Nikkei Electronics Asia -- February 2007
Tech Feature
PS3 IC Packaging: Innovation for Volume Production

E-Mail Article
Tweet This
Digg This
Share this with friends on Facebook
Buzz Up!
Jan 26, 2007 16:04 Nikkei Electronics Asia
"We could accept no compromises on the performance of the Cell microprocessor, the RSX graphics chip or other components. At the same time, we had to achieve volume production yields appropriate for consumer electronics, at a million or two million units a month. We had no choice but to revamp the packages," explained a spokesperson for Sony Computer Entertainment Corp (SCE) of Japan, as the company revealed details about the integrated circuit (IC) technology developed for its PlayStation 3 (PS3). The company developed a
multichip module (MCM) for the RSX and an internal build-up board for higher speeds in the Cell microprocessor.

Main Board Size
The RSX packs the graphics draw chip and four graphic memory chips (512-Mbit GDDR3 synchronous dynamic random access memory, or SDRAM) into an MCM (see Fig). Data is swapped between the draw chip and SDRAM at a transfer rate thought to be as high as about 1.4Gbps/pin. The reason for using an MCM was to minimize the path length between the two, maintaining the high transfer rate while reducing main board size. With the draw chip and memory interconnected on the main board, according to a source at SCE, "It would have been necessary to connect them with a 128-bit data bus of equivalent length, which probably would have resulted in a bigger main board."
This is why the line width and spacing is larger on the PS3 main board than in graphics cards with GDDR SDRAM used in personal computers, for example. SCE explained: "We are purchasing PS3 main boards from a number of suppliers to achieve high-volume production. Thinner leads and tighter spacing could cut board manufacturer yields and increase costs."
Implementing a 128-line data bus width on the PS3 with those fatter leads and wider spaces, according to SCE, would have increased the lead length to about 10cm, and would likely have caused main board growth. If main board line spacing were reduced to prevent this, crosstalk between leads would have become a problem.

Designed for Outsourcing
The use of the MCM kept the lead length between the draw chip and memory to within a few cm. The footprint of the MCM used in the RSX is 42.5mm square, significantly smaller than the main board real estate that would be used if the chips were mounted in a plane.
To keep costs down, standard parts were used as much as possible in the MCM structure, memory selection and other key decisions. The "most standard design" was selected for the MCM, according to SCE, with the draw chip and four memory chips mounted flat on the
interposer. One major reason for the decision was that this type of MCM has been manufactured in volume by SCE and other firms, and equipment was already in place. RSX assembly was implemented on a manufacturing line owned jointly by Sony Corp and Toshiba Corp, both of Japan. The firm had to adopt a standard MCM structure in order to outsource assembly.
Instead of bare chips or wafer-level packages, for example, SCE chose standard packages for the graphics memory chips, explaining they offered high reliability and the highest yield in MCM assembly.
SCE did develop a new package for the Cell microprocessor, however, featuring a core layer about 400um thick and using a thin build-up board as the interposer. The thinnest core layers in build-up boards used in microprocessors for personal computers with operating frequencies of several GHz are about 800um, meaning that SCE slashed the thickness of the core layer in half.  

by Motoyuki Ooishi