilm passive components as a building block in the 3D IPAC telemetry module.

Demonstration of 20µm pitch micro-vias by excimer laser ablation in ultra-thin dry-film polymer dielectrics for multi-layer RDL on glass interposers

Abstract: This paper describes the first demonstration of 8–10µm diameter micro-vias at 20µm pitch in ultra-thin dry-film polymer dielectrics to achieve high-density and low-cost redistribution layers (RDL) on panel-based glass and organic interposers A polymer dielectric dry-film, ZEONIF ZS100, at 10µm thickness was double side laminated on thin and low CTE glass and organic substrates Micro-via arrays at 20µm pitch were formed by 248nm KrF excimer laser ablation using mask projection scanning, and metallized by a semi-additive process (SAP) using electroless and electrolytic copper plating, with no chemical-mechanical polishing to form fully filled via structures Fully-filled micro-vias at 20um were achieved using processes scalable to large panels for low-cost and high-density 25D and 3D interposers

High-Temperature And Moisture-Ageing Reliability of High-Density Power Packages For Electric Vehicles

Abstract: Reliability and failure mechanisms associated with high-density, high-temperature (175-250 °C) electronics in harsh environments were investigated. Test-structures were built to emulate both high-power and high-density packages. Different material sets were evaluated for various failure modes such as delamination and corrosion. Design guidelines for high-density and high-power automotive packages are provided with emerging and leading-edge packaging materials. A 40% reduction in the adhesion strength was seen after high-temperature ageing at 200 °C along with a discoloration of the polymer. The experimental results indicate a need for enhanced interfaces and improved material properties for long-term high-temperature reliability.

Thermal aging reliability of socketable, surface-modified solder BGAs with and without polymer collars

Abstract: Ball Grid Array (BGA) package designs are increasingly used in surface mount applications while Land Grid Array (LGA) designs are predominantly used in socketing. The need to converge to a single package design has been driving the need to enable socketable BGAs. BGA spheres with a noble metal surface provide a stable mechanical contact interface with the socket paddles. These noble contact interfaces, however, have to remain intact through the socketing life of the product, considering accelerated testing temperatures of 100-120 °C. Under such conditions, it has been reported that the solder from the ball-attach joints undergoes solid-state wicking along the surface of the ball, leading to complete Au dissolution and potential undesirable intermetallic formation with the socket paddles, along with a drop in ball shear strength due to depletion of the solder from the joints. This paper discusses the use of polymer collars to address this challenge and improve the thermal aging reliability of packages with surface-modified BGA interconnections. The polymer collars were spin-coated on the package, which was aged alongside a reference package with no collars, at accelerated test temperatures of 100 °C and 120 °C, respectively. XPS studies showed that after 650 h of aging, no Au signal but a strong Sn signal was observed in the package without collars, which confirmed complete solder wicking to the top of the ball, while the Au signal remained for the package with the collars, confirming that polymer collars are effective in inhibiting solid-state solder wicking from the ball-attach joints. The joints with polymer collars also showed mechanical stability throughout thermal aging with a 3X improvement in joint shear strength.

Modeling, design, and demonstration of ultra-miniaturized and high efficiency 3D glass photonic modules

Abstract: This paper presents the modeling, design, and demonstration of an ultra-miniaturized 2.5D optical transceiver module using ultra-thin glass interposers with electrical and optical through vias. The 3D Glass Photonics (3DGP) technology with double sided attach of electrical and photonics ICs can achieve ultra-high bandwidth with improved power efficiency at lower cost than other photonic integration such as silicon photonics and organic boards. Thin glass substrates with 60um diameter through vias were fabricated with copper plated electrical vias and polymer-filled optical vias, formed simultaneously. Re-distribution layers were fabricated on top of these integrated vias for electrical interconnections. The 2.5D optical module produced this way features flip-chip bonded VCSEL and driver chips. Initial measurements of the optical vias showed 1.2 dB of loss.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Microfibre–nanowire hybrid structure for energy scavenging

Abstract: Nanodevices don't use much energy, and if the little they do need can be scavenged from vibrations associated with foot steps, heart beats, noises and air flow, a whole range of applications in personal electronics, sensing and defence technologies opens up. Energy gathering of that type requires a technology that works at low frequency range (below 10 Hz), ideally based on soft, flexible materials. A group working at Georgia Institute of Technology has now come up with a system that converts low-frequency vibration/friction energy into electricity using piezoelectric zinc oxide nanowires grown radially around textile fibres. By entangling two fibres and brushing their associated nanowires together, mechanical energy is converted into electricity via a coupled piezoelectric-semiconductor process. This work shows a potential method for creating fabrics which scavenge energy from light winds and body movement. A self-powering nanosystem that harvests its operating energy from the environment is an attractive proposition for sensing, personal electronics and defence technologies1. This is in principle feasible for nanodevices owing to their extremely low power consumption2,3,4,5. Solar, thermal and mechanical (wind, friction, body movement) energies are common and may be scavenged from the environment, but the type of energy source to be chosen has to be decided on the basis of specific applications. Military sensing/surveillance node placement, for example, may involve difficult-to-reach locations, may need to be hidden, and may be in environments that are dusty, rainy, dark and/or in deep forest. In a moving vehicle or aeroplane, harvesting energy from a rotating tyre or wind blowing on the body is a possible choice to power wireless devices implanted in the surface of the vehicle. Nanowire nanogenerators built on hard substrates were demonstrated for harvesting local mechanical energy produced by high-frequency ultrasonic waves6,7. To harvest the energy from vibration or disturbance originating from footsteps, heartbeats, ambient noise and air flow, it is important to explore innovative technologies that work at low frequencies (such as <10 Hz) and that are based on flexible soft materials. Here we present a simple, low-cost approach that converts low-frequency vibration/friction energy into electricity using piezoelectric zinc oxide nanowires grown radially around textile fibres. By entangling two fibres and brushing the nanowires rooted on them with respect to each other, mechanical energy is converted into electricity owing to a coupled piezoelectric–semiconductor process8,9. This work establishes a methodology for scavenging light-wind energy and body-movement energy using fabrics.

Self-powered nanowire devices.

Abstract: The lateral and vertical integration of ZnO piezoelectric nanowires allows for voltage and power outputs sufficient to power nanowire-based sensors.