Issue link: https://iconnect007.uberflip.com/i/1042925
OCTOBER 2018 I FLEX007 MAGAZINE 49 the manufacturer assume the requirements. Flex circuits have many moving variables, so providing as much detail as possible is critical to successful manufacturing. Material Material selection is important in design- ing a flex circuit. How much a circuit will flex depends on the type of material used to make the board. Although the thickness of the board will determine the flexibility, the specific mate- rial used will enhance the quality and overall life cycle of the flex circuit. Due to the unique design of a flex circuit, it is recommended to use the same material in the prototype and the final volume-production product. Most manu- facturers in the U.S. will use polyimide made by DuPont. Manufacturers outside of the U.S. may use other material suppliers due to cost and availability. The purpose of testing is to see how many cycles the flex circuit can withstand. Figure 3 details a general rule on a flex circuit bend radius as a guideline. The absolute way to determine how much a flex circuit can bend or how many cycles it will withstand is to stress test it. The type of copper used on a flex circuit is another critical material. There are two types of copper, electrodeposited (ED) copper and rolled annealed (RA) copper. RA copper is pre- ferred for flex. The copper is rolled onto the flex material and is very malleable. Thus, it is also important to call out the grain direction on the fabrication drawing. The copper grain Figure 2: Rigid-flex circuit consisting of 4-layer rigid with 2-layer flex connecting each rigid section. Figure 3: Rigid-flex circuit consisting of 8-layer rigid with 2-layer flex with controlled-impedance traces and filled microvias. Figure 4: Flex circuit bend radius guideline.