Rigid and flex PCB bonded boards are not your typical circuit boards. The process of combining thin layers of flex and rigid substrates, followed by lamination, presents both exceptional challenges and opportunities. When designers first ventured into designing rigid-flex printed circuit boards (PCBs), they quickly realized that much of what they had learned about traditional PCB design was inadequate.
Gone were the days of designing a flat, two-dimensional PCB. Instead, they were faced with the task of creating a three-dimensional structure with flexible interior traces that could be bent and folded. However, this transition brought about a remarkable transformation, resulting in more powerful PCB designs. By utilizing rigid and flexible composites, designers could replace multiple connectors, cables, and ribbon cables with a single component, leading to enhanced performance and increased stability. They harnessed the potential of a single component, optimizing the available space by skillfully bending and folding the traces, much like crafting an origami swan.
In essence, the designers of rigid and flex PCB bonded boards have unlocked new possibilities and unleashed the potential for more robust and versatile PCB designs. By embracing the unique characteristics and capabilities of these composite boards, they have revolutionized the concept of PCB design, ushering in a new era of innovation and efficiency.
Literally speaking, the term “flexible circuit” may evoke thoughts of an alternative to multi-wire ribbon cables. Above the flexible PCB substrate lies the circuit layer, which is connected end-to-end. This type of connection is commonly employed in inkjet printers, linking the print head with the control panel. Within flexible circuits, this inherent flexibility is referred to as “dynamic flex”. In dynamic flexible applications, flexible circuits are typically (though not exclusively) single panels, aiming to achieve optimal performance and maximum reliability. Flexible circuits are often favored for interconnections between subsystems, such as connecting print heads to control panels.
Throughout the lifespan of a flex PCB, it must endure bending, folding, and assembly with minimal deflection, known as “flex-to-install”. Depending on the application’s requirements, various types of flexible installation structures exist, ranging from single-layer to multi-layer designs. Restricting deflection during the lifecycle aids in reducing stress on the conductors and allows for additional layers.
In cases where a single-sided module installation is necessary during flexible installation, a treatment strategy involves incorporating rigid materials into the flex PCB to reinforce specific areas. This design is referred to as “rigidized flex”. Rigid materials, typically FR4, lack conductors and are primarily employed to strengthen the base or connection regions of the component. Rigid-flex boards combine the advantages of both flexible PCBs and rigid materials, but they tend to be relatively costly.
An alternative to rigid-flex boards is the use of bonded rigid and flexible boards. Rigid materials do not require etching or plating, only drilling and addition according to the circuit requirements. This can help reduce the processing time of printed circuit boards.
In the flexible installation process, when double-sided module installation is required or an ultra-thin printed circuit board is needed, choosing a combination of soft and hard boards may be the only viable solution. A rigid and flex PCB bonded board comprises both a rigid layer and a flexible layer, resulting in a multilayer printed circuit board. A typical (four-layer) rigid-flex PCB consists of a polyimide core with copper foil on both sides. The outer rigid layer comprises single-sided FR4 laminated onto both sides of the flexible core, creating the multilayer PCB structure.
Rigid-flex boards have widespread usage, but their processing time and production cost are higher due to the utilization of different materials and production processes. The manufacturing technology for the flexible layer is entirely distinct from that of the outer FR4 layer when producing a multilayer rigid and flex bonded board. Layers composed of different materials must be laminated together, followed by drilling and electroplating. Consequently, the production of a typical four-layer rigid-flex PCB may take five to seven times longer compared to a standard four-layer rigid PCB.
Rigid and flex PCBs are commonly employed in consumer electronics like digital cameras, camcorders, and MP3 players. They also find usage in high-end airborne weapon navigation systems. Research indicates that rigid-flex PCB bonded boards are predominantly used in the production of military aircraft and medical equipment.
In the flexible installation process, when double-sided module installation is required or an ultra-thin printed circuit board is needed, selecting a combination of soft and hard boards may be the most viable solution. A rigid and flex PCB bonded board consists of both a rigid layer and a flexible layer, making it a multilayer printed circuit board. A typical rigid-flex PCB (four-layer) comprises a polyimide core covered with copper foil on both sides. The outer rigid layer is composed of single-sided FR4 laminated onto both sides of the flexible core, creating the multilayer PCB structure.
While rigid-flex boards are widely used, their processing time and production cost are higher compared to standard rigid PCBs due to the utilization of various materials and production processes. The processing technology for the flexible layer is completely different from that of the outer FR4 layer when manufacturing a multilayer rigid and flexible bonded board. Layers made of different materials must be combined through lamination, followed by drilling and electroplating. Consequently, it may take five to seven times longer to produce a typical four-layer rigid-flex PCB compared to a standard four-layer rigid PCB.
Rigid and flex PCBs are commonly employed in consumer electronics, such as digital cameras, camcorders, and MP3 players. They are also utilized in high-end airborne weapon navigation systems. Research indicates that rigid-flex PCB bonded boards are most commonly used in the manufacturing of military aircraft and medical equipment.
The utilization of rigid and flex PCB bonded boards has brought significant benefits to the design of military aircraft, improving connection reliability while reducing weight. The advantages of a smaller overall size are also noteworthy. In the realm of medical devices, such as pacemakers and cochlear implants, the ability of rigid and flex PCB bonded boards to bend and fold in tight spaces greatly enhances reliability. Just imagine the consequences if a pacemaker failed because the wires connecting the battery became detached.
The incorporation of stiff-flex bonding plates allows for direct connection of the battery to the circuit layer, enabling its installation anywhere within the component.
Designers working with rigid and flexible composite boards often prioritize the use of rigid and flex PCB bonded boards as their primary choice, as it is the only way to achieve their product objectives. They may employ rigid designs as prototypes to test their design ideas and subsequently utilize rigid and flex PCB bonded boards to create new products.
An infrared system is set to be installed on a micro-aircraft or unmanned aerial vehicle. The system must cover the monitoring range of a 5-cubic-inch handheld digital camera and have a payload capacity of up to 3 ounces. Achieving this requires a 50 percent reduction in space and a 95 percent reduction in weight while maintaining the original level of functionality and reliability.
One of the major challenges is reducing the total weight from three pounds to under three ounces. The only viable solution was to eliminate the rigid PCB components connected by multiple connectors and transition to rigid and flex PCB bonded boards. In the early stages of the design process, I collaborated with Mondo, who was new to using rigid and flex PCB bonded boards. However, he excelled at every step. During the production process, he outsourced the PCB design project to an experienced rigid-flexible board application designer and involved the PCB manufacturer from an early stage.
Although rigid and flex PCB bonded boards are more expensive than traditional rigid and flexible boards, they provide the ideal solution for this project. Instead of connecting multiple PCBs, we are utilizing interconnections of flexible substrates. This is the key to reducing both space and weight, which is precisely what we require.
Thanks to the flexible and adaptable nature of rigid and flex PCB bonded boards, we can create custom circuits that maximize the available space within the component. Mondo leveraged this capability to reduce the overall system footprint. Since this design will not be mass-produced, even though the production cost is higher, the revenue generated outweighs the cost.
Design Considerations
When designing rigid and flex PCB bonded boards, it is crucial to consider the characteristics of the manufacturing process and the variety of materials used. A designer cannot simply create a typical circuit design for a four-layer rigid PCB and expect it to work seamlessly with a rigid-flex board. This is because the dimensional stability of polyimide is over three times worse than that of ordinary FR4. Once the copper is etched away, the flexible material will experience significant shrinkage. Most manufacturers are aware of this property and make accurate estimations to bring the board within dimensional tolerance during the machining process, such as drilling and online addition. If the designer fails to consider potential manufacturing issues, they may realize at the last minute that the design needs to be updated to accommodate the manufacturer’s special processing requirements.
For optimal results, it is recommended to maximize the minimum ring hole size for plating through holes on the flexible layer and ensure that all wire-to-pad and wire-to-trace connections have teardrops added. When using a flexible inner layer to connect areas of a rigid circuit, careful consideration must be given to supporting floating rigid areas of the board during the manufacturing process. This is because portions of the rigid layer are removed to expose the flexible layer underneath. However, removing too much rigid material can make the board brittle.
Typically, we use pressing for flexible layers as it is more suitable for thin polyimides. Areas on the flexible layer can also be removed to reduce contact with the wiring points in the final assembly. These “layer-free” areas must be taken into account in the design tool. Additionally, it may be necessary to avoid placing wiring and/or components near the edges of rigid areas to prevent them from hanging over the edges.
Design Suggestions
Flex PCBs offer the advantage of being bendable and foldable, which helps prevent circuit failures and reduces the risk of conductor breakage. To minimize conductor pressure in the bending region, consider the following design recommendations:
- Route wires through the bending area perpendicular to the curved axis.
- Keep circuit chamfer, width, and hole path outside the bending area.
- Use grid copper instead of solid copper.
- Cross-beam adjacent layers side-by-side, also known as “I-beaming.”
By adhering to these design suggestions, the likelihood of conductor breakage can be reduced, enhancing the reliability and durability of flex PCBs.