"This uses already established techniques. We put multiple chips on a flexible substrate side-by-side, [then fold the assembly] with a mechanical tool to create a package the size of your thumb," McWilliams said.

Multichip packages are commonly used in the cell-phone industry to combine an SRAM and a flash into a single package. But these combinations generally use packaging techniques that wire bond two chip-scale packages. The wire bonding approach makes it difficult to go beyond two dice in a package while remaining thin enough to meet the height requirements of mobile systems. The Tessera folded-dice package allows three or more dice to be placed vertically, or two chips to be stacked and then placed alongside two other stacked dice, all within a single package.

Tessera said the package's lighter weight makes the approach more robust, since if a system is dropped, the lighter weight creates less force on the solder balls that connect the dice to the package.

The approach uses standard lapping technology to thin the dice to about 125 microns. Three dice, stacked in the tape and molded, measure 1 mm, which McWilliams said "is the magic number that people want. Our approach is substantially thinner [than alternatives], and we can use a wider variety of dice of different sizes." A DSP, flash and SRAM could be combined in a single package, McWilliams said, or several flash memories could be combined with a system IC for an MP3 player.

Tessera claims to be first to market with a stacked packaging approach that goes beyond wire bonding, though many companies have demo'd R&D-level packaging that's equally thin.

Tessera is a factoryless, packaging-design company that licenses its intellectual property. It pioneered the microBGA (ball-grid array) chip-scale package, which is now coming into widespread usage for flash and RDRAM memories. Intel adopted Tessera's microBGA chip-scale package for Intel flash products in 1996.