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APRIL 2018 I FLEX007 MAGAZINE 29 and is used almost exclusively in these applica- tions. Although screen printed and photo-imaged solder mask are sometimes used as dielectrics, it is rare to find these alternative materials used in applications requiring high-flex life, because they tend to be brittle and are likely to craze or crack when folded or repeatedly bent. In some applications, it is necessary to use photo-imaged solder mask for its high-resolution capability for component openings. But if these parts are also to be dynamically flexed, polyimide is again used in the flexing region of the circuit, creating a circuit with dual insulation materials and processing. Errors in Copper Selection There are two basic types of copper foil: elec- trodeposited (ED) and rolled annealed (RA). They have significantly different grain structures and bending properties. Using ED copper for high-flex applications may cause infant mortality. RA cop - per offers significantly better flex life. Also impor- tant is the copper grain direction on RA copper, as flex life is much higher when the fold or bend line is perpendicular to the grain. Copper thick- ness of one ounce or thinner is also a good design practice. Copper selection errors can also occur with flex circuits of two layers or more when creating plated through-holes. Avoiding the addition of electroplated copper in a flex region is possible by masking during copper plating. This restricts electroplated copper to the copper pads and the vias. This fabrication method avoids the addition of electroplated copper in a flex zone and keeps copper thicknesses to a minimum. This is a pretty common process known as "pads only plating" or "button plating." Both these features can be key to a robust design and are often called out specifically as requirements on a customer's drawing. FLEX007 This column originally appeared in the Flex007 Weekly Newsletter. Dave Becker is vice president of sales and marketing for All Flex Inc. Flex Survey: Challenges In a recent survey, we asked our readers who work with flex the following question: What are the biggest challenges you face when working with flex or rigid-flex circuits? Here are a few of the replies, slightly edited for clarity. Bend radii of flex circuits need to be taken into consideration. On a multilayer rigid-flex cir- cuit, you will have compressive stresses on the inner layers of a bend and tensile stresses on the outer layers. There are minimum bend radii that you can specify. These must be defined early in your design. If you have impedance-controlled flex circuits, consider double-sided flex material rather than single-sided flex material. If the flex is dynamic (a moving flex), the copper should be rolled annealed rather than electro-deposited, which is more brittle. —Steve Knobel, specialist PCB designer with Denel Dynamics Customers who do not understand that these are not rigid boards. The design considerations are not the same. —Steve Kelly, general manager with PFC Flex Circuits Limited When designing flex circuits, my biggest chal- lenge is determining which materials are available from the vendor and their respective thicknesses when trying to hit a specific bend radius, for exam- ple. I also face issues with my software (Cadstar), as it doesn't seem to like curved traces when rout- ing in a dynamic environment that allows push/ pull and spring back. —Jarrod Schulte, engineering support special- ist with Cadwell Industries

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