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68 DESIGN007 MAGAZINE I MAY 2023 about 10 Gbps. For speeds above 10 Gbps, your manufacturer should recommend alternative materials. PTFE and LCP are the current popular choices. In addition to the base materials, different adhesives and epoxy films can be used in the lami- nation process that provide low Dk and thinner material stacks. In addition, there are high-speed coverlays to be considered and tested, depending on the Dk values required. › Shielding: Material stacks may require shielding either for EMI or for support of high-speed low-loss solutions. You can also add more copper layers, cross hatching, metallized epoxies, and some of the new ferrous cover materials. Adding materials to the stack will always affect the bending of the circuit, so keep that in mind. › Bend radius: IPC standards recommend the bend radius of a circuit to be 10X the thickness of the material. is likely affects material choices and design. e bend of • UL requirements: Does the flex need to be UL-approved? What material stacks does your vendor have in their UL library? UL approval is affected by the flammability of the adhesives used in their proposed stack. • Flex stack: Polyimide and copper adhe- sive-based or adhesive-less? Adhesive- based materials use acrylic adhesive to adhere the copper to the polyimide mate- rial. e acrylic adhesive has disadvantages such as via cracking and squeeze-out, and it is more apt to absorb moisture. • Electrical Characteristics: › Power: Copper thickness, temperature, and adhesive requirements to meet your current requirements. › Impedance/signal integrity: Your manufacturer should recommend trace, space, and polyimide thickness, as well as appropriate adhesive requirements. › Signal speed/low loss: In most cases, standard flex materials can be used up to Figure 1.