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60 DESIGN007 MAGAZINE I MAY 2023 Article by Mark Gallant DOWNSTREAM TECHNOLOGIES (Editor's note: is is the second of a three-part series. To read Part 1, click here.) Flex and Rigid-flex: Different Layer Types Everyday rigid FR-4 PCBs have a well-known layer stackup recipe: dielectrics, PCB conduc- tor layers, plane layers, mask, and silkscreen (nomenclature or legend). More advanced layer types may include embedded or screened components or cavities with bonded bare die. Flex and rigid-flex stackups include those similar to rigid PCBs, such as dielectrics, con- ductor, mask, and silkscreen layers, but that is where the similarity ends. ere are many additional layer types present for this genre of PCB. ey include types like coverlay, adhe- sive, conductive film, conductive foil, conduc- tive adhesive, bondply, and stiffener. Challenges of DFM Analysis for Flex and Rigid-flex Design, Part 2 • Coverlay: An external polyimide film that encapsulates and protects flexible conductor layers. • Adhesive: Bonding agent used to bond copper foil to a polyimide substrate or coverlay. • Conductive film: Transparent but electrically conductive film found in LCDs, touchscreens, and other optical applications. • Conductive foil: in sheet of copper or silver commonly applied to a flexible PCB as shielding. • Conductive adhesive: Adhesive applied for electrical interconnect, thermal or structural bonding applications.

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