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In particular, with the requirements for fine-pitch and highperformance interconnects in advanced packaging, ECAs with nanomaterials or other nano-technology are becoming more and more important due to the special electrical, mechanical, optical, magnetic, and chemical properties that nano-sized materials can possess.
There has been extensive research for the last few years on materials and process improvement of ECAs, as well as on the advances of nanoconductive adhesives that contain nano-filler, such as nano-particles, nanowires, or carbon nanotubes and nano monolayer graphenes.
This book consists of nine Chapters, each representing a specific field of interest.
Chapter 1 discusses an overview of electronic packaging and the evolvement of different types of conductive adhesives. Chapter 2 describes the latest development of nano-materials, nanotechnology and their applications in microelectronics packaging.
Electrical Conductive Adhesives with Nanotechnologies
Chapter 3 reviews the key polymeric materials used in conductive adhesives and the analytic approaches for ECA characterizations. Chapter 4 deals with the recent advances in materials, processes, and applications of isotropically conductive adhesives ICAs , particularly focusing on the fundamental understanding and improvement of materials properties for ICAs and nano-ICAs. Chapter 5 discusses the recent development and applications of anisotropically conductive adhesives ACA with the emphasis on the nano-materials implementation for improved performance.
Chapter 6 describes the latest materials and processing development of nonconductive adhesives NCA. Chapter 7 discusses the details of conductive nano-inks and their applications in transparent electrodes, printed electronics, and other packaging areas. Chapter 8 focuses on the recent research and development of materials and applications of intrinsically conducting polymers.
And finally, Chapter 9 summarizes the recent advances of conductive adhesives with nanotechnology and discusses the challenges and opportunities for continuing the work on nanoconductive adhesives. We have attempted to include most major areas with the latest references which should be useful to our audience who work in this vast growing discipline.
With the advances of microelectronics packaging and nanotechnology, there is always a constant need of improved materials and technology. This is a challenge that requires the continuous and active collaborative efforts between materials scientists, chemists, physicists, device and package design engineers.
We express our gratitude to many of our colleagues and friends in the field of microelectronics packaging, conductive adhesives and nanoscience and nanotechnology. Many of their published works have been cited in this book, including work published by many other experts in the fields. Wong Contents 1 Introduction However, an IC alone does not form a complete system and it must be integrated with other components into a system-level board. The packaging has four main functions: 1 signal distribution, mainly involving topological and electromagnetic considerations; 2 power distribution, involving electromagnetic, structural, and material aspects; 3 heat dissipation thermal management , involving structural and material considerations; 4 and protection mechanical, chemical, electromagnetic of components and interconnections.
Furthermore, design for x where x stands for performance, environment, manufacturability, and reliability at the front end and system test at the final stage prior to the system shipment are also important functions for electronic packaging. The challenge for the package is to provide all crucial functions required by the microelectronic system without limiting the performance of the individual part. From the bare chip fabricated from the silicon wafer to the final product ready for use, the whole system can be divided into various levels of the packaging.
The first-level packaging provides the interconnection between an IC and the package. Due to the complexity of modern electronics packaging, there are many levels of interconnects with first-level interconnects defined as the connections between the IC and the IC carrier.
Currently, two types of first-level interconnection dominate the industry: 1 wire bonding and 2 flip chip attachment. Li et al.
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These range from tape automated bonding TAB which, at times, has seen significant usage within certain product lines to novel interconnection schemes, involving deposited thin films , compliant G-shaped springs , laser-deposited written conductors [4—6], copper pillar with solder cap [7—9], pure copper interconnect , and carbon nanotube CNT based interconnect .
Pressure contacts using de-formable conducting polymers or elastomers have been used where the need to easily remove and replace the IC is a primary concern. Wire bonding is the process of providing electrical connection between the silicon chip and the external leads of the semiconductor device using very fine bonding wires. The wire used in wire bonding is usually made either of gold Au or aluminum Al.
There are two common wire bonding processes: Au ball bonding Fig.
Ball bonds thermo-compression or thermosonic. Continued X X a Fig. Ultrasonic bonds wedge bonds. Continued Gold ball bonding starts when a gold wire is fed through what is called a capillary. The capillary holds the gold wire as an electronic flame-off EFO is used to melt the end of the wire, forming a gold ball with a free-air ball diameter ranging from 1. Free air ball size consistency, controlled by the EFO and the tail length, is critical in good bonding.
The free- air ball is then brought into contact with the bond pad. Adequate amounts of pressure, heat, and ultrasonic forces are then applied to the ball for a specific amount of time, forming the initial metallurgical weld between the ball and the bond pad as well as deforming the ball bond itself into its final shape. The wire is then run to the corresponding finger of the leadframe, forming a gradual arc or "loop" between the bond pad and the leadfinger.
Pressure and ultrasonic forces are applied to the wire to form the second bond known as a wedge bond, stitch bond, or fishtail bond , with the leadfinger.
The wire bonding machine or wire bonder breaks the wire in preparation for the next wire bond cycle by clamping the wire and raising the capillary. During aluminum wedge bonding, a clamped aluminum wire is brought in contact with the aluminum bond pad.
Ultrasonic energy is then applied 1. The wire is then run to the corresponding lead finger, against which it is again pressed, and a second bond formed by applying ultrasonic energy to the wire.
The wire is then broken off by clamping and movement of the wire. The gold ball bonding is non-directional and, as a result, is much faster than aluminum wedge bonding, which is why it is extensively used in plastic packaging. Gold ball bonding on gold bond pads, however, may be employed in hermetic packages.
Unlike Al—Al ultrasonic wedge bonding, Au—Al thermosonic ball bonding requires heat to facilitate the bonding process. The Al bond pad is harder than the Au ball bond, and it is impossible to make good bonding between them through purely ultrasonic without causing wire, bond pad, or silicon substrate damage.
Heat application also improves bonding by removing organic contaminants on the bond pad surface. The tape automated bonding TAB technology is a usual microelectronic industrial process. TAB involves bonding a gold-bumped die to a leadframe circuit built on a flexible tape material, such as polyimide or polyester Fig. The TAB is used primarily in the flat panel display industry to mount driver chips between the glass of the display and the input circuitry behind the display, because of its low dimension interconnection.
It is also used in optical applications such as the sensors of stepper motors. In this format, the die and some of the leads are punched from the tape, the leads are formed, and then the package is mounted in a method similar to a J-Lead package.
In some cases, the leads are not formed, but are soldered directly to the board. TAB is typically a single-sided polyimide-based circuit, although a more expensive two-metal or four-metal tape is available.
A copper metallization layer is bonded to the polyimide in one of two ways, either the copper is electrodeposited to the tape or an adhesive is used to bond rolled copper to the tape. The tight pitch of TAB is very advantageous and allows for high-density circuits for high pin count devices. There are two methods to achieve the connection between the die and the circuit.
The first one is a single-point thermosonic bond. Single-point bonding requires each bond pad position to be individually bonded using heat, time, pressure, and ultrasonic applied to the TAB lead which is directly over its unique gold-bumped bond pad. This process does not require specific tools but on the other hand, it takes several times.
Unit 57 - Electronics
A specially designed tool bonds all the leads to the die using force, temperature, and time. It has the highest throughput.
Then, the devices can be encapsulated, delivered in a reel-to-reel or singular format.
Another advantage of this process is the minimized size of the circuits: the resulting height is the sum thickness of the die, the bumps, and the copper track. In contrast, wire bonding uses face-up chips with a wire connection to each pad. A comparison of wire bonding and flip chip is shown in Table 1. Flip chip components are predominantly semiconductor devices; however, components such as passive filters, detector arrays, and microelectromechanical system MEMs devices are also beginning to use flip chip bonding technologies.
Schematic representation of the flip chip bonding process. Flip chip is the simplest minimal package, smaller than chip-scale packages CSP because of its chip size.
Eliminating bond wires reduces the delaying inductance and capacitance of the connection by a factor of 10 and shortens the path by a factor of 25— The result is high-speed off-chip interconnection.
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Wire bond connections are limited to the perimeter of the die, driving die sizes up as the number of connections increases, while flip chip connections can use the whole area of the die, accommodating many more connections on a smaller die.
Area 8 1 Introduction connections also allow 3-D stacking of dies via through silicon via [TSV], wire bonding, anisotropic conductive adhesive film ACF as well as flip chip interconnections and other components.
Even though materials other than solder balls have been developed, the term bump is still widely used by the industry; however, the primary functions of bumps remain the same. First, the bump provides the conductive path from chip to substrate.
The bump also provides a thermally conductive path to carry heat from the chip to the substrate. In addition, the bump provides part of the mechanical mounting of the die to the substrate. Finally, the bump provides a spacer, preventing electrical contact between the chip and the substrate conductors, and acting as a short lead to relieve mechanical strain between board and substrate. This UBM layer replaces the insulating aluminum oxide layer and also defines and limits the solder-wetted area.
Solder is deposited over the UBM by evaporation, electroplating, screen printing solder paste, or needle depositing. Plated nickel-gold bumps are formed on the semiconductor wafer by electroless nickel plating of the aluminum bond pads of the chips.
Handbook of Nanophysics: Nanotubes and Nanowires
After plating with the desired thickness of nickel, an immersion gold layer is added for protection, and the wafer is diced into bumped die. Plating copper bumps posts on wafers have also been demonstrated .
This technique makes a gold ball for wire bonding by melting the end of a gold wire to form a sphere. The gold ball is attached to the chip bond pad as the first part of a wire bond. To form gold bumps instead of wire bonds, wire bonders are modified to break off the wire after attaching the ball to the chip bond pad.
The gold stud bump process is unique in being readily applied to individual single die or to wafers.