Issue link: https://iconnect007.uberflip.com/i/1042925
OCTOBER 2018 I FLEX007 MAGAZINE 35 • The overall number of layers in the rigid section(s): rigid sections can have differing amounts of layers of circuitry, but they should all end up the same thickness • Desired copper thicknesses in the rigid section(s): innerlayers and outer layers with plating • Number of layers in the flexible section(s) • Desired copper thickness in flexible section(s) • If there are more than three layers in the flex section, do you want them bonded together or loose-leaf? • The desired overall thickness of the rigid board(s): over laminate, plating, or solder mask? • Materials desired (FR-4, polyimide, etc.) • Final finish • Special requirements (UL, RoHS, REACH, lead-free and halogen-free assembly, etc.) With this, your fabricator should be able to provide you with a suitable material layup as a starting point, which you can refine from there. Also, remember that many of the most popular laminates available on the market do not have a corresponding no-flow prepreg. Rigid-flex manufacturers have to use no-flow prepreg to keep the uncured resin from flowing onto the flexible areas of the board during lamination. Your fabricator can recommend laminates with corresponding no-flow prepregs that will meet your requirements. Another good starting point for material layups is our Valu Build brochure [1] . Valu Builds offer simple, stable, and robust material layups for rigid-flex that are also very economical. They are ideal for someone who is just starting rigid-flex, looking for a good entry point, and seeking the lowest cost in a rigid-flex design. Detailed Print Board designers and their fabricators often see the print as a list of requirements, which it surely is. But much more than that, the print communicates to the fabricator what your desires are as the designer. Without that com- munication, the fabricator isn't always sure what you want. This is especially true for rigid-flex designs. Gerber files show your data and what you desire for holes, etc., but often it is not possible to tell where the rigid-to-flex transition areas are in the Gerber layers. This is where a detailed print—often much more so than an equivalent hardboard design—shows your fabricator precisely what you want. Any dimensions across the flexible portions of the board should be referenced dimensions only. The flexible areas will expand and con- tract with temperature and humidity changes, so dimensions across the flexible portions of the board should be for reference only. Rigid-to-Flex Transition Area Design rules change around the flex-to-rigid transition area. There is a keep-out area on both sides of the rigid-to-flex transition line, particu- larly on the rigid side of that line. The keep-out area varies by the fabricator, but most fabrica- tors will want you to keep all pads, traces, and vias a certain distance away from that line. In our case, we want all traces and the edge of pads at least 0.025" from the line and the edge of all drilled holes more than 0.050" from that line. The reason is due mostly to cut-back cover- layer manufacturing (sometimes referred to as bikini processing). In high-reliability rigid-flex design and manufacturing, the coverlayer and bondply do not extend all the way through the rigid part of the board. They will typically extend 0.025" to 0.100" into the hard boards, but that also varies by the fabricator. The reason for using cut-back coverlayer is that the acrylic adhesive used to bond the cover- layer has a relatively high Z-axis coefficient of thermal expansion (CTE) rate—much higher than the surrounding laminates. During ther- mal cycling—think of RoHS and lead-free assembly temperatures—the expansion can put too much pressure on the vias and cause them to crack. Also, the acrylic adhesive does not drill, prep for plating, or plate well. Over - all, it is not desirable in the rigid sections of your board. There are times when it is not possible to use the cut-back coverlayer technique, and you must use full-sheet coverlayer/bondply.