Rigid-flex PCBs offer many advantages over conventional rigid PCBs. One of the most significant advantages is the reduction of solder joints, which ensures higher connection reliability. In addition, the use of Rigid-Flex PCBs offers space and cost-saving advantages over rigid PCBs, as fewer components and materials are required. In addition, testing of Rigid-Flex PCBs is easier because all sub-circuits are interconnected, eliminating the need to test each component individually. Rigid-flex printed circuit boards are also characterized by high-density laminate construction, hole filling and plating, hole stacking, and board and surface flatness requirements. These features not only improve electrical and mechanical properties, but also reduce footprint, save costs and improve thermal performance.
Substrate Selection
Flexible polyimide materials and FR4 boards.
Typically, flexible polyimide is a heat-resistant film that is laminated to a copper-clad substrate to form a flexible layer that can bend and stretch while maintaining its structure.
FR4 board is a rigid layer placed on top of the flexible material to provide a strong, durable, and reliable substrate. The combination of these two materials allows for the manufacture of rigid-flex panels that are capable of bending and flexing while still providing the strength needed to meet the requirements of the application.
Rigid-flex circuit board design specifications
1) Line design requirements for flexible areas:
1.1)The line should avoid sudden expansion or reduction, and a tear shape is used between thick and thin lines:
1.2)In compliance with the electrical requirements, the pad should be taken as the maximum value. The pad and conductor connection with rounded transition lines, avoid using right angles, independent pads should be added to the disk toes, which can strengthen the role of support.
2) Dimensional stability: add copper design as much as possible.
Design as many solid copper foils as possible in the waste area
3)Design of the covering film window
a)Add manual alignment holes to improve alignment accuracy
b) Window design to consider the scope of the flow of glue, usually the window is larger than the original design, and the specific dimensions of the ME to provide design standards.
c) Small and dense windows can use special mold design: rotary punching, jump punching.
Advantages and disadvantages of rigid-flex combination board
Advantages: At the same time, with the characteristics of the FPC and PCB, they can be used for products with special requirements, to save the internal space of the product, reduce the volume of the finished product, improve product performance is very helpful.
Disadvantages: production processes, production difficulties, lower yield rate, resulting in more expensive, longer production cycle.
Production process
1. Substrate preparation
Clean copper-clad laminates (plasma cleaning), surface treatment (roughening/activation)
2. Pattern making
Line pattern transfer (LDI/photolithography), precision etching (differential control)
3. Hole processing
Mechanical/laser drilling, hole metallization (copper immersion + plating)
4. Protective layer processing
Cover film lamination (vacuum lamination), local reinforcement (FR4/metal sheet)
5. Shape processing
Laser/punch cutting, rigid-flexible transition zone molding
6. Final Inspection
Electrical performance test, reliability verification
Key process description:
Lamination temperature: 180±5℃, minimum line width: 50μm, hole copper thickness: ≥18μm, dimensional tolerance: ±0.05mm
Common application areas
1. industrial, medical equipment, a hard and soft combination of circuit boards
Most industrial parts require precision, safety, and non-perishability. Requirements: high reliability, high precision, low impedance loss, complete signal transmission quality and durability. Due to the high complexity of the process, the production volume is small, and the unit price is quite high.
2. Mobile phone applications
Commonly found in folding cell phone turnstiles, camera modules, keypads, RF modules, etc..
3. Consumer electronics
DSC and DV are representatives of the development of soft and hard boards, which can be divided into two main axes: performance and structure. In terms of performance, flexible and rigid boards can connect different PCB rigid boards and components in three dimensions. Therefore, at the same line density, the total area of the PCB used can be increased, relatively improving its circuit carrying capacity and reducing the signaling limit of the contacts and the assembly error rate. Since the soft and hard boards are thin and light and can be bent for wiring, it helps a lot to reduce the size and weight.