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But the ideal package solution is up for debate.

While the fashion industry may have a ban on thin models for its runways, in consumer electronics, thin is in. But how does one make a thin portable product and at the same time include the greater functionality consumers demand?

Apple set off the consumer preference for thin in September 2005 when more than one million iPod nanos sold in the first 17 days of its release. Less than 0.27" thick, the iPod nano spurred a flood of thin products, especially in mobile phones. Instead of highlighting features such as standby time or talk time, advertisements for cellphone promote the "cool" of being thin. How thin can they go? It depends on which package is used.

Stacked die packages and WLPs. Today's cellphone is a window into packaging trends. Stacked die packages have been around for a while, and there is no shortage of these products in today's phones – as many as two or more per unit. In Japan, Sharp pioneered much of the work in stacked die packages, shipping the first SRAM and flash memory packages in the 1990s. Increasingly, these stacked die packages contain more than two die – mostly memory. And they are downright anorexic: Motorola's RAZR V3, the thinnest phone of its time, contained a 1.0 mm thick two-die stacked package and at least 14 wafer-level packages. The height of the WLPs ranged from 0.27 to 0.5 mm, offering the lowest profile possible. Greater numbers of companies are using small, low-profile WLPs in mobile phones.

Thinner die stacks are also being introduced, despite the difficulties with handling, dicing and packaging thin wafers. A four-die stack containing NAND and SDRAM from Samsung found in the Nokia Vodafone 804SS is just 1.0 mm thick. To achieve the low-profile, two of the die are thinned to 70 µm, the other two to 80 µm.

Stacked die packages. While stacked die packages offer a way to increase the functionality in a limited board area by moving in the z direction, the issues become problematic when stacking logic and memory. If a stacked die memory package is not functional, throwing it away is an inexpensive proposition. For packages containing stacked logic and memory, the need for known good die (KGD) memory is essential. Obtaining KGD is a logistics and business concern for companies that do not make their own memory. For this reason, stacked die packages are becoming increasingly popular for many consumer applications.

It often takes more than 10 years for a packaging technology to become mainstream, and the stacked package concept is no exception. Japanese companies such as Hitachi, Matsushita, Mitsubishi, NEC, NTT, Toshiba and others introduced the first stacked packages in a variety of memory applications. Computer makers such as Unysis also used stacked memory for computer applications years ago, and companies such as Staktek made stacking TSOPs into big business for memory products found in PCs. For years military/aerospace applications have used stacked packages, supplied by companies such as Irvine Sensors and Vertical Circuits in the U.S. and 3D Plus in France. These companies ship many stacked modules today, some containing logic and memory.

While many of the early packages for consumer applications were focused on stacking memory, options for stacking memory and logic have also been developed. Motorola's Advanced Package Development and Prototype Lab in Austin, TX, developed a package called a 3-D BGA DCA TAB COB MCM technology (using every acronym in the packaging vocabulary). Motorola introduced a stacked BGA approach as early as December 1992. Toshiba's paper-thin package, targeted at consumer applications, was introduced in 1999. Some of these package concepts have evolved into the package-on-package (PoP) that Amkor and others are promoting today.

Amkor's PoP. In collaboration with several key players, Amkor's PoP has been under development for the past four years. The PoP has been in production for approximately a year, and today's applications include mobile phones, digital cameras and MP3 players. The package was developed because it offers several advantages over stacked die packages, including the flexibility of using memory from a variety of suppliers and testing prior to assembly. While it is slightly larger and thicker than a stacked die package, requires co-design for the top and bottom packages, and costs more than stacked die packages, many companies are finding it the best solution from the standpoint of total cost, including test, logistics and other factors. With some versions of the package and new technology developments, it is possible to stack die as thin as 75 µm.

Considerable time and effort has been put into PoP infrastructure development. JEDEC standards for the pin-out footprints for the top-stacked package have been developed, new packaging stacking equipment from as many as five equipment suppliers is now available, and five major EMS companies are in production or development with board-level assembly PoP. At least 10 major OEMs in the handset and digital still camera markets are adopting PoP.

Clearly a variety of packages are under consideration. Will an increased number of modules such as PoP be used? Will companies use more single chip packages with finer pitch? Will WLPs offer the smallest form factor solution? It is almost certain that the consumer products of tomorrow will contain multiple package configurations, and no one solution will meet all needs.

E. Jan Vardaman is president of TechSearch International, Austin, TX; jan@TechSearchInc.com. Her column appears bimonthly.

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