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68 DESIGN007 MAGAZINE I NOVEMBER 2019 Author's Note: Read Part 1 here. 2.5D Interposer Design A typical 2.5D substrate application supports the interfacing of one or more high-density semiconductors. While the upper surface will accommodate a majority of semiconductor re - distribution and/or die-to-die interface for mul- tiple die applications, the primary I/O chan- nels and power and ground terminals are trans- ferred to the bottom surface of the interposer through plated or filled microvias for routing to an array pattern of terminals designated to interface with the intermediate package sub - strate or host PCB structure requiring signifi- cantly lower circuit density. Although the over- all circuit density of the 2.5D interposer is sig- nificantly greater than the mainstream HDI cir- cuit board, the commercial CAD tools already available for PCB circuit routing should accom- modate most interposer development activity. In preparation for developing the interposer, designers are advised to prepare a description of the proposed substrate, detailing the semi- conductor element(s) physical parameters, in- tended use environment, and timeline antic- ipated for development. The 2.5D substrate fabricator can then recommend a suitable base material (organic, silicon or glass) and define the fabricator's via forming methodologies, metalization process capabilities, and circuit geometry limitations. Base Materials The primary base materials utilized for the 2.5D interposer applications include glass-re- inforced organic laminates, silicon (wafers or panels), and ruggedized glass panels. Design Challenges for Developing High-density 2.5D Interposers, Part 2 Designers Notebook by Vern Solberg, CONSULTANT