Flipping Over Flip-Chip

Electronic News, Jan 22, 2001 by Jan Vardaman

Demand is growing for interconnect technology

The number of new packages introduced in the last decade is astounding. The movement from leads to balls has been one of the most dramatic developments. Inside the package, flip-chip is increasingly used as an interconnect method for both performance and form-factor reasons.

Flip-chip is expanding in high-performance applications driven by the needs of microprocessor, ASIC, and high-end DSP devices. In many cases these devices need multimetal layers, some are pad limited and area array is needed. The flip-chip expansion is also driven by form factor where die sizes are small and packaging cost must be as minimal as the package itself. For high-performance applications, the die sizes are typically large and the power dissipation great, with thousands of I/O for many applications. Flip-chip-in-package (FCIP) applications include microprocessors, fast SRAMs, high-end DSPs from Texas Instruments, almost every ASIC supplier's high-end products, and many of IBM's semiconductor products ranging from fast SRAMs to processor and ASICs. Chipsets are also expected to see increased use of flip-chip for improved electrical performance reasons.

What's new in flip-chip technology is its expansion in the interconnect realm previously dominated by wire bond. It is the potential expansion of flip-chip into the midrange pin-counts that represents a shift in the adoption of the technology and a maturing of the industry. A recent example is the introduction of LSI Logic's flip-chip package targeted at applications with between 300 and 1,150 leads that require higher electrical and thermal performance than wire-bonded package technology offers.

Another new development is the proliferation of bumps for small die (less than 2mm x 2mm) sometimes considered wafer-level packages. Both of these developments represent not only improvements in flip-chip infrastructure, but also cost reductions in the technology.

Workstation/Servers and NetworkSystems

For the highest pin-count packages, flip-chip is the overwhelming interconnect choice for many future products. Mounted inside BGA packages, flip-chip interconnected devices are enabling increased processing power in midrange and high-end computer systems such as workstations and servers. Also increasing in pincount and complexity are the packages used for network system providers such as routers, switches, and hubs. These system makers continue to drive the high pincount BGAs. Packages with more than 1,500 I/Os are increasingly common. The highest pin-count plastic package at Silicon Graphics is a 1,521-ball (1.0mm pitch). At Sun Microsystems Inc., the highest pin-count is 1,849 balls. By the end of 2001 more than half of the BGAs used by Sun Microsystems (nasdaq: SUNW) will be flip-chip split between ceramic and organic substrates. In five years 90 percent of the devices will be flip-chip with organic substrates preferred. Multilayer laminate substrates, many with microvias, support high I/O count ASICs. C eramic column grid array (CCGA) packages with more than 1,000 solder columns have been in volume production for some time-and the pin-counts are also increasing. The highest pin-count ceramic package at Silicon Graphics is a 1,657 I/O CCGA. The package is 42.5mmx42.5mm with 1.0mm column pitch. Packaged by IBM, this pin-count is also used in router boards by network system makers such as Cisco Systems Inc.

The workstation segment is not the domain of just high pin-count BGAs. Fast SRAMs and packaged in BGAs with flip-chip inside will continue to increase in volume. Both IBM Corp and Motorola Inc. have been shipping fast SRAMs with flip-chip interconnect in high volumes for several years.

PCs and Game Machines

Flip-chip interconnect is also found in high volumes in the PC market in the form of FCIPs. For several years, microprocessor speeds have required the use of flip-chip interconnect to achieve the performance specifications designed into silicon. FCLPs include products such as AMD's processor family, Intel's CPUs, and Motorola's (nyse: MOT) PowerPCs. In the near future, chipsets are also expected to use flip-chip as an interconnect method inside the package.

Even machines such as Sony's popular PlayStatinon2 will use flip-chip in the future for high-speed memory devices. Today's processor and graphics chips use wire-bond interconnect, but flip-chip may be in the future for these applications as well.

Flip-Chip vs. Wafer-Level Packages

Low lead count (less than 50) devices such as op amp devices, voltage regulators, controllers, EEPROMS, and even some flash memory are increasingly using bumps as the interconnect method to the board. While these devices will be marketed as wafer-level packages, they clearly resemble flip-chip bumped devices and in many cases use the same process. The major difference is that under fill is not required on the board for thermal stress reasons. These devices do, however, provide the small form-factor required for an increasing number of applications. Whatever the name, bumped devices will grow rapidly in the near future.