Issue link: https://iconnect007.uberflip.com/i/1507356
20 DESIGN007 MAGAZINE I SEPTEMBER 2023 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 prepregs to keep the resin from flowing onto the flex sub- stack, causing a portion of the flex region to become brittle and creating a potential failure mode. No/low-flow prepregs are typically avail- able in 106 and 1080 glass styles. As the names suggest, low-flow prepregs flow a bit more than no-flow prepregs. Based on my research, low/ no-flow prepregs are available from the following manufacturers: 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 sub- stack can nest into the rigid board and be held onto by the outer low-flow prepreg (Figure 3). Flex cores and bond plies, polyimide films, and coverlays don't have glass weaves, but as noted, no-flow prepregs are glass-reinforced. Once you understand the basics, you may be able to have a more meaningful conversa- tion with your fabricator. Nick points out that successful rigid-flex designers need to under- stand what materials in the stackup exist and where in the part. For example, flex cores extend everywhere throughout the stackup. Conversely, the flexible bondply and coverlay are in the flex regions only, and just extend a short distance into the rigid zones, avoiding any plated through-hole areas. It is important to keep any of the flexible adhesive out of the plated through-holes in the rigid sections as it has a low Tg and high CTE, meaning it cre- ates extra stress on the plated through-holes if present. Mark Finstad added the following advice on material lead times: "There are common materials, less com- mon materials, and some downright exotic materials available to build rigid-flex circuits. Figure 2: Two conductive layers with a flex core between them, as well as adhesive and coverlay on the outside. Figure 3: Coverlays from flex substacks are nested within a transition zone in the rigid substack.