Die Products: Ideal IC Packaging For Demanding Applications

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.

Stacked CSP: Usually, 14 IC devices reside in a 2.5G cellular handset, but there won't necessarily be 14 packages. An ongoing trend in the cell-phone industry is "stacking" more than one IC in one package. Toshiba, Fujitsu, Sharp, Mitsubishi, and NEC significantly reduced circuit-board area by introducing stacked packages in the late '90s. The first applications combined flash and SRAM memory.

These multichip packages (MCPs) help cell-phone manufacturers keep their handsets as small as possible, while increasing performance and functionality.³ Today's portable products, particularly cell phones, may contain memory devices from multiple sources in one package. For example, the flash memory chip may come from a completely different vendor than the SRAM memory chip.

The best candidates in portable electronics for combining in a stacked-chip format are the memory and main processor chips. Today, memory chips make up approximately 45% of the total die area in cellular-phone handsets, while the processor chips occupy approximately 20%.

It's very common now to find flash and SRAM chips combined inside a Stacked CSP, which reduces the packaging area to 25% of the total (a savings of 20%). If the main baseband processor chips come in this package, the assembly area occupied by IC devices in a phone could be cut by approximately 40%. This would shrink the required assembly area and the number of components handled by the assembly shop.

Several leading IC packaging companies have developed the capability of stacking more than two chips. Fujitsu Microelectronics, for example, recently became able to stack as many as eight chips in one package (Fig. 4)⁴. To achieve this level of stacking, the company uses its chip-thinning process, which Fujitsu claims can thin chips down to 0.025 mm. This is necessary to keep the total package thickness to less than 2 mm, which is what most designs try to achieve.

Solder Flip-Chip: After several years of much talk and many development programs, solder flip-chip technology is finally seeing some use in multichip packages. Several vendors of small, commodity semiconductor devices and integrated passives now offer their components as bare die with solder balls attached, ready to be directly connected to a circuit assembly. These components are usually small enough to make underfilling necessary.

Multifunction Integration: As the cellular handset begins to incorporate Bluetooth, Internet access, GPS, video, and other applications, additional modules and IC components will be required beyond those needed to serve basic telecommunications functions.

For instance, one of the new 3G phones uses i-Mode video-clip and music transmission service incorporated into a videophone (Fig. 5). Introduced in 1999, i-Mode is a wireless Internet access service. It's accessed over special digital cellular handsets that look like conventional phones but include an "I" button for instant Internet access. The i-Mode handsets usually have a somewhat larger display than a standard cell phone.

This 3G phone features a single, fixed digital camera that allows viewing from two directions: one for real-time videoconferencing and the other for taking photographs or making videos. The phone also features a large 132- by 162-pixel color display and has a data transfer rate of 384 kbits/s (packet transmission) or 64 kbits/s for videophone calls. Every major IC device (except one) in this phone took advantage of some form of multichip packaging.

The ability to put large processors along with memory and analog I/O into a multidie package has moved the IC packaging industry into a new realm—true system-in-a-package (SIP) devices. Die suppliers, IC packaging houses, and traditional surface-mount contract assemblers are all developing technology and adding capacity for these technologies now. Suitable for a broad range of applications, SIP configurations promise a new wave of packaging innovation in the near future.