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The most common inquiry we receive concerning flex circuits pertains to the extent to which a flex circuit can be bent: “How far can I safely bend a flexible circuit?”
According to IPC standards, the recommended minimum bend radius is 10 times the thickness of the material. IPC-2223 includes a section that provides reasonable guidance on bend radius calculations. However, additional factors must be considered when designing flex circuits for high-reliability applications.
1. The grain direction of the copper plays a crucial role, particularly in achieving tight bend radii.
2. The stiffness of a flex circuit is primarily determined by the copper weight, but the Kapton thickness also significantly impacts flexibility.
3. The precise location of the bend is critical to ensuring reliability. Below are further considerations for enhancing the reliability of flex circuit applications.
Make the bendable section thinner overall
1. Reduce the base copper weight and corresponding adhesive thickness, or decrease the dielectric thickness.
2. Utilize adhesiveless base materials. Compared to adhesive-based substrates, adhesiveless materials typically reduce the initial thickness of each substrate by 12–25 µm (0.0005”–0.0010”). 3. Eliminate copper plating on conductors in the flexing area (dynamic region) by employing selective plating (e.g., pad plating or button plating only), thereby enhancing the flexibility of the circuit.
Design the flexible circuit to be durable during bending
1. Conductors should be staggered across layers rather than stacked directly on top of one another to enhance flexibility.
2. The thickness and width of conductors must remain consistent in bending regions.
3. Ensure balanced conductor weights and material thicknesses on both sides of the neutral bend axis.
4. The bend radius of a flex circuit should ideally be approximately 10 times the material thickness and maintain a minimum distance of at least 500 µm (0.020″) from plated-through holes.
5. Integrate tear stops or relief features for slits within the circuit to mitigate the risk of tears initiating and propagating from the slit ends, which are critical failure points.
