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54 The PCB Magazine • November 2016 by John Dangler, IBM; Jeffrey Taylor, LENOVO; and Cynnthia Verbrugge, AMPHENOL Editor's Note: This paper was originally pre- sented at IPC APEX EXPO 2016 and published in the proceedings as: Unique Implementation of a 15-Layer, Unbonded/Looseleaf, Bookbinder Rigid- Flex with Backdrill and LGA Interconnect. Abstract While flex and rigid-flex (IPC-6013 Types 1 through 4 [1] ) have always been important in 3D packaging to help resolve space constraints and meet other design requirements, the continued push for denser packaging and higher perfor- mance has only increased the demand for more complex interconnects. Often, we forget that a flex is more than a mechanical solution but that it is a critical part in controlling signal integ- rity and meeting other electrical performance requirements. Any design must be manufactur- able, reliable, and meet cost constraints. We will be examining a packaging solution for a server application that met all require- ments through a combination of key design points, including: • Rigid-flex (IPC-6013 type 4) • 15-layer cross-section • Unbonded/looseleaf construction • Bookbinder construction • Backdrill • LGA (land grid array) interconnect Cost, manufacturability, reliability, signal in- tegrity, thermal and mechanical requirements were all considered during development. The collaborative efforts of mechanical development, signal integrity modeling and input, qualifica - tion engineering, production engineering, cost engineering, sourcing team support and manu- facturability and cost feedback from the fabrica- tor were key to creating a final design that was an optimum balance considering various trade-offs. More than a simple stack of circuit materi- als, the unbonded/looseleaf, bookbinder cross section and LGA interconnect was able to meet the tight rigid-flex mechanical bend radius re- quirements and the small interconnect foot- print requirements. The flexibility of the rigid- flex met the system mechanical requirements related to tolerances between the two mating LGA interconnect areas. Critical signal integrity requirements were met through the selected cross-section, backdrill and the utilizing an LGA interconnect solution. This paper includes details as to how we went from concept to initial development, to design iterations and prototyping, through qualification into a final product in volume production. Unique Implementation of a Rigid-Flex Circuit FEATURE

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