Issue link: https://iconnect007.uberflip.com/i/1449048
FEBRUARY 2022 I DESIGN007 MAGAZINE 79 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 dielec- tric 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 flex cores are less expensive and generally used for single- or double-sided or low-layer-count flex applications. Low-flow and no-flow prepregs are used on the rigid substacks in lieu of traditional pre- pregs to keep the resin from flowing onto the flex substack, which would cause a portion of the flex region to become brittle and a create a potential failure mode. No/low-flow prepregs are typically available in 106 and 1080 glass styles. As the names suggest, low-flow pre- pregs flow a bit more than no-flow prepregs. Based on my research, low/no-flow prepregs are available from the following manufactur- ers: AGC-Nelco, Arlon, Isola, TUC, Showa Denko (formerly Hitachi), and Ventec. Low-flow prepregs are typically stacked one above the other, with the outer ply going to the rigid edge, while the inner layer is recessed by 50 mils so that the coverlay from the flex substack can nest into the rigid board and be held onto by the outer low-flow prepreg (Figure 7.2). Flex cores and bond plies, polyimide films, and coverlays don't have glass weaves, but as noted, no-flow prepregs are glass-reinforced. Design Considerations You can choose from different flex circuit strategies depending upon your application environment. Operating temperature range and duration along with thermal cycling requirements will be crucial in your decision whether to use rigid-flex or not. For your design, the number of cycles and the bend method, whether flex-to-install or dynamic flexing, is also important. Flex-to- install indicates that the board needs to be bent to be installed in an end-product or system. Dynamic flex is when it gets bent repeatedly Figure 7.2: Coverlays from flex substacks are nested within a transition zone in the rigid substack.