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OCTOBER 2018 I FLEX007 MAGAZINE 27 5. Creating holes located only in flexible sections of rigid-flex boards: In rigid-flex circuits, it is generally desirable to limit the drilling of through-holes to those areas where the rigid and flex areas are laminated together. However, owing to the diversity of design needs, there are occasions when holes are needed in the flexible areas. This requirement can have a significant impact on both the pro- cessing and the physical performance of the design. Figure 3 illustrates an example of this. The processing aspect is related to the fact that at a minimum, a separate drilling step is required and if the hole requires plating, all process steps associated with plating must be carried out. The prospective impact on cost is readily apparent. It is well worth discuss- ing the design with the fabricator's engineers before locking in such approaches to see if there might be a better solution. If not, at least the manufacturing engineer might be able to help suggest ways to make the circuit easier to manufacture and more reliable. With regards to reliability, any holes in the flexible sec- tions should be kept out of the area which will experience the most flexing as these can cause stress risers and non-uniform performance. 6. Rigid-flex circuits with more than ten layers or three flexible cores: It can hope- fully be appreciated by the reader that the greater the number of layers in a rigid-flex circuit, the greater the opportunity for errors, especially the compounding of small errors which are insignificant in and of themselves, but which grow with the increase in handling and processing. It is not possible to make blan- ket design recommendations on this topic, because the potential variety of designs is vir- tually infinite. Thus, it is highly recommended that the designer check with the fabricator's manufacturing engineers to get their thoughts on what issues might come up and what prob- lem preemptive design choices might be made. 7. Flex section as an outer layer: One of the simplest forms of rigid-flex are those with flex- ible sections on the outer surfaces rather than on the inside. It is possible to make a two-layer rigid-flex circuit with a flex circuit on one side and none on the other, with the two sides inter- connected by plated through-holes. To assure that the flex areas remain flexible after process- ing, there are special processing requirements, such as making sure that the rigid and flexible Figure 3: Rigid-flex with PTH in both the flex area and rigid area. MASK PLATED COPPER LAYER 1 LAYER 2 LAYER 3 LAYER 4 RIGID LAMINATE CORE NO-FLOW PREPREG COVERLAYER 1 OZ COPPER ADHESIVELESS POLYIMIDE FLEX 1 MIL COVERLAYER MASK RIGID SECTION FLEX SECTION PLATING ON RIGID AREA PLATING CYCLE 2 PLATED THROUGH VIAS IN FLEX PORTION PLATING CYCLE 1 PLATED COPPER 1/2 OZ COPPER 1/2 OZ COPPER RIGID LAMINATE CORE NO-FLOW PREPREG 1 OZ COPPER

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