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SEPTEMBER 2023 I DESIGN007 MAGAZINE 19 additional plane layer reduces the bend radius, so there are lots of new tradeoffs to make when you're designing for flexibility. ere are some important dif- ferences relative to rigid PCBs that you should consider when designing a rigid-flex project. You can connect flex cores using a loose-leaf or bonded approach (Figure 1). With a loose-leaf design, you have air gaps between individual flex cores, and you get a less expensive and more flexible design. Bonded designs require an additional sublami- nation with bondply and prepreg, resulting in a stiffer design. Although at least 20x thicker than the loose-leaf design, it improves stripline impedance control because you have tightly controlled current return paths. Nick mentioned that while impedance requirements may drive thicker dielectrics, thicker materials will result in less flexibility. Also, while any line width can be etched on flex layers, wider traces are mechanically more robust and withstand more bending and flexing. Mark added that if a rigid-flex circuit is designed with only the electrical requirements in mind, you could end up with an electrically perfect rigid-flex with traces that crack when you bend it. So, all those great electrical prop- erties are meaningless if the circuit cannot then be formed to its final configuration with- out causing damage. Most successful rigid-flex designs are a balancing act to ensure that you make tradeoffs between both the electrical and the mechanical performance. From a cost perspective, Nick offered that designers should be aware of how rigid-flex boards are fabricated. For example, oen not all the layers are needed in all the rigid sections. However, if you remove the copper layers and associated dielectrics from some rigid sections and not others, you will be driving up manu- facturing costs. It is cheaper to have all rigid sections using the same stack of materials. You can etch away copper you don't need but keep all the dielectrics. How does material selection impact stackup planning with rigid-flex? Before you can select materi- als, you need to know what the options are. With rigid PCBs, we have cores, prepregs, and solder mask for dielectrics. In the rigid- flex world, cores are replaced by flex cores, prepreg offerings include both "no- flow" and "low-flow" varieties, and there are bondplies, coverlays, and stiffeners. Copper in rigid-flex designs is adhered to flex dielectrics in various ways. Adhesive may be added where copper is bonded directly onto the base material. Stiffeners are some- times added to reinforce a flex area for compo- nent placement or routing holes. Copper can either be electrodeposited (ED, less flexible, lower cost) or rolled annealed (RA, more flex- ible, higher cost). Flex planes typically have a hatched pattern etched into them because the reduction in copper makes them more flexible. Coverlays are flexible materials, typically on the outside of a flex substack. ey protect and insulate the flex circuitry on the surfaces, pre- venting it from liing. Coverlays are typically made from acrylic, polyimide, or polyester. A typical coverlay construction (Figure 2) has a polyimide dielectric on top and an adhesive on the side facing the copper. Bondplies are similar to coverlays, but they are used on inner stripline layers, with adhe- sive on both sides and the polyimide dielectric in the middle. If you were gluing two flex cores together, this is where a bondply would come in. Flex cores typically span all substacks, carry- ing the copper from one end to the other. Com- mon brands for flex cores are DuPont Pyralux or Panasonic Felios, with lots of sub-flavors within these two product families. Flex cores without adhesive are used for high-perfor- mance rigid-flex applications. Adhesive-based Bill Hargin