3D flexible circuits ease packaging

Flexible circuit design is fast becoming a preferred electronic circuit packaging practice in creating products such as mobile flip-phones or laptop computers. With marketing teams striving to reduce product sizes and create more ergonomically pleasing products, PCB designers must educate themselves on other forms of circuit packaging.

Today's flexible circuits have the trace-routing densities, current-carrying requirements and surface-mounted device component populations typical of FR4 PCBs. This circuit packaging method is also flexible enough to conform to the abstract shapes of product enclosures. This implies that they can bend and wrap around objects, and they can be used in applications where regular movement of the complete product is required, such as in mobile flip-phones.

It also allows the entire circuitry of a product to be created using just one flexible circuit rather than multiple rigid PCBs connected with ribbon cables and wires. This is often achieved by connecting two rigid PCBs permanently with a flex circuit.

When it comes to designing, there is no substitute for experience. The primary source for flex-circuit design guidelines is your fabrication house. It has created thousands of circuits and has rule kits to meet the electronic requirements. The following tips should help as well.

Do

• Document thoroughly. It's critical. It is important to start off by outlining key areas of the flex circuit, including details for the flex layer cross section or stack-up, cover layer and stiffener areas.

• Consider the application type at the early stages of design. The nature of the application will have a big impact on the materials and components used and design methods implemented.

• Remember the manufacturing constraints associated with flexible boards. Specific pad stacks must be created to prevent copper from PCBs.

• Ensure an overall 0.15cm clearance to the edge of the board to guarantee good lamination with no separation. Use fillets or hold-down tabs at pad ends to minimize the risk of interfacing problems between the circuit trace and the pad.

• Be careful when designing the trace structure. Observe proper layer biasing. Where possible, try to route perpendicular to a curve.

• Where possible, embed parts on internal layers of the board to help overcome the complexity inherent in designing 3D flex circuits.

• Consult IPC standards like the IPC-2223 Sectional Design Standard for Flexible Printed Boards and the IPC-6013 Qualification and Performance Specification for Flexible Printed Wiring.

Don't

• Allow cracking. It can be a real problem in flex circuits. Careful selection of materials and components can minimize that, but designers also need to take particular care in the placement of parts and other design structures.

• Allow bends to exceed the minimum radius set by the manufacturer. Manufacturers set a minimum radius to board material for a reason—it indicates the level of stress and flex the particular composite can endure.

• Ignore manufacturing specifications, such as hole tolerances, which detail the maximum diameter for a hole based on the material, layers and flexing required.

• Create hole openings on stiffeners that are +.010cm under the finished pad size, as this may cause issues associated with creating short circuits.

• Stack routes on adjacent layers. Route compression can be a problem when bending circuits. Instead, stagger routes to avoid the I-beam effect (stress and compression that causes cracking).

- Andy Buja
Zuken Inc.