PCB Blog - Make PCB boards easier to manufacture and reduce processing costs

Rigid-flex PCB board are not an ordinary circuit board. The process of combining thin layers of flexible and rigid substrates into a single component presents unique challenges and opportunities. When designers set out to design block rigid-flex PCBs, they found that most of what they had previously learned about PCB design was problematic. What they design is no longer a two-dimensional plane floor, but a three-dimensional internal connection that can be bent and folded. I dare say, this will be a more powerful PCB. Designers of rigid-flex boards replace composite printed circuit boards with multiple connectors, cables and ribbon cables with a single component for greater performance and stability. They limited the scope of the design to a single component, bending and folding lines like a stack of paper swans to optimize the available space. Common Term Literally, "flex circuit" feels like a replacement for multi-wired ribbon cable. On top of the flexible flat substrate are the circuit layers, which are attached to each other end-to-end. This connection is often seen between the print head and the control board of an inkjet printer. In flex circuit terminology, this continuous flexibility is referred to as "dynamic flex". In dynamic flex applications, flex circuits are often (but not limited to) single-sided, in order to achieve high performance and strong reliability. Flex circuits are used for interconnections between various subsystems, such as connecting printheads to control boards.

During the life cycle of a flex circuit, it must be bent, folded, and assembled with little deflection, which is called "flex-to-install". Flexible installations are available in a variety of configurations, from single-layer to multi-layer, depending on the needs of the application. The limited flexing during the lifetime is good for limiting the stress on the conductor and for making more layers. During flex installations, where single-sided component installation is required, the strategy is to position and laminate rigid materials into the flex circuit to reinforce specific areas. This type of flex circuit design is referred to as a "rigidized flex". Rigid materials (typically FR4) do not contain conductors and are primarily used to reinforce the base or connection area of a component. Rigid-flex boards have the advantages of flexible circuits and rigid materials, but the cost is higher; rigid-flex boards can be used as a substitute for rigid-flex boards. Rigid materials need not be etched or plated, but only need to be drilled and added as lines, reducing PCB processing time. During flex installation, if double-sided module installation is required, or you need ultra-thin printed circuit boards, a rigid-flex board may be a viable solution. The rigid-flex board has both a rigid layer and a flexible layer, and is a multi-layer printed circuit board. A typical (four-layer) rigid-flex printed circuit board has a polyimide core that is covered with copper foil on both the upper and lower sides. The outer rigid layers consist of single-sided FR4 that are laminated into both sides of the flex core to assemble a multi-layer PCB. Rigid-flex boards are widely used, but due to the mixed use of various materials and multiple production steps, the processing time of rigid-flex boards is longer and the production cost is higher. When making multi-layer rigid-flex boards, the processing of the flex layer is very different from the outer FR4 layer. The layers made of different materials must be brought together by lamination, then drilled and electroplated. Therefore, it may take 5 to 7 times longer to make a typical four-layer rigid-flex PCB than to make a standard four-layer rigid PCB

Application of rigid-flex board
Rigid-flex boards are commonly found in consumer electronics such as digital cameras, camcorders, and MP3 players. It can also be used in high-end aircraft-mounted weapon navigation systems. According to my research, rigid-flex boards are often used in the manufacture of military aircraft and medical equipment. Rigid-flex panels bring enormous benefits to the design of military aircraft because it reduces weight while increasing connection reliability. Of course, the benefits brought by the overall smaller size cannot be ignored. Implantable medical devices -- such as pacemakers and cochlear implants -- also benefit from the rigid-flex board's ability to bend and fold in tight spaces, as well as its vastly improved reliability. You might as well imagine what it would be like if the pacemaker didn't work because the wire to the battery came off. With the rigid-flex board, the battery can be directly connected to the circuit layer and can be installed anywhere in the assembly.

Application example of rigid-flex board
Designers in rigid-flex applications rely on rigid-flex as the way to achieve product goals. They may use rigid designs as prototypes to test their design concepts before creating new products using rigid-flex boards. The infrared system will be installed on a micro air vehicle or unmanned aircraft, requiring the system to cover the monitoring range of a 5 cubic inch handheld digital camera with a payload of no more than 3 ounces. That is, while maintaining the original level of functionality and reliability, the space is reduced by 50% and the weight is reduced by 95%. The challenge was how to reduce the total weight from 3 pounds to under 3 ounces. The solution is to remove the rigid PCB board assembly connected by multiple connectors and switch to rigid-flex boards. I met Jeff early in the design process, and although this was his first time using a rigid-flex board, he got it right every step of the way. During the production process, he outsourced the PCB design project to an experienced rigid-flex application designer, and involved PCB manufacturers early in the process. The authority said: "Although the cost of rigid-flex board is more expensive than traditional rigid board, it provides an ideal solution for this project. We use the interconnection of flexible substrates instead of connecting equipment of multiple PCB boards. , which is the key to reducing space and weight, which is exactly what we need.” Since the rigid-flex board is flexible and foldable, it can be used to make custom circuits, making optimal use of the available indoor space. Jeff took advantage of this and reduced the space occupied by the entire system. Since this design is not mass-produced, the benefits outweigh the costs by comparison, despite the higher production costs.

Design Considerations When designing a rigid-flex panel, the characteristics of the manufacturing process and the variety of materials used must be considered. Designers cannot simply create the typical types of traces used on a four-layer rigid PCB and expect the same results as a rigid-flex board. Because the steric stability of polyimide is more than 3 times worse than that of FR4. Once the copper is etched away, the flexible material shrinks considerably. Most manufacturers understand this property of the material and estimate it accurately, so when the PCB goes into the machining process (drilling and adding as traces), they try to get the board as close to dimensional tolerances as possible. If the designer does not consider possible manufacturing problems, it is likely that it will not be realized until the moment that the design must be updated to accommodate the manufacturer's special machining process. To achieve the desired results, consider adding teardrops to all trace-to-pad and trace-to-trace junctions by maximizing the size of the plated via ring on the flex layer. If a flexible inner layer is to be used to connect areas of a rigid circuit, careful consideration must be given to how to support the floating in-board rigid areas during manufacturing, as areas of the rigid layer will be removed to expose the underlying flexible layer. However, removing too much rigid material can make the board fragile. We usually die cut the flex layer as die cutting is more suitable for thin polyimide. Areas on the flex layer may also be removed to reduce contact with routing points in the final routing. These "no-layer" areas must be taken into account in the design tool, and the design may need to place no-routing and/or component-free areas to prevent some components or lines from hanging over the edges of rigid areas.

Design Recommendations Since flex circuits can be bent and folded, avoid creating bad traces that increase the likelihood of conductors cracking open. Here are a few suggestions for reducing conductor stress in the bend area: route through the bend area perpendicular to the bend axis; keep line chamfers, width changes, and vias outside the bend area; use a grid of copper instead of Solid copper; side-by-side routing across adjacent layers, "I-Beaming"; and many similar suggestions around designing flex circuits, too many to list here. Check out the open publications in Recommended Reading for more details and design highlights. Conclusion The opinions of some manufacturers, most of them suggest that customers give up rigid-flex boards and switch to rigid-flex boards, because the former increases the production cost of PCB boards and cannot meet the price requirements of customers. One of the key steps in achieving high volume production of rigid-flex boards is to get manufacturers involved early in production. At the same time, we believe: "The three main factors that cause the cost of rigid-flex boards to rise are raw materials, board utilization, and yield. If we can cooperate with designers at the beginning of the project, we can help them achieve low-cost Design and avoid some costly mistakes." When choosing a supplier of flex and/or rigid-flex, take a look at their number of major projects in this area. Get an idea of their average skill level to determine which skill level most of their boards are at. The boards you design should be in the midrange rather than the high end of their processing capabilities, which is especially true for rigid-flex board selection. An ideal rigid-flex board design evaluation team should be composed of mechanical and electronic engineers, PCB board designers and PCB board processing engineers. Mechanical engineers understand the mechanical constraints in system components. The PCB board process engineer in the team can investigate the change of camber and the addition of reinforcement materials, etc., to change the actual number of panels and the cost of production. Remember that the cost per PCB board is inversely proportional to the number of images on the board. If a rigid-flex printed circuit board looks like a multi-legged spider and doesn't nest well on the board, the cost per component increases significantly. If it is decided to use rigid-flex board to make PCB board, then the designer should be more flexible in thinking to improve mass production and reduce investment.