How to Design a Rigid Flexible PCB?

Posted on 2020-07-09

Design a robot with a rigid PCB board without considering protecting the PCB from vibration failures caused by mechanical resonance. These faults can cause serious problems, such as broken insulators and capacitors, broken wires, discontinuities in PCB traces, cracks in solder joints, delamination of PCBs, electrical shorts, and disconnection of plating barrels to pads. In order to eliminate these failures, rigid flexible PCB need to be used.

What is a rigid flexible PCB?

A printed circuit board in which a rigid circuit substrate and a flexible circuit substrate are laminated together to solder the components on the rigid component and replace the wired connection with a bent portion. The rigid part can be like a traditional rigid PCB. The components can be soldered on both sides of the circuit board, and multilayer connections can be made. The flexible part can be connected in multiple layers, but the components can be soldered on it, because the flexible part is used to connect only between the rigid circuit parts.


Eliminating connectors from the design introduces the following attributes in the circuit:

  • Signal transmission from one part to another without loss and jitter (noise);
  • Eliminate connection problems such as cold junctions;
  • Free up space and reduce weight;
  • Vibration-proof circuit and can be installed in applications with moving parts;

Design rigid flexible PCB:

A variety of software can be used to design rigid flexible PCB. But it provides the best 3D visualization of rigid flexible PCBs and is strongly recommended. When designing rigid and flexible parts, the most important thing is to choose the copper trace width according to the application. The formula for calculating the trace width of the rigid part is:


However, “I” is the current, “ΔT” is the temperature rise, and “A” is the area of the trace. To calculate the width from the area obtained from the above equation.
Width = area / (thickness * 1.378).
For the inner layer of the PCB, use k = 0.024, for the outer layer use k = 0.048.

Refer to the table below for the copper trace width of the bent part.

Limits in mm for nominal sizes in mmLimits in mm for nominal sizes in mm

Trace width (inches) The maximum value. 1 ounce of copper (A) rises by 10ᵒC current The maximum value. 2oz copper (A) rises 10ᵒC current
0.005 0.25 NA
0.010 0.6 1.0
0.015 1.1 1.8
0.020 1.3 2.0
0.025 1.5 2.5
0.030 1.8 2.9
0.050 2.5 4.0
0.070 3.2 5.0
0.100 4.0 6.9
0.150 9.8
0.200 6.9 12.0
0.250 8.6 13.5

This indicates that due to the different thickness, area and dielectric constant of the material, different trace widths of the same amount of current must be used in the rigid and curved sections.

Simulation of flexible PCB:

The paper doll prototype is very important when designing flexible circuits. This simple practice can help designers prevent many errors by displaying problems related to bending as early as possible, and can save time and money. This can help designers predict the bending radius and choose the correct direction for the copper traces to prevent tearing or discontinuities.

Design copper traces with offset:

Keeping extra copper in the design can increase the dimensional stability of the flexible circuit. For single-layer and double-sided flexible designs, it is a good practice to offset the design around the copper traces. The addition or removal of additional copper depends only on the application. But if the designer has additional copper with offset, the trace with offset should be preferred for mechanical stability. In addition, this can reduce the amount of etched copper, which is environmentally friendly in terms of chemical use.


Stapler structure in multi-layer flexibility:

The staggered length design is usually adopted to facilitate the design of multilayer flexible circuits. In this technique, the designer slightly increases the length of each subsequent flexible layer, which is usually 1.5 times the thickness of a single layer. Doing so can prevent the center of the bending layer in a multilayer flexible circuit having separate layers from bending. Through this simple method, the tensor strain and I-beam effect established on the outer metal layer can be eliminated, which may be a key issue in dynamic applications.


Track corner wiring:

Some of the problems associated with wire routing in flexible circuits include keeping the number of crossings to a minimum so that the number of layers can be reduced to save costs. And the second one is the bending angle of the trace in the flexible circuit design. The trace should be bent and folded at the corners, because sharp corners can capture the solution during etching and may be over-etched and will be difficult to clean after processing. When there are copper traces on both sides of the flexible circuit, the designer should design a space of 2-2.5 times the width of the trace to avoid any electrical short circuit and proper etching. Considering these instructions can improve signal propagation and reduce reflections when turning.


Rigid bending transition:

The minimum distance from the rigid to flexible transition area to the edge of the clearance hole and plated through hole should not be less than 0.0748 inches. When designing the distance between the inner and outer edges of non-plated through holes and cuts, the final residual material should not be less than 0.0197 inches.

Rigid-flexible interface plated through holes:

The recommended minimum distance between the rigid section and the plated through hole of the rigid flexible interface is greater than 0.125 inches. Violation of this regulation may affect the reliability of plated through holes.

The engineers of Kingford have high quality and sufficient ability to seek guidance on the design standards of rigid flexible circuits and the effectiveness of the designed circuits. If you need to design rigid flexible PCB, you can contact us directly!