# Rigid flex Printed Circuit Boards in Robot Designing

Designing robots with rigid PCB boards without considering about protection of PCB against vibration failures that can rise due to mechanical resonances. These failures can lead to serious problems like cracked insulators & capacitors, broken leads of components, discontinuity in PCB traces, cracks in solder joints, delamination of PCB board, electrical short circuits and plated barrel-to-pad disconnection. To eliminate these failures, flexible rigid printed circuit boards need to be used.

## What is rigid-flex PCB?

Printed circuit board in which rigid circuit substrate and flexible circuit substrate are laminated together to solder components on rigid part and replace wired connections with flex portion. The rigid portion can be like conventional rigid PCB’s in which components can be soldered on both sides of the board and multilayer connections can be made while flex portion can make connections in multiple layers but components can be soldered on it because flexible portion is meant for connectivity between rigid circuit portions only.

The elimination of connectors from the design introduces following properties in the circuit:

• Signal transmission from one portion to the other without loss and jitter (Noise)
• Eradication of connection problems like cold joints
• Frees up space and reduces weight
• Make the circuit vibration proof and installable in applications with moving parts.

## Designing rigid-flex PCB:

Multiple softwares are available for designing rigid flex PCB’s but Altium offers best 3D visualization of rigid flex PCB and is highly recommended. While designing rigid and flex portion the most important thing is copper trace width selection depending on the application. The formula for calculating trace width in rigid portion is,

Whereas, “I” is current, “ΔT” is temperature rise and “A” is area of trace. To calculate width from the area obtained from above equation is,

Width= area/(thickness*1.378)

For internal layers of PCB use k= 0.024 and for external layers use k= 0.048

Now for copper trace width in flex portion, consult the table below.

 Trace width (inches) Max. Current for 10ᵒC rise in 1 oz copper (A) Max. Current for 10ᵒC rise in 2 oz copper (A) 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 5.9 9.8 0.200 6.9 12.0 0.250 8.6 13.5

This shows that for same amount of current different trace widths have to be used in rigid and flex portion due to difference in thickness, area and dielectric constant of the materials. Engineers at Rayming PCB and Assembly are always available to consult regarding proper trace width and favorable material depending on your operating frequency and application.

### Mock-ups for flexible PCB:

Paper doll mockups are of great importance in designing flex circuits. This simple practice can help designers prevent many errors by showing problems associated with bends early and can save time and money both. This can help designers in predicting the bend radius and in choosing correct orientation for copper traces to prevent tearing or discontinuity.

### Designing copper traces with bias:

Maintaining extra copper in the design increases dimensional stability of the flex circuit. Designing with a bias around copper traces is a good practice for single layer and double-sided flex designs. Adding or removing extra copper solely depends on the application but if the designer with extra copper with a bias, traces with bias should be preferred for mechanical stability. Moreover, doing this reduces the amount of copper to be etched that is environment friendly in terms of chemical usage.

### Bookbinder construction in multilayer flex:

Use of staggered length design is commonly employed for ease in designing multilayer flex circuits. In this technique designer adds slightly to the length of each succeeding flexible layer that is usually 1.5 times the individual layer thickness. Doing this prevents buckling of the center of bend layers in multilayer flex circuits with separate layers. Through this simple method tensor strain built up on outer metal layers and I-beam effect can be eradicated that can be a critical concern in dynamic applications.

### Trace corner routing:

Some of the issues related to conductor routing in a flexible circuit includes keeping the number of crossovers to minimum so that layer count can be decreased to save cost and second one is bend angle for traces in flex circuit design. Traces should be routed through bend and fold at corner points because sharp corners can trap solution during etching and can over etch and will be difficult to clean after processing. When copper traces are on both sides of the flex circuit, designer should design spaces of 2-2.5 times of trace width to avoid any sort of electrical short and proper etching. Considering these instructions can improve signal propagation and reduces reflection at turns.

### Rigid to flex transition section:

The minimum distance from the rigid to flex transition area to the edges of clearance holes and plated through holes should not be less than 0.0748 inches. While the final residual material should not be less than 0.0197 inches during designing the distance between interior, exterior edges of non-plated through holes and cutouts.

### Rigid-flex interface to plated through holes:

The minimum distance recommended between plated through holes of rigid section and rigid flex interface is more than 0.125 inches. Violating this may compromise plated through holes reliability.

Engineers at RayPCB are highly qualified and competent enough to seek guidance regarding design standards of rigid flex circuits and validity of designed circuit. You can send your designs anytime directly to ENG@raypcb.com or sales@raypcb.com to get instant quote.