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64 FLEX007 MAGAZINE I JANUARY 2019 rigid-flex part stable, similar to a hardboard, during the assembly processes (Figure 4). The rigid sections of the board are usually routed with conventional router bits and/ or end mills depending on the thickness and material composite, and they are usually routed on conventional PCB routers similar to hardboards and flex circuits. The flexible sections of the boards can be machined using lasers, conventional routers, steel-rule dies, and specialized CNC equip- ment. When using laser and steel-rule dies, care must be taken to be sure that those cuts match up well with the routed edges on the rigid sections of the board. Flex and rigid- flex boards have a greater degree of dimen - sional movement than hardboards, so usually your perimeter file(s) will need to be scaled to match the material movement of the com- posite—both the conventional router for the hardboard areas and laser or mechanical rout- ing of the flexible areas of the board. A rigid-flex board usually has far more fea- tures to cut in the rigid and flex sections than you would see in hardboards or even most flex boards. A typical job will spend hours and sometimes days routing the flex and rigid sections out. This is particularly true in arrays with a large number of parts (Figure 4). Scor- ing can be used in rigid-flex manufacturing, but often does not give desirable results. The panels score just like a hardboard would, but there is still flexible laminate buried in the material layup. The flex material bends rather than breaks, which can make it difficult to get the parts out of the array. Strain Relief or Beading High-reliability rigid-flex boards often use strain relief or beading along the flex-to-rigid interface (IPC 2223, 5.2.9). Beads of a some- what soft resin are placed by hand along the transition points where the flex goes into the hardboard sections. The beads reduce the amount of stress—particularly on circuits— at the transition points. It is used in dynamic flex applications where a board is expected to last hundreds of thousands of flex cycles with- Figure 4: Rigid-flex boards in an array for assembly.