Package-on-Package Space Savings with Flexibility
BY AKITO YOSHIDA
Today’s consumer electronics demand higher performance from smaller form factors. Since the first stacked-die memory product was introduced to the market almost 10 years ago, 3-dimensional (3D) packaging, especially the multichip package (MCP), has been widely adopted in many compact applications, such as cellular phones and digital cameras. Currently, OEMs insist on rapidly expanding package-on-package (PoP), which provides the space-saving of 3D technology with the added flexibility not present in traditional multichip and system-in-package configurations.
With the introduction of quarter-micron LSI in the early 1990s, most people dreamed that the majority of applications could be configured by system-on-chip (SoC), where any set of functions could be put onto a single die. This is because the design rule for Si devices had shrunk continuously, according to Moore’s law. In fact, it seemed that a sufficient number of transistors could be fabricated on a die, even for complex systems. However, when Si node approaches less than 0.1-µm in dimension, it is expected that this road will become less traveled. This is because the wafer process typically is optimized for a given device to get the best performance. For example, if one tries to embed a memory function onto a logic die, the original manufacturing conditions for the individual die are no longer valid, and new device-integration work is required. This would impact time-to-market schedules. Also, as process steps increase in SoC, the mask set used for wafer fabrication becomes prohibitively expensive, not to mention the capital expense for the most advanced Si fab. As a result, many IDMs are focusing on SoC products only where high-volume manufacturing is expected. Those devices are application-specific standard product (ASSP) microprocessors.
Figure 1. Stacked packages.
In the 1990s, flash memory appeared to store data and programs needed for more complex consumer applications. During that time, MCPs have gained in popularity as 3D packaging methods for stacking multiple memory die were developed. The results for OEMs are higher-density or complex memory functions, while eliminating the sacrifice of valuable real estate. NOR flash and SRAM were adopted in the beginning stage, but a variety of memory devices are competing in the current market, including NAND flash and DRAM. However, the MCP solution provides its own set of challenges to OEMs, especially when they want memory and logic devices from different IDMs. It is rare that logic- and memory-device manufacturers are one in the same, because few of them could afford to continue developing both devices. When this occurs, OEMs have been forced to place such devices in individual packages side-by-side on a system board, until now. The advent of the PoP package is providing an attractive solution to those OEMs.
PoP is a simple concept - one package is stacked on top of another one instead of placing them side by side. By vertically connecting through chip scale package (CSP) solder balls, real-estate area on a system board can be reduced drastically. This simplicity is also preferred by OEMs because it enables logistical flexibility in much the same way as individual components. OEMs can stack devices purchased from different suppliers, memory density selection can be finalized just before launching an application, and second-sourcing options will be available.
There were initial hurdles. New infrastructure for package stacking was necessary, and the standardization of pin-out footprints (for the top stacked package) was important to allow the previously described flexibility. These things at first made OEMs reluctant to drive the PoP solutions, but the situation has reversed. Package-stacking equipment is now commercially available, and memory-package standardization work is under way in an industry heavy with support from memory suppliers.
Both PoP and MCP solutions equip consumer applications with higher performance and compact sizes. The distinction is that PoP gives OEMs the ownership of packaged devices with open choices for suppliers, while MCP devices inherently drive integrated IDMs to retain ownership. This difference begs the question: what path will lead to the consumer applications of 2010?
AKITO YOSHIDA, product manager for package-on-package, may be contacted at Amkor Technology, 1900 S. Price Road, Chandler, AZ 85248; 480/821-2408, Ext. 5746; e-mail: firstname.lastname@example.org.