Guide to Rigid - Flexible PCB Design and Reinforcement

Author: Jack Wang

Rigid - flexible PCBs combine the advantages of flexible and rigid boards and play a crucial role in the manufacturing of electronic devices. Their design is quite different from that of ordinary flexible or rigid boards, and the following key points require special attention. 

I. Design Key Points of Flexible Partition Lines 

1. Thick - Thin Line Transition

To prevent the circuit from being damaged due to sudden expansion or contraction, the connection between thick and thin lines should be designed in a torn - like shape to avoid sudden changes in the line. 

2. Corner Treatment

Smooth corners should be adopted instead of sharp corners to ensure the stability and reliability of the circuit. 

II. Design Specifications for Gaskets and Pads 

1. Under the premise of meeting the power transmission requirements, the size of the gasket should be maximized as much as possible. 

2. The transition line at the connection between the pad and the wire should abandon the right - angle design and adopt a smooth transition method. - Independent pads should be provided with fillets to enhance the support effect and improve the firmness of welding. 

III.Strategies for Ensuring Dimensional Stability 

In the design, the copper - based part should be increased, especially in the waste area. More solid copper berths should be planned to reduce the risk of deformation and improve the dimensional stability of the entire rigid - flexible PCB. 

IV. Design Tips for Film - Covering Windows 

1. Setting of Manual Alignment Holes

Adding manual alignment holes can significantly improve the alignment accuracy during the film - covering process and ensure the accuracy of the production process. 

2. Consideration of Window Size

The window design needs to fully consider the flow range of the glue. Generally, the window opening should be slightly larger than the original design, and the specific size is determined according to the design standards provided by ME. 

3. Special Window - Opening Molds

For small and dense window - opening requirements, special mold designs such as rotary punches and jump punches can be used to achieve precise processing. 

V. Design Key Points of the Rigid - Flex Transition Area

1. Line Transition Requirements

The lines in the transition area need to achieve a smooth transition, and the line direction should be perpendicular to the bending direction to ensure the normal operation of the circuit during bending. 

3. Principle of Wire Distribution

The wires should be evenly distributed throughout the bending area to avoid local concentration or sparseness. 

4. Wire Width and Process Limitations

The wire width in the bending area should be maximized as much as possible. At the same time, PTH design, Coverlay, and non - flowing PP design are strictly prohibited in the transition area to prevent affecting the bending performance. 

VI. Design Rules for Flexible Areas with Air - Gap Requirements 

1. Through - Hole Limitations

In the flexible area that needs to be bent, through - holes are strictly prohibited to prevent the circuit from being damaged during bending. 

2. Setting of Protective Copper Wires

When space allows, protective copper wires should be added on both sides of the circuit. If space is insufficient, protective copper wires can be added at the inner R - corner of the bending part. 

3. Shape of Circuit Connection

The connection part of the circuit should be designed in an arc shape to adapt to the bending requirements. 

4. Optimization of Bending Area

Without affecting the product assembly, the larger the bending area, the better the effect. 

VII.Other Key Precautions 

The tool holes of the flexible board, such as punching holes, ET holes, and SMT positioning holes, are strictly prohibited from being shared to avoid affecting accuracy and performance. 

VIII.Comprehensive Analysis of FPC Reinforcement Materials

FPC (Flexible Printed Circuit) is widely used in electronic products. However, due to its low mechanical strength and easy cracking, it often needs to be combined with reinforcement materials to enhance its strength. Common FPC reinforcement materials are as follows: 

1. PI Reinforcement

The tolerance can be accurately controlled within ±0.03mm, with high precision and good high - temperature resistance (130°C - 280°C). It has a rich range of thickness specifications, including 0.075mm, 0.1mm, 0.125mm, 0.15mm, 0.175mm, 0.2mm, 0.225mm, 0.25mm. 

2. Steel Plate Reinforcement

This method requires manual assembly, is relatively complex to operate, and has a high cost. The thickness of steel plate reinforcement is generally 0.1mm, 0.2mm. 

3. FR4 Reinforcement

When the thickness is less than 0.1mm, the tolerance can be controlled within ±0.05mm; when the thickness is 1.0mm, the tolerance is ±0.1mm. The thickness specifications are 0.1mm, 0.2mm, 0.3mm, 0.4mm, 0.5mm, 0.6mm, 0.7mm, 1.6mm. 

Each reinforcement material has its own advantages and disadvantages: PI has a small tolerance but insufficient hardness, FR4 has a large tolerance when it is thick, and the steel plate is hard but difficult to rework. In practical applications, an appropriate reinforcement solution needs to be selected according to specific requirements. 

If you are confused by the above complex design points or hesitant in the selection of FPC reinforcement materials, don't worry! No matter what problems you encounter, you can contact PCBMASTER at any time!

Author: Jack Wang