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98 DESIGN007 MAGAZINE I AUGUST 2020 Flex Feature by Olga Scheglov, CID, CID+ EPTAC CORPORATION Have you ever designed a flexible printed circuit (FPC)? Many seasoned PCB layout designers and electrical engineers have never designed a flex or rigid-flex circuit (Figure 1). The purpose of this article is to raise aware- ness about possible FPC construction, essen- tial FPC layout requirements, restrictions, and limitations induced by FPC stack-up materials, while highlighting the importance of commu- nication with the flex fabrication and assembly providers. FPCs offer advantages that rigid PCBs and round wiring cables cannot provide: flexibil- ity, weight and volume reduction, reduced pro- duction cost, uniform electrical properties, and the ability to be used in dynamic applications where parts of the product must move con- stantly. They can also be bent, folded, and fit into a 3-D space. Each FPC manufacturer has different equip- ment, capabilities, and personnel; each has different FPC layout requirements that the flex designer should understand. Sometimes, the flex designer may conduct an on-site vendor assessment. Communication between the FPC designer, fabricator and assembly provider is key in the success of the final product. Design of the flex printed circuits shall be done in respect to the fabrication rules of this specific fabricator. This aspect will minimize the list of produc- tion technical questions and the time spent for clarification, compromises and consequences caused by pushing production limits. This list of technical questions regarding problems found in the design during DFM review is known as a TQ. These problems may compromise the level of producibility and con- sequently the reliability of the product. The same FPC design for different vendors could be Flexible Circuit Design Rules for Beginners Figure 1: Flexible printed circuits.