Issue link: https://iconnect007.uberflip.com/i/1073397
20 FLEX007 MAGAZINE I JANUARY 2019 Curls and Waves in More Than '80s Hair I always asked the customer to give me 3D drawings of the FPC that showed how the FPC should be used in the application. During the 1980s, many people requested semi-flexible, or bend-to-stay, circuits. Rogers introduced a material called Bendflex, and I recommended this material for applications where a single bend with no sharp folds was required. However, this material did not receive enough attention for Rogers to continue producing it, so it was taken off the market after a decade or so. I still have not seen material with the properties we had with Bendflex unless you choose an aluminum base. Surviving Dynamic Movements I remember one success story related to the early design of disc drives reading and erasing heads. I brought Steve Holding from Flex- Ability to Norway for a brainstorming meeting with our local disk drive OEMs, and we started investigating what was needed to survive dynamic movements with sufficient flexibility in combination with a new level of track width and density. We tested out how available copper structures and grain direction affected flexibility and dynamic movements, and tuned the design and material choice to meet the required bend cycles. These early developments ended up at a global product owner and were transferred to Asia for mass volume production. Since Day 1 of my first contact with flexible circuits and DFM, early involvement in guiding product development has been the key to success—and it still is! Some Things Never Change Back in the days, we created some simple design rules that I still find very useful. Some of the parameters are very conservative today, but if you want to produce a low-cost flex, these parameters still apply: • Flexible materials are subject to slight shrinkage during the manufacturing process; thus, allowance must be made when determining minimum distances between groups of holes • Circuit pad design should take into account a method of anchorage; this can be by pad filleting, Mickey Mouse ears, or utilizing the plating in the hole on the PTH circuits • Ninety degree track bends should be avoided • Route tracks between holes tangentially to gain maximum space; where possible, route these tracks on the component side and increase solderable land on the solder side • Avoid having rectangular holes, as this can increase tooling costs; this applies to both base and overcoats • Lands around via holes should be as large as possible • Avoid via holes in bend areas • Where the circuit is required to bend, tracking should follow the direction of the bend; do not track across a bend and design the circuit to be single-sided in areas with dynamic bends • Generally, covercoat apertures should be designed to be smaller than through-hole component lands by an approximate 0.2-mm diameter; when considering the hole size in relation to the solderable land area, the same rules as those used for a rigid PCB apply • Ground plane areas should be cross- hatched to improve bondability when using copper screens, and consideration given to silver polymer-loaded inks when overall circuit thickness is critical • All inside corners must have a radius; in these cases, a copper tear stop should be incorporated into the design as an added safety feature to guard against tearing or cracks propagating across a flexible circuit • When routing your traces in a double- sided flex, it is best to stagger the traces to prevent an I-beam effect; with the conductors stacked, you run the risk of compression issues when bending to install