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Stiffeners Since the flexible substrates above are not rigid, we need to stiffen some areas to hold components. The materials used are called stiffeners and have a good variety of selections: Polyimide (PI): Extra layers of PI can provide a cheaper stiffening solution. However, PI is still quite flexible, so tougher materials may be required for more stiffness. Fiber-glass with Epoxy-resin (FR4): This is the same material found in rigid PCBs. It is more rigid than your standard PI. Stainless steel, Aluminum, etc.: Depending on the manufacturer, you can have other stiffening materials. Speak with the manufacturer about your requirements and after performing deep research, you will know which material is suitable for your application. These metal stiffeners are typically more expensive than FR4 or PI. Adhesives Used to connect your flex material to copper and stiffeners and other material layers as needed. Acrylic: Commonly used for bonding flex layers and rigid sections. Offers good adhesion and flexibility. Epoxy: Provides higher temperature resistance and better chemical resistance than acrylic. Conductors Copper: Most commonly used conductor and typical thicknesses range from ⅓ to ½ oz. You want to stay in the 3 mil trace and spacing range for flex material in general. However, it's typical to have flex PCBs in the 2 mil trace width and space range. Note that flex copper is often rolled annealed copper. It's smoothed and manipulated to be more flexible to withstand various bends, unlike typical copper found in rigid PCBs. However, that makes the copper smoother, thus requiring better adhesive selection. We also have increased manual processes and thermal bonding required to combine the layers. Plating Newer designers may not be aware that plating of 1-2 mils is performed on standard PCBs. In flex PCBs, you don't have typical mass plating. You would define the extra plating as needed for vias and specific sections of the PCB. Now that we have a thorough understanding of the materials, their challenges and guidelines on how to handle them, let's discuss when we should work with these challenges vs standard rigid PCB problems. Challenges Limited Bending Cycles Static vs. Dynamic Flex PCBs: There are two main categories of flex PCBs: static and dynamic. Static flex boards are designed to bend only a limited number of times (typically less than 100) during assembly. Dynamic flex boards can withstand repeated bending cycles (tens of thousands) but require a more robust design. Minimum Bend Radius: When thinking about flex PCBs, the first question you may have in mind is 'how much bend is acceptable?'. First we need to understand the concept of a bend radius and why you want it to be large as opposed to small. Designing for a proper bend radius is crucial. A tighter bend radius can cause the copper circuit to fracture and lead to circuit failure. Ensure that the bend radius meets the minimum requirements specified by the flex material manufacturer and IPC-2223 to avoid stress concentrations and ensure reliable flexing. Note that the tighter a PCB flexes and bends, the smaller its bend radius. 10 www.cadence.com Rigid Flex Design Guide