A rigid-flex PCB is a new type of printed circuit board that combines the durability of rigid PCB and the flexibility of flexible PCB (FPC). Among all types of circuit boards, rigid-flex PCB offer the strongest resistance to harsh environments, making them popular among manufacturers of industrial control, medical, and military equipment. WonderfulPCB is also gradually increasing the proportion of rigid-flex PCB in its total production.
The advantages of rigid-flex PCB are their excellent properties from both rigid PCB and flexible FPCs. They can be folded, bent, and save space, while still allowing the complex welding of components. Compared to traditional cables, they offer longer lifespan, more reliable stability, and are less prone to breaking, oxidation, or detachment, greatly improving product performance. However, rigid-flex PCB have some drawbacks: their production involves numerous processes, they are difficult to manufacture, have a low yield rate, require a large amount of material and labor, making them expensive and with a longer production cycle.
What are the applications of rigid-flex PCB?
1.Industrial Use – This includes applications in industries such as military and medical fields. Most industrial parts require precision, safety, and durability, making the required properties for rigid-flex PCB high reliability, high precision, low impedance loss, excellent signal transmission quality, and durability. However, due to the complexity of the process, production volume is small, and the unit price is relatively high.
2.Mobile Phones – Common applications of rigid-flex PCB in mobile phones include the hinge of foldable phones, camera modules, keyPAD, and RF modules.
3.Consumer Electronics – In consumer products, DSC (digital still cameras) and DV (digital video) are representative devices driving the development of rigid-flex PCB. They connect different PCB hard boards and components in three dimensions, increasing the total usable area of the PCB while maintaining the same circuit density. This improves circuit capacity and reduces signal transmission limitations and assembly error rates. Additionally, since rigid-flex boards are light, thin, and flexible, they contribute to reducing product size and weight.
4.Automotive – In vehicles, rigid-flex PCB are used in applications such as connecting buttons on the steering wheel to the motherboard, linking screens and control panels in vehicle video systems, controlling buttons on the car door panels, reverse radar imaging systems, sensors (air quality, temperature, humidity, and special gas control), communication systems, satellite navigation, backseat control panels, and external vehicle detection systems.
Key Points in Rigid-Flex PCB Manufacturing
The creation and development of FPCs and PCB gave birth to the rigid-flex PCB, which is formed by combining flexible circuit boards and rigid circuit boards through processes like lamination. The key point in rigid-flex PCB manufacturing lies in the lamination process, especially at the junction between the flexible and rigid sections. While standalone PCB or FPC lamination processes are mature, combining these two types in rigid-flex boards remains a challenge for manufacturers.
- Use of vacuum lamination machines ensures continuous pressure and temperature for optimal adhesion and material bonding.
- Appropriate coverlay materials must be selected: soft coverlay may show metal traces and patterns on the surface, while too hard a material may cause under-pressure and bubbles.
Challenges in Rigid-Flex PCB Manufacturing
Rigid-flex PCB involve complex processes, and certain key technologies and challenges are difficult to control. The differences in structure and material between flexible and rigid boards result in significant differences in their dimensional stability, making the choice of suitable materials critical for proper alignment.
For the flexible section:
- Soft materials need to be guided through the production line using a carrier plate to avoid jamming and wastage.
- Precise handling of individual layers is critical for alignment, especially due to polyimide materials’ sensitivity to strong alkaline solutions, which can cause swelling.
- Lamination quality can be improved by using suitable buffer materials like polypropylene film or PTFE sheets to improve bonding between layers.
For the rigid section:
- Ensuring uniform grain direction of glass fiber cloth and eliminating thermal stress during lamination to prevent warping.
- Controlling expansion and contraction during lamination, especially for flexible sections.
- Flex windows can be processed using either pre-milling or post-milling methods, depending on the board structure and thickness.
Impact of Raw Material Price Increases on Rigid-Flex PCB Costs
Since September 2020, CCL (copper clad laminate) prices have significantly increased, driven by a shortage of raw materials and strong downstream demand. The rise in raw material costs, particularly copper, glass fiber, and resin, has pushed up CCL prices by up to 100%. However, this price hike has had a relatively small impact on rigid-flex PCB costs, because material costs account for a smaller portion of the total cost compared to regular PCB.
Quality Control Points in Camera Module Rigid-Flex PCB Manufacturing
Camera module rigid-flex PCB are especially difficult to manufacture due to the small spacing (2-3 mil) between COB (chip-on-board) PAD and the need for surface treatments like ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold), which can lead to side etching. To address this, two challenges must be resolved:
- Fine Line Etching – To handle small COB PAD sizes, LDI (laser direct imaging) exposure machines should be used, as they offer higher resolution than traditional machines. This helps avoid misalignment during exposure.
- Solder Mask Side-Etch Control – Finer solder mask ink should be used to reduce pores in the ink, which would otherwise lead to high side-plating rates and short circuits during surface treatment.
In conclusion, rigid-flex PCB prototyping and manufacturing involve unique challenges due to their material structure and applications, requiring adjustments in every production step to optimize processes and parameters.
