What is a rigid-flex PCB How to design a rigid-flex PCB

Design a robot with a rigid PCB board without considering protecting the PCB from vibration failures caused by mechanical resonance. These failures can cause serious problems such as broken insulators and capacitors, component disconnection, PCB routing discontinuities, solder joint cracks, PCB layer delamination, electrical shorts, and plating barrel to pad disconnects. To eliminate these failures, a flexible rigid printed circuit board is required.

What is rigid and flexible PCB?

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

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

Signal transmission from one part to another without loss and jitter (noise)

Eliminate connection issues such as cold junctions

Free up space and reduce weight

Makes the circuit vibration-proof and installable in applications with moving parts.

Designing rigid and flexible PCBs:

A variety of software can be used to design rigid flexible PCBs, but Altium provides the best 3D visualization of rigid flexible PCBs and is highly recommended. When designing rigid and flexible sections, the most important thing is to choose the copper trace width based on 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 by the equation above,

Width = area / (thickness * 1.378)

For the inner layers of the PCB, use k = 0.024 and for the outer layers use k = 0.048

Now about the copper trace width of the bend

This indicates that different trace widths of the same amount of current must be used in the rigid and curved parts due to the different thickness, area and dielectric constant of the material. Rayming PCB and assembly engineers can always consult the correct trace width and favorable materials according to your operating frequency and application.

Simulation of flexible PCB:

Paper doll mockups are very important when designing flexible circuits. This simple practice can help designers prevent many mistakes by showing bending-related issues early, and can save time and money. This helps designers predict the bend radius and choose the correct direction for the copper traces to prevent tearing or discontinuities.

Designing copper traces with offsets:

Keeping extra copper in the design increases the dimensional stability of the flexible circuit. For single- and double-sided flexible designs, it is good practice to bias the design around copper traces. Adding or removing additional copper depends only on the application, but if the designer has biased additional copper, a trace with bias should be preferred for mechanical stability. In addition, this reduces the amount of etched copper, which is environmentally friendly for chemical uses.

What is a rigid-flex PCB How to design a rigid-flex PCB Huaqiang PCB

Stapler structure in multilayer flexibility:

Interleaved length designs are often used to facilitate the design of multilayer flexible circuits. In this technique, the designer slightly increases the length of each subsequent flexible layer, which is typically 1.5 times the thickness of a single layer. Doing so prevents the center of the bending layer in a multilayer flexible circuit having separate layers from bending. With this simple method, the tensor strain and I-beam effects established on the outer metal layer can be eliminated, which may be a key issue in dynamic applications.

What is a rigid-flex PCB How to design a rigid-flex PCB Huaqiang PCB

Track corner routing:

Some issues related to 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 is the bend angle of the traces in a flexible circuit design. The trace should be bent and folded at the corners, as the sharp corners can capture the solution during etching and may over-etch 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 trace width to avoid any electrical shorts and proper etching. Considering these instructions can improve signal propagation and reduce reflections when turning.

What is a rigid-flex PCB How to design a rigid-flex PCB Huaqiang PCB

Rigid bending transitions:

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

Rigid-Flexible Interface Plated Through Hole:

The recommended minimum distance between rigid sections and plated through holes for rigid flexible interfaces is greater than 0.125 inches. Violations may affect the reliability of plated through-holes.