Section: TECHNOLOGY NEWS

A new multilayer, multichip module (MCM) technology that eliminates solder and wire bonding has been developed by Integrated System Assemblies (ISA), Woburn, MA, under contracts from ARPA and the CIA's Office of Research and Development. The process, which shares some similarities to flexible circuit production, can be used to create finished electronic systems that take up less volume than the initial chips, since dice are thinned before assembly. ISA says its AMCM technique is well-suited for applications involving high pincount and high-density I/O, such as PCMCIA cards.

The AMCM process (see figure) starts with bare circuit dice, which are thinned to about 150 microns using a backgrinding process. The thinned chips are tested, placed edge-to-edge on an alumina or aluminum nitride substrate with a very thin die attach adhesive, and encapsulated in a proprietary polyimide-like dielectric material. This material is planarized, and via holes to the IC bond pads are formed using an excimer laser. A sputtering process metallizes the top surface, and conventional lithography and etch techniques are used to define interconnects. This process can be repeated for additional interconnection, and a pad grid array can be formed on top of the final layer for connection to a larger system.

Several of these modules, known as "plies," can be stacked on top of each other and interconnected in similar fashion by drilling "deep vias" through the polymer encapsulant. In these cases, a quartz substrate is substituted for aluminum. The module is tested, and then removed from the quartz layer, leaving a very thin package. Plies are typically 225 microns thick, and have been successfully stacked eight high in an integrated 1-Gbit solid state video recorder. The adhesive is a standard product from the flexible circuit industry.

Chuck Eichelberger, an ISA founder and inventor of the AMCM process, noted several advantages to the process. Interconnection reliability is very high because of the pure materials and clean process used in their deposition, the finished package is robust because of the encapsulating material, and heat dissipation from the outer plies is excellent. "You could put a processor chip on the bottom layer to get rid of the heat, and then have lower-power chips on the subsequent layers," noted Eichelberger.

In addition, chips of any size and shape can be interconnected, unlike other schemes in which chips are stacked and interconnected along their edges. Because of the chip thinning and packing efficiencies, finished modules actually occupy less volume than the original chips.

Eichelberger said that the AMCM will ultimately offer performance comparable to thin-film MCM-D designs, but with pricing in the range of laminated MCMs. Discussions are underway with a variety of potential customers, including the developer of the video recorder, which can hold more than 15 seconds of full-motion color video in a three-square-inch package, 1.5 mm thick. ISA, which uses a commercial EDA software package for design of individual plies, and internal procedures for ply interconnection, is providing design services and low-volume production, and plans to offer high-volume manufacturing in the near future.

DIAGRAM: The manufacturing process for ISA's AMCM multichip module.

~~~~~~~~

By Peter N. Dunn