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Transferable Redistribution Layers (TRDL)
Keywords: Redistribution, Flip-Chip, Interconnect
Redistribution layers (RDL) to relocate pads have been around since the early 1990s. RDL allowed legacy parts that were designed as wire-bond die to be flip-chip soldered to Printed Wiring Boards (PWBs). This technology shrinks overall board size as well as shortening electrical connection lengths. Since the introduction of this technology in the early 1990s, RDL has been performed at the wafer level through a back end of line (BEOL) process performed directly on the active surface of the wafer. Transferable redistribution layers (TRDL) are the next evolutionary step for RDL and provide a viable solution to complex RDL. The TRDL technology allows for multi-layer RDL to be transferred from a non-active substrate to an active surface. TRDL is particularly important to prototyping and quick-react situations. The TRDL wafers can be processed in parallel with obtaining the active components, thereby drastically reducing the time from design to completed components. The TRDL process builds the RDL on a non-active surface which allows the designer to pick traditional RDL materials as well as new and novel materials. Traditional RDL uses polyimides and benzocyclobutene (BCB) as dielectric materials but TRDL opens the possibilities to dielectrics best suited for the design. Metals that are traditionally not allowed due to contamination problems could be allowed in the separate TRDL process. This technology can simplify board interconnect routing to escape high I/O parts, especially since TRDL is not limited in layer count. TRDL allows for more unique routing and final board schemes mainly due to the less restrictive process parameters. The nature of the TRDL technology allows for testing of the RDL to be performed prior to subjecting the active surface to processing. By testing the RDL layers prior to transferring to the active surface, the probability that costly rework or redesign would be required is greatly reduced. TRDL is built on a temporary substrate such that the new footprint is created first, progressing through layers of RDL until the final surface has a footprint that mirrors the active component. The temporary substrate with TRDL is then flipped and mated to the active component. Through advanced transferable methods, the temporary substrate is removed, leaving the new footprint on the active component. This technological evolution now even allows RDL to be performed on single die. Single die transferable RDL (SDTRDL) is accomplished by mating the single die directly to the TRDL and completing the transfer process.
David Herndon & Suzanne Dunphy, Electrical Engineer
Harris Corporation
Melbourne, Florida

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