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Development of Ultra-Thin Microelectronic Multi-Chip Assemblies
Keywords: Ultra-thin, multilayer substrates, Flexible electronic assemblies, Flexible electronics reliability
Ultra-thin, flexible microcircuit assemblies using thinned die (down to 20m in thickness) and multilayer, thin film substrates (up to 4 conductor layers (five dielectric layers) at an overall thickness of approximately 10m) have been fabricated. Such circuits offer significant promise in many advanced packaging applications such as: biomedical implant and monitoring devices, wearing apparel electronics, conformal antennas and receiver/transmitter systems, and appliqués of all types. The work reported in this paper is the next step in producing the “ultimate” thin electronic assembly. In previuos efforts [1], we used one- or two-conductor layer flex circuit board (50 micrometers thick). Now we have developed a thin-film polyimide substrate containing four conductor layers and five dielectric layers with a nominal thickness of 10 micrometers. This substrate was fabricated on an oxidized silicon carrier using a conventional polyimide dielectric thin-film multilayer scheme with copper metallization and chromium adhesion layers. An etch barrier layer was placed between the initial polyimide layer and the oxided carrier surface to allow separation of the thin film substrate from the carrier after assembly. The novelty of the new approach is not only the replacement of the conventional flex substrate with the thin-film multilayer substrate, but also in the module assembly where all the assembly operations are performed prior to substrate release from the carrier. This improvement eliminates the need for a special vacuum fixture to hold the substrate flat during assembly—thus significantly simplifying and speeding up the operation. The released chip-substrate system is truly flexible and easily conformed to a curved surface. Without the thinned die the substrate itself is extremely pliable and can easily form bends of 90 degrees or greater without any damage. Because the substrate is fabricated on a silicon carrier it can be made in relatively any size depending upon the diameter of the silicon wafers available. In addition, since the substrate can contain at least four metal layers, the ability to fabricate integral capacitors and low-value resistors is inherent in the process. Similarly, planar antenna elements, RF and optical wave guides can be constructed. Given that the polyimide layers are the same or greater thickness than our very thin die, it is also possible to fully embed these die within the substrate itself, thus forming truly integrated flexible thin film assemblies with overall thicknesses much less than 50 micrometers. 1. Banda, C., Johnson, R.W., Zang, T., Hou, Z., and Charles, Jr., H.K., “Flip Chip Assembly of Thinned Silicon Die on Flex Substrates,” IEEE Transactions on Electronic Packaging Manufacturing, Vol. 31(1), January 2008, pp. 1-8.
Harry K. Charles, Jr., Chief Engineer
JHU/Applied Physics Laboratory
Laurel, MD

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