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56 The PCB Magazine • November 2016 While this product may look very much like a technology test vehicle, it successfully and el- egantly solved a real-world challenge. Introduction In developing any server, especially high density, scalable, high performance blade serv- ers, significant electronic packaging challenges are encountered. High speed, dense intercon- nect between blades is often required to achieve desired system performance. In the application described in this paper, challenging space, signal integrity, cost, scal- abilty and usability requirements were defined for signal interconnect between blades. An in- terconnect solution using rigid-flex cables, LGA interconnect and scalability cards was proposed (Figure 1). Construction While flex may be ideally suited for appli- cations requiring interconnects within a tight form-factor, not all flex constructions are up to the job of delivering high density intercon- nects when space is at a premium. This appli- cation required we bend 13 conductive layers 90 degrees within a span of 15.25 mm. Adding to the difficulties associated with such a severe bend was the impedance requirements on the five signal layers. Impedance control and good signal integrity properties depend on thick in- nerlayer dielectrics resulting in a significantly thicker flex than standard non-impedance con- trolled circuit. Why Type 4 (rigid-flex) Special considerations are required when building high layer-count FPCs (flexible print- ed circuits). A standard construction multilayer (IPC-6013 type 3) is adequate for most applica- tions; however, as layer count rises, or more im- portantly circuit thickness increases, special at- tention must be paid to the materials used with- in the stack. Of primary importance is CTE (coef- ficient of thermal expansion) match between all materials within the stack. As seen in the Figure 2, a large portion of the stack-up is comprised of acr ylic adhesive from the bond plys. In Figure 3, we can see the materials that make up the typical IPC type-4 multilayer. Suc- cessive layers of dielectric, adhesive, and con- ductive layers make up the flex. When it comes UNIQUE IMPLEMENTATION OF A RIGID-FLEX CIRCUIT Figure 1: Original interconnect concept. Figure 2: Typical type 3 construction. Figure 3: Typical type 4 rigid-flex construction.

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