Not so long ago, defense and aerospace applications were the traditional drivers of advances in electronic packaging technologies, or "the way electronic systems are assembled." At the time, cost was a secondary consideration, because reliability, size, and weight were at a premium. But then in the 1990s, the world changed due to the emergence of digital content.
Nowadays, consumers expect continuing improvements in cellular phones, digital cameras, portable computers, PDAs, and other high-volume, feature-rich applications. So, small size, high performance, and low cost are all equal attributes. These factors have now become the "drivers" for the advanced semiconductor packaging technology.
The most promising advanced techniques rely on packaging technologies called die products. Notably, manufacturers of leading-edge consumer-electronics devices are adopting die products for space and weight savings, functional integration, cost, and gaining an advantage in time-to-market. The table shows how various approaches stack up against each other.
In particular, the high-volume cell-phone market offers the ideal environment for these innovations. Cell phones place particularly rigorous demands on packaging technology. Performance and reliability must be high, size must be small, and cost must be low. Die products are also ideal for other low-cost consumer applications.
The term "die products" describes IC devices that are true die-size packages. That is, when a die shrink occurs, the device size as seen by the customer will shrink as well. This means that the new wafer-level packaging technologies in use today are included in the definition of die products, as well as the more traditional wire-bonded and flip-chip devices. Most flavors of wafer-level packaging employ identical assembly processes. So from a user point of view, they all appear simply as "flip-chip" mounted devices.
To give an idea of the market size for die products, consider the communications area. Between 2000 and 2005, almost a fivefold increase in die-product usage is expected.
About Cellular Technology: Cellular-phone service was initially offered in Sweden in 1981. These phones were based on analog cellular systems, characterized by a single user per channel. They lacked the capability to support data services. To overcome these limitations, digital cellular systems were developed and introduced in the early 1990s. These second-generation systems, called 2G, could put up to six users at a time on one channel, allowing many more subscribers on a given transmission bandwidth. Being digital, they also facilitated data services, such as encryption capability, text messaging, and e-mail.
The advanced digital services available today, the so-called 2.5G technologies, implemented techniques like packet switching to speed transmissions. The 2.5G handsets are particularly useful for data-intensive applications such as Internet access.
On the horizon is the so-called 3G (third generation) cellular technology, which will include very high data rates, improved voice quality, more efficient use of the available spectrum, and support for new services and technologies. To illustrate how this will affect the semiconductor industry, consider again the communications-equipment industry, including cellular handsets. Although cell-phone handsets make up only 20% of the communications-equipment market, they account for over 40% of the value of IC devices consumed in that market. As such, cell-phone handsets provide a rapidly widening market for advanced die products (see "The Shrinking Cell-Phone Handset," p. 50).
Die products can be categorized according to the technology used to assemble them into an electronic module. Some common techniques are:
Chip-On-Board (COB): The most mature technology that relies on die products is COB. Historically, it has been defined as an assembly method that uses a "bare" die mechanically attached to a substrate with epoxy, then electrically connected via wirebonding to pads on the substrate (Fig. 1). To protect the device, it's covered with a specially designed epoxy compound. The die is identical to those assembled into various single-chip package formats. COB mounting technology has a long history in the hybrid-circuit industry and continues to be the most popular method for mounting die products. It's extensively used in very low-cost consumer products with fewer components (e.g., toys and calculators).
Flip-Chip And Wafer-Level Packaging: The highest growth rate for die products is found with applications characterized by assembling bare die in a "face-down" orientation, commonly known as "flip-chip bonding." This definition extends to include many of the burgeoning wafer-level chip-packaging technologies. In a typical flip-chip assembly, mechanical and electrical connections are made concurrently. The solder balls on a flip-chip reflow to pads on the substrate to provide a mechanical attachment mechanism to the substrate, as well as the electrical connection to the circuit.
Flip-chip mounting has be-come the mainstream for high-power IC devices, such as Intel's Pentium or AMD's Athlon. It can handle the power demand, distribute the I/O, and provide the "cleanest" electrical path for the high-speed signals. As a "batch" process, the balls are attached to the ICs while still in wafer form. Each of the IC's I/Os is then simultaneously attached to the substrate. Flip-chip attach technologies promises to be the lowest-cost attachment method. For all of these reasons, flip-chip attach onto a substrate for multichip packages is gaining favor in the industry.
Adhesive Flip Chip On Flex: Aniso-tropically Conductive Film (ACF) adhesive flip-chip bonding to flexible circuits is becoming a popular alternative to other direct chip-attach technologies, such as COB and Tape Automated Bonding (TAB). It's commonly used in display modules where the technology was first used to attach display-driver ICs to glass for cell phones as well as other applications.
Now the technology is being extended to attaching the die to a flexible circuit. The die is bumped, using either copper or gold bumps, then a film of thermosetting adhesive is "loaded" with conductive particles. The flex circuit has pads at the locations of bumps on the chip. When the film is placed over the pads on the substrate, the chip is "pressed" into the film and heated.
Upon cooling, the die is drawn to the substrate by the film's shrinking, and the conductive particles get lodged between the bumps on the die and the pads on the substrate. The chip is then mechanically attached and electrically connected to the flex. Figure 2 shows a chip-on-flex (COF) implementation from a latest-generation cell phone.
Another recent example of ACF in cellular handsets is a very aggressive double-sided ACF flip-chip implementation. The packaging is noteworthy because these are fairly large devices attached to either side of a high-density laminate substrate. Then the entire device attaches to the motherboard as a BGA package.
Figure 3 shows a picture of the device and an illustrative cross-section of the package. This was the first use of adhesive flip-chip bonding of a large processor-type IC used in cell-phone packages. It was made even more notable by having two chips bonded on both sides of a substrate. Also noteworthy, the chips were extraordinarily thin.
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