This article will give a detailed discussion of flexible printed circuits.
It is expected that after reading this article, one should understand the following:
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A flexible printed circuit (FPC) involves conductive tracks on a thin, pliable base material. Unlike traditional rigid PCBs, these "flexible printed circuits" are engineered to bend or twist. They may incorporate surface-mounted parts such as resistors and LEDs and typically feature connectors or terminations for integration with other systems. The choice of substrate is vital, differing from standard PCBs in design, production methods, and functionality. Materials like polyimide are frequently used for their distinct properties in constructing these circuits. An FPC is created by laminating a copper foil onto a resin substrate, with layers joined through adhesive, heat, or pressure to create a unified board. The construction includes conductive, insulating, and adhesive layers to ensure proper operation and insulation.
Flexible PCBs are perfect for applications requiring lightweight, slender, and high-density circuitry. As companies aim to maximize functionality with minimal resources, flexible PCBs offer the benefits of three-dimensional wiring and the ability to conform to specific spaces and uses. Moreover, they generally provide superior heat dissipation compared to traditional PCBs, thereby enhancing the longevity and performance of electronic devices.
Flexible printed circuits and conventional PCBs share similar electrical roles, yet they differ substantially in design and structure. The primary difference is the ability of flexible circuits to bend or twist, unlike their rigid counterparts. This flexibility necessitates diverse manufacturing approaches, materials, and designs.
Standard PCBs focus on thermal and chemical properties of adhesives irrelevant to flexibility. In contrast, flexible circuits require adhesives that stretch and offer flexibility. Therefore, the adhesive in flexible circuits must possess distinct thermal, mechanical, and chemical characteristics compared to those used in rigid PCBs.
The foundational layer in a PCB needs strength and is typically reinforced with glass. The most common material used is a flame retardant epoxy resin and glass fabric composite, providing crucial stability, mechanical, and thermal resistance. However, it's rigid and inelastic. Conversely, flexible circuits commonly use polyimide for its flexibility, although this material offers limited mechanical support and dimensional stability.
Regarding copper film, there are two varieties. The first uses electrodeposition, while the second involves rolling and annealing. The rolled and annealed type is more flexible, ideal for FPC applications, whereas PCBs typically use electro-deposited copper with limited flexibility. Thus, the performance of a flexible circuit heavily relies on the copper type used.
Designing flexible PCBs requires a distinct approach compared to rigid ones. Consider these guidelines and factors when creating flexible printed circuits.
Choosing a manufacturer for flexible PCBs necessitates ensuring they employ current technology to address material characteristics, movement, and thermal coefficients vital for crafting robust circuits that meet specifications.
Additional considerations when selecting a manufacturer include guaranteeing product quality, as the investment is significant. Circuit boards should feature high-density designs, effective communication paths, and good conductivity to enhance thermal dissipation. Moreover, the manufacturer must be capable of producing flexible PCBs to any required shape, ensuring they meet essential criteria:
Bend radius characteristics are crucial for flexible circuit manufacturers. Establishing the bend radius early in the design process ensures the circuit can withstand bending without damaging copper traces. The bend radius defines the minimum curvature of a flexible section and should avoid features like slots or cutouts that could weaken it.
Selecting cover materials during PCB fabrication significantly affects durability and performance. Choosing materials suited to the PCB's specific application is essential. Quality matters whether flexible printed circuit boards are for personal or commercial use. Manufacturers should choose cover materials based on performance, specifications, and the board's operating environment.
In PCB design, "wire routing" describes the path electrical connections follow. This step is critical for PCB design, especially for flex PCBs, as the growing bend radius increases strain. Routing traces through low-stress areas enhances PCB safety, helping prevent short circuits and reducing electrostatic discharge risks.
Attention must be paid to features that might cause discontinuities, especially in bending zones. Internal stress when twisting or stretching flexible circuits can lead to insulation or conductor fractures. Avoiding features like beams and reducing copper thinning minimizes these risks.
Layer stacking in flexible PCBs is pivotal due to multiple layers being stacked. Position the most pliable layers centrally to mitigate movement and maintain flexibility.
Delamination, or "layer detachment," is a common issue in PCB manufacturing, potentially damaging circuits or causing faults. Using appropriate solder paste and considering flexibility requirements can help prevent delamination. Incorporating features that counteract delamination is essential in the design.
In flexible printed circuit board design, don't solely rely on drawings for adhesive width. Design should specify overall PCB width, including dielectric materials affecting impedance. Designers should prepare comprehensive drawing packages, focusing on critical features while omitting unnecessary details to balance accuracy and cost efficiency.
Flexible printed circuits are made of a variety of materials. Copper is one of the primary materials used in flexible printed circuits as a conductor material. Other conductors that are employed include silver ink, Constantan°, and aluminum. Another material used in these specific circuits is polyimide. These components set flexible printed circuits apart from conventional printed circuit boards.
The production of flexible printed circuits is a systematic process divided into several stages. Manufacturers generally adhere to four main steps, each encompassing a range of specific procedures.
The first phase focuses on preserving the base material, with polyimide being the primary material used for flexible circuits. To optimize the use of polyimide, circuits should be arranged as closely together as possible using the nesting method. The manufacturing process for flexible circuits includes the following procedures:
Create cross-hatched ground areas when there is adequate electrical allocation. Additionally, reducing the weight of the circuit board enhances its flexibility.
After the initial stages, fabrication begins with a focus on the boards. The process starts with determining the conductor width and spacing. For thin polymer films, a minimum conductor width of 375 micrometers is required. Both nominally thick polymer films and silver-based polymer films carry the desired percentage of circuit current. The diameter of holes in flexible PCBs may vary based on the design and application requirements.
Edge distance is a critical factor in the production of flexible circuits. Proper spacing of laminate layers ensures that the product fits seamlessly into its enclosure. Filleting is a technique used to distribute stress and increase the pad's surface area, which enhances the overall design. Designers should fillet the pads and land termination points on their flexible circuits. Plating through holes is recommended to create a robust solder joint.
Since some flexible PCBs are more rigid than traditional PCBs, surface treatment can improve their mechanical properties during manufacturing. Plasma chemical vapor deposition is used for surface preparation, specifically through vacuum-assisted plasma deposition (PVD). When applied to an FR4 substrate, a flame-retardant epoxy, this treatment enhances dielectric strength and extends the product's lifespan.
In this scenario, a replacement plated through-hole can be created. Manufacturers typically prepare these through-holes using copper.
Manufacturers can design small holes and a PCB layout that enhances flexibility. Advances in technology now allow for the creation of holes as small as 30 micrometers.
After laminating the circuit layouts, manufacturers perform cutting and trimming to accurately remove excess laminate material. This step is completed before the final assembly of the product.
During this process, manufacturers address issues related to cover layers and cover coatings. The cover layer functions as a solder mask, preventing solder from causing short circuits and protecting the printed circuits from both internal and external damage. It also prevents the circuits from becoming externally electrified.
Some common cover layers used in this process include:
Before the flexible circuit board can be put into service, it undergoes essential quality control procedures. The following processes are carried out during this phase:
Electrical test procedures are performed on the circuit board to identify any shorts or open circuits. These tests detect any short circuits or defective components that may have arisen during production, ensuring the product's quality. After completing all procedures and a final visual inspection, electrical testing is conducted to confirm the effectiveness and reliability of the finished product. It is also essential to verify the functionality of the board’s connections, circuitry, and internal components to ensure there are no short circuits or poor contact points among the electronic components.
Flexible printed circuits come in various types and configurations, often adhering to different standards. They are also categorized based on the number of layers. The specific types of flexible circuit boards depend on their configurations and layer arrangements.
High-density flexible printed circuit boards are characterized by their advanced connectors and superior performance compared to conventional flexible circuits. They feature complex designs and a high number of components, resulting in enhanced performance. Additionally, these boards use thinner substrates than standard ones, significantly improving both their performance and compactness.
The single-sided flexible circuit is a basic type of flexible circuit board that consists of a single layer of polyimide film bonded to a thin copper sheet. The copper layer is etched according to precise design specifications to form the circuit pattern. Additionally, the PCB is covered with a polyamide layer for protection and enhanced insulation.
These circuit boards are single-sided, as the name would imply; however, the copper sheet is accessible from both sides. It is a straightforward single-sided flex circuit, providing access to the conducting material from both sides. The single copper layer can be accessed twice thanks to specialized laser and processing methods.
These circuit boards feature conductors on both sides of the polyimide layer, creating a double-layered configuration. Trace patterns tailored to specific requirements can be designed on each side of the substrate film. Copper-plated holes are used to establish connections between the two conductive layers.
This circuit integrates features of both double-sided and single-sided flexible circuits within a multi-layered design. It includes complex interconnections, shielding, and surface-mounted technologies. This approach effectively manages crossovers, reduces crosstalk, meets impedance requirements, provides additional shielding, and supports high component density.
Hybrid printed circuit boards, as the name suggests, combine features of both rigid and flexible PCBs. They integrate the advantages of rigid and flexible circuits, with connections made using plated holes. This type of board offers flexibility in certain areas while providing rigidity for components in less flexible regions. The primary benefit of hybrid circuits is their ability to enhance circuit functionality. Rigid sections are particularly useful for mounting components like connectors and chassis, while flexible sections provide vibration resistance and adaptability.
The automotive industry has greatly benefited from flexible electronics, which expand the design possibilities for dash panels, infotainment systems, and other vehicle electronics. Unlike traditional flat and boxy displays, flexible electronics offer a wide range of design options.
Furthermore, flexible electronics allow for more compact packaging, enabling components to fit into smaller engine compartments compared to conventional rectangular boxes that occupy more space. This flexibility allows car designers to prioritize making the vehicle more fuel-efficient, comfortable, and aerodynamic, and then adapt the electronics to fit the available space.
Flexible circuits are employed in various computer peripherals, such as connecting signals to the moving print heads of printers and the read/write heads of disk drives. They are also widely used in consumer electronics, including cameras, portable entertainment systems, calculators, and fitness trackers. In manufacturing and medical equipment, flexible printed circuits are essential for managing numerous interconnections within compact spaces. Cellular phones are another prominent example of devices that utilize flexible circuits.
Flexible circuits play a crucial role in the expanding industry of wearable technology, including devices like fitness trackers, smartwatches, and compact real-time medical monitors.
Because every human body has unique contours, flexible electronics are particularly well-suited for this application. Sensors can now adapt to the natural curves of the skin, eliminating the need for equipment to be worn tightly to ensure proper contact.
Recent research in flexible electronics has focused on medical applications. Beyond step trackers and calorie counters, advancements are being made in developing flexible devices for monitoring blood pressure, oxygen levels, glucose, and even blood alcohol content.
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