Issue link: https://iconnect007.uberflip.com/i/1507356
50 DESIGN007 MAGAZINE I SEPTEMBER 2023 the board. One layer is used for the return and the other layer is used for your signals. e return is commonly a GND plane and hatched in the flex area of the board. Vias are not usu- ally placed in this area either but may be used if needed. Another rigid-flex approach is to first create a flex board and then laminate or adhere a rigid board (or just a stiffener) to the flex. is is done when you need more signal layers in the flex portion, the flex needs to be longer, and/or it needs to withstand multiple flexing without breaking. e rigid portions are processed sep- arately and have the components assembled to them. In general, components should not be mounted on the flex portion. Flexing solder joints is just a bad thing. Now you've got the basics, what's next? Here are a few guidelines, in no particular order: Layer stackup: Keep the flex layers to a min- imum of two for increased mechanical flexibil- ity and reduced costs. Electrical design constraints: What needs to be on the flex and what doesn't? Trace widths may need to vary. ey may need to be very long. Make sure that the signals can han- dle these conditions. Also, designers should have a good understanding of the heat sinking capabilities of the flex portion. Mechanical design constraints: You will need more clearance to the edges of the board, cutouts, slots, other traces, etc., as well as more space around bends and corners. Any slots or holes that must be cut will need "relief " fea- tures so that the material doesn't tear. Component placement and finishes: As I mentioned, components in general, should not be mounted on flex, but there are exceptions. Some components can be mounted on the flex portion with a stiffener material attached beneath them to prevent solder joint break- age. e finish used to protect the open copper traces is usually an overlay material or a thicker solder mask—not LPI. Both of these have larger clearance areas around the components. Conformal coatings can be used to protect the bare copper aer soldering. Testing and verification: Because of the unique structure of rigid-flex circuits, testing and verification are critical parts of the pro- cess. Test points must be strategically placed, not too close to edges or components, and not covered by solder mask. Ask your fabricator for more information.