Discussions about chip architecture and processing technology were the main focus at the IEEE’s recent 21st Non-Volatile Semiconductor Memory Workshop (Monterey, CA, Feb. 12-16), but a panel discussion on packaging and test, including the need for standards for multi-chip packages (MCPs), reminded the participants that the final manufacturing steps are essential for enabling successful memory products.

The majority of the 300 attendees were in attendance at an evening panel discussion on NVM packaging and test challenges. Alan Niebel, the president of workshop organizer Web-Feet Research, opened the event with a review of market projections for NOR and NAND flash, highlighting the 300% growth expected from 2005 to 2010 for multi-chip NAND flash revenue, driven by consumer applications such as USB flash drives, digital cameras, and MP3 players.

Key to the 40%-50% growth in MCPs during 2005 was fast product development times, enabled by standard solutions, said Melissa Grupen-Shemansky, director of design engineering at Spansion. She cited a two-week leadtime for samples of standard products, compared with four to eight weeks for a customer MCP product. Co-design is an emerging trend in memory packaging, with bond pad layout on the chip, for example, designed for a logical match for the BGA substrate layout and pinout. Backward compatibility of evolving products is also important, and standardization is a key part of that effort, she noted.

Fellow panelist Moriyoshi Nakashima, founder of memory supplier Genusion, discussed the technical and business challenges of memory MCP, notably additional test cost resulting from the many package variations typically associated with MCPs. Each new test set (testing fixture, burn-in boards, etc.) can cost $3-$5 million for production volumes around one million units/month. A possible compromise in the trade-off among flexibility for customers, cost reduction, and commoditization of products, Nakashima suggested, is “platformization”-a variation on standardization in which common test and burn-in hardware accommodates multiple MCP designs. An initial investment would cover this platform test capability for various packages; with just a few wisely chosen footprints, the variety of MCP products could increase significantly. An example shown was a package-on-package structure in which the footprint of the bottom package matches the footprint of a common individual package. This approach keeps suppliers and users from being restricted to a small set of standard packages, and reduces the test cost typically associated with custom packages.

On the test front, Pi Chao, director of the product application group at probe card maker FormFactor, reviewed issues related to known good die for MCPs. Specific failure mechanisms vary with the type of memory technology, so different test algorithms are required for different types of memory elements, he noted. With many nonvolatile memory technologies being developed, this is one challenge that might elude standardization. Development of new test solutions will need to be taken into account when doing a cost analysis of an emerging memory technology, he said.

Kurt Gusinow of Agilent pointed out that the cost of test has actually been driven down by 30%/Mb annually since 1990 because of how quickly memory density has grown. Like Nakashima of Genusion, Gusinow also discussed a platform test approach with hardware that accommodates multiple package designs as a strategy to continue the aggressive test cost reduction trend.

The debate over standardization for products in which differentiation is critical is expected to continue. Dissenting voices in the IEEE workshop debate noted such issues as the disconnect in the product life cycles vs. the pace of typical industry standardization efforts. Another issue hindering the development of standards is the volumes now involved in many products-800 million cell phones/year, for example-and at some level the argument that standards are needed to recognize economies of scale begins to falter. The consensus was that the marketplace will ultimately sort out where standards make sense, and with new approaches such as “platformization,” the benefits could be realized in a broad application space.
- Jeffrey Demmin, Contributing Editor