Essential FPC Design Guidelines for Engineers – Part 2: Tips for Reliable Flexible PCB Designs

In Part 1 of our Flexible PCB (FPC) design series, we covered the basics every engineer needs to know. Now, in Part 2, we dive into more design techniques that ensure your FPCs are not only functional but also reliable, durable, and optimized for performance. Whether you're designing for wearables, medical devices, or automotive applications, these guidelines will help you tackle complex challenges and create high-quality Flexible PCBs.

Panelization Design  

1. If an entire panel consists of steel sheets, it becomes heavy, making FPC prone to stretching and deformation. This makes SMT assembly difficult. The minimum gap between reinforced boards should be 3mm. Slot width should be 0.5mm, and connection points should be 1mm wide, with a connection every 15mm. When placing the order, specify that each piece should be separated with paper and sandwiched between cardboard.  

2. Connection points should not be placed on gold fingers, as this will make the gold fingers uneven.  

3. Too few connection points may cause boards to detach. Each PCB should have at least two connection points. Connection width should be at least 0.8mm. Adjust based on board size—larger boards require more connections.  

4 Too many connection points on small boards make depaneling difficult. If SMT is not needed, use 0.3mm-wide connections for easy manual separation.  

5. Low panel utilization increases costs. Optimize panel width to 119mm or 240/250mm or use third-party panelization.  

6. Small boards may be sucked away during laser dust removal. Boards smaller than 20×20mm should be panelized for shipment or have panel separation handled by JLC.  

Reinforcement Design  

Reinforcement adds rigid materials to specific areas of FPC for easier assembly. PI reinforcement is suitable for gold finger insertion. FR4 is for lower-end applications. Steel reinforcement offers good flatness and does not deform, making it suitable for chip mounting.  

1. Steel reinforcement should not be used for plug-in holes as it may cause short circuits. Steel has weak magnetism and should not be used with Hall elements. Steel is also not recommended for gold finger insertions.  

2. If gold finger insertion is required, specify the total thickness. The connector datasheet typically provides this. PI reinforcement thickness should not be calculated simply by subtracting FPC thickness from total thickness. Use JLC’s Gold Finger PI Thickness Calculator: https://tools.jlc.com/jlcTools/#/calcGoldfingerPIThickness.  

3. Reinforcement cutout design: Reinforcement should avoid underlying component holes or pads. Customers should ideally define this themselves. JLCPCB defaults to avoiding pads by 0.3mm. If the remaining reinforcement width after cutout is less than 2mm, JLC will remove the entire reinforcement section. No further inquiry will be made unless specified.

4. Gold finger reinforcement should be at least 1.0mm longer than gold finger pads to prevent breakage.  

5. Electromagnetic shielding film may conduct on both sides. If different networks exist below, cancel the shielding film.  

6. Steel reinforcement on pads causes short circuits.  

7. FR4 reinforcement below 5mm width may break or carbonize. Use PI or steel instead. Adhesive backing should be at least 3mm wide.  

8. Reinforcement or adhesive must not be near SMT pads, or solder paste printing will be impossible. If unavoidable, apply reinforcement or adhesive after SMT assembly.  

Board Thickness Notes  

1. The board thickness selected during ordering includes coverlay, copper thickness, and PI substrate thickness. If there are non-copper or non-coverlay areas, the final thickness may be thinner. Consider this in design.  

2. FPC impedance is difficult to calculate precisely using simulation software. Reference JLC's empirical width values, but always prototype first. This applies to double-sided boards with 0.11mm thickness.  

Designing reliable and high-performance Flexible PCBs requires a combination of advanced techniques, careful planning, and collaboration with experienced manufacturers. By following these essential guidelines, you can create FPCs that meet the demands of even the most challenging applications.

These design expertise addresses potential manufacturing issues, helping engineers verify manufacturability before production, reducing iterations, and minimizing costs.