Design Guidance

2022-11-08 BEST

Special Considerations for Rigid-Flex

• Rigid-flex is the ideal solution for applications with multiple rigid PCBs having SMT components on both sides and requiring interconnects between the rigid PCBs.

• Before designing a rigid-flex circuit, make certain that it is truly what you need. If the circuit only has a few layers, stiffeners is a less expensive alternative to rigid-flex.

• It is most cost effective to build a rigid-flex with an even number of layers. All rigid portions of the circuit should have the same number and stack-up of layers.

• Observe aspect ratio (hole depth/hole diameter) limits.

• BEST TECH builds circuits up to 16 layers, but costs increase significantly above 10 layers.

• Expect a trim tolerance similar to that of a steel rule die from hole-to-border and border-to-border. Hole placement within a cluster of holes and from cluster-to-cluster will have a ±0.005" (0.13mm) tolerance within a single rigid area.

• Minimum inside corner radius of 0.031" (0.79mm) is standard, but smaller radius are available.

• Unbonded layers can increase flexibility in multi-layer flex circuits, but this option is more expensive. Specify unbonded layers only in areas of the circuit that will bend.

• BEST TECH can provide an epoxy fillet on stiffener edges that will bend or flex.

• For rigid-flex circuits, it is less expensive to have plated through-holes in the rigid portions only.

. BEST TECH can provide blind and buried vias in rigid-flex circuits

Special Considerations for Rigid-Flex

Fab Drawing: Rigid PCBs vs Flex PCBs

Rigid PCB Fab Drawings

• Board outline with dimensions

• Any other critical information that is not standard

• Drill map and drill legend for quality checks

• All board part data, or indication for manufacturer to use their standard

Flex PCB Fab Drawings

• Dimensions and locations of stiffener (reinforcements) and rigid areas

• Thicknesses of each portion of the board AND/OR materials to be used

• Application: Static or Dynamic

• Locations where board will flex and flex the most, if not obvious