ABSTRACT
Th e brain of modern electronics is the integrated circuit (IC) on the semiconductor chip. In order for the brain to control the system, interconnects need to be established between the IC chip and other electronic parts, power and ground, and inputs and outputs. Th e fi rst-level interconnect usually connects the chip to a package made of either plastics and or ceramics, which in turn is assembled onto a printed circuit board (PCB). Th ree main interconnect techniques are used: wire-bonding, tape automated bonding (TAB), and fl ip-chip. In a wire-bonded package, the chip is adhered to a carrier substrate using die-attach adhesive with the active IC facing up. A gold or aluminum wire is bonded between each pad on the chip and the corresponding bonding surface on the carrier as shown in Figure 15.1. Th e chip and the wire interconnections are usually protected by encapsulation. TAB, on the other hand, uses a prefabricated lead frame carrier with copper leads adapted to the IC pads. Th e copper is usually gold-plated to provide a fi nish for bonding to the IC chip pads. Th e chip is attached onto the carrier and either thermosonic/thermocompression bonding or Au/Sn bonding is used to establish the interconnect. Both wirebonding and TAB interconnects are limited to peripheral arrangement and therefore low input/output (I/O) counts. Flip-chip, however, can utilize the entire semiconductor area for interconnects. In a fl ip-chip package, the active side of an IC chip is faced down toward and mounted onto a substrate [1]. Interconnects, in the form of solder bumps, stud bumps, or
adhesive bumps, are built on the active surface of the chip, and are joined to the substrate pads, in either a melting operation, adhesive joining, thermosonic, or thermocompression process. Figure 15.2 shows an example of solder-bumped chip surface for fl ip-chip interconnect. Since fl ip-chip was fi rst developed about 40 years ago, many variations of the fl ip-chip design have been developed, among which, the controlled collapse chip connection (also known as C4) invented by IBM in 1960s is the most important form of fl ip-chip [2]. Compared with conventional packaging using wire-bonding technology, fl ip-chip off ers many advantages such as high I/O density, short interconnects, self-alignment, better heat dissipation through the back of the die, smaller footprint, lower profi le, and high throughput, etc. Th e outstanding merits of fl ip-chip have made it one of the most attracting techniques in modern electronic packaging, including MCM modules, high-frequency communications, high-performance computers, portable electronics, and fi ber optical assemblies.
