Design Principles for Flexible Printed Circuit Boards:
Designing a flexible Printed Circuit Board (PCB) requires careful planning and consideration to ensure the board's performance, reliability, and manufacturability. Here are the main factors to focus on when designing a flexible PCB:
## Stackup Construction
The layer count and design of the PCB play a significant role in its flexibility and thickness. Single-layer designs, around 0.1 mm thick, are the most flexible, while multilayer designs can be up to 0.3 mm or thicker. For dynamic bending applications, it is advisable to minimise layers and use staggered vias to reduce stress points. Copper weight should also be lighter, such as 0.5 oz, for improved flexibility and durability. Traces should be designed to run perpendicular to the bend axis.
## Materials
Common materials for flexible PCBs include polyimide and polyester, known for their flexibility and resistance to environmental stressors. In rigid-flex designs, FR4 or polyimide can be used for rigid sections. No-flow prepregs should be used in lamination to prevent resin from seeping into flexible areas.
## Dimensional Drawing
Precision and tolerancing are crucial when it comes to the dimensional drawing of a flexible PCB. The drawing should account for the flexibility and potential deformation of the PCB during and after manufacturing. Including tolerances for material thickness and trace width is essential. Sharp corners should be avoided as they can lead to stress fractures; rounded corners should be used instead to enhance flexibility.
## Testing Requirements
Flexural testing should be performed to ensure the PCB can withstand repeated flexing without delamination or trace damage. A minimum bend radius of 6x the flex thickness is recommended for simpler designs, and 12x for more complex ones. Environmental testing in various conditions (temperature, humidity) should also be conducted to ensure the PCB's performance and durability in different scenarios.
## Additional Design Considerations
Signal flow and routing should be carefully planned to prevent interference and enhance signal integrity. Component placement should avoid high-stress areas to prevent cracking or delamination. Stiffeners may be used to add structural support to specific areas of the flex PCB, especially where components are mounted.
By carefully addressing these considerations, you can design a flexible PCB that meets the needs of your application while maintaining reliability and performance.
In addition, it is essential to consider the type of plating (panel plating, pattern plating, pads only plating) for multilayer PCBs. The PCB may be a semi-static flex PCB, flexed a maximum of 20 times, or a dynamic flex PCB, regularly flexed and twisted. The drill symbol chart indicates all the finished hole sizes, as well as the hole size tolerance, for your circuit board design. The flexible PCB stackup construction and layer order, the dimensional drawing and tolerances, and the minimum pad size on double-sided flexible products should also be considered.
Over-specifying the test requirements might increase the overall circuit cost, so it is essential to balance testing requirements with cost considerations. The manufacturers would like the designers to provide class type (class 1, class 2, class 3), wiring type, and installation use requirements. Marking requirements for PCBs, PCB stiffeners' location and dimensions, and the minimum size of plated through holes should also be specified.
By providing vital information to the manufacturers, they can fabricate the flexible PCB as per the designer's expectation without any time delay. Selecting the right PCB materials for your flex PCB design is important as it can impact the overall performance of the flex PCB. Flex PCB materials include flexible dielectric material, conductors (copper type), PCB Stiffeners, and other specific materials.
Physical and electrical testing requirements, such as dimension checks, electrical continuity, inductance, capacitance, ionic cleanliness, flexibility, plating thickness, insulation resistance, should be specified. The manufacturers would like the designers to specify the type of ink (durable white ink, traditional epoxy ink handstamp) for various board markings.
In conclusion, designing a flexible PCB requires careful planning, consideration, and attention to detail. By following best practices and considering all the factors outlined above, you can create a reliable and high-performing flexible PCB for your application.
Controlled impedance should be considered during the signal flow and routing process to maintain proper signal integrity and prevent interference in data-and-cloud-computing applications. The technology used for plating, such as panel plating, pattern plating, or pads only plating, is crucial for multilayer PCBs to ensure signal consistency and performance.
