This article will give a detailed discussion of flexible printed circuits.
It is expected that after reading this article, one should understand the following:
And much more…
In simple terms, a flexible printed circuit (FPC) consists of conductive pathways on a thin, pliable substrate. These "flexible printed circuits" are designed to be bendable or twistable, setting them apart from traditional rigid circuit boards. They may include surface-mount components like resistors and LEDs, and typically have connectors or termination points for integration with other components. Choosing the right substrate is crucial, as it differs significantly from standard printed circuit boards in terms of design, manufacturing methods, and functionality. Flexible substrates, such as polyimide, are used to construct these circuits due to their unique properties. A flexible PCB is made by laminating a copper foil onto a resin substrate, with layers bonded using adhesive, heat, or pressure to form a cohesive board. This construction includes conductive, insulating, and adhesive layers to ensure proper insulation and functionality.
Flexible circuit boards are ideal for applications that demand lightweight, compact, and thin printed circuits with high wiring density. As companies strive to achieve more functionality with fewer resources, flexible PCBs offer the advantage of three-dimensional wiring and adaptability to fit various spaces and applications. Additionally, flexible printed circuits provide enhanced heat dissipation compared to traditional printed circuit boards, contributing to the long-term reliability and performance of the electronics that utilize them.
Flexible printed circuits and traditional printed circuit boards (PCBs) have several key differences. While both serve similar electrical functions, their mechanical and structural designs vary significantly. A primary distinction is that flexible printed circuits can bend and twist, unlike rigid PCBs. This flexibility necessitates different manufacturing processes, materials, and designs for each type.
Printed circuit boards generally emphasize thermal and chemical properties related to adhesives because they do not require flexibility. In contrast, flexible printed circuits must accommodate bending, so the adhesives used must be flexible and able to stretch. Consequently, the adhesive for flexible circuits needs to have distinct thermal, mechanical, and chemical properties compared to those used in rigid PCBs.
The base layer of a printed circuit board must be strong and typically reinforced with glass. In this instance, a class of flame retardant material formed of an epoxy resin and glass fabric composite is the most frequently utilized to produce it. The substance employed offers crucial stability and excellent mechanical and thermal resistance. However, it is rigid and cannot bend. On the other side, polyimide is mostly employed as the basic material for flexible circuits. This material has the advantage of having good flexibility, but it cannot offer mechanical support. Its dimensional stability also has some limitations.
The discussion then turns to copper film. There are two types of copper film. Electrodeposition is used for the first, and rolling and annealing are used for the second. The more flexible of the two, rolled and annealed, is ideal for a flexible printed circuit application. However, printed circuit boards are primarily produced using electro-deposited copper, although it has limited flexibility. Therefore, the performance of a flexible circuit greatly depends on the type of copper used.
Designing flex PCBs involves a somewhat different process compared to rigid PCBs. Here are some guidelines and considerations to keep in mind when working with flexible printed circuits.
When selecting a manufacturer for flexible PCBs, it's essential to ensure they use up-to-date technology to account for material properties, movement, and thermal coefficients, all of which are crucial for producing durable circuit boards that meet client specifications.
Additionally, consider the following factors when choosing a flexible PCB manufacturer: the manufacturer should guarantee product quality, as the investment in these often costly products should be worthwhile. The circuit boards should feature high-density designs, efficient communication lines, and excellent conductivity to enhance thermal dissipation.
Furthermore, the manufacturer should be capable of producing flexible PCBs in any required shape. Most reputable producers should meet these fundamental criteria:
Flexible circuit manufacturers carefully consider bend radius characteristics. It's important to establish the bend radius early in the design phase to ensure that the circuit can withstand bends without damaging the copper traces. The bend radius specifies the minimum curvature of the flexible region and should be free from features like slots, cutouts, and mounting components that could compromise its integrity.
The choice of cover material during the fabrication of a printed circuit board (PCB) greatly influences its durability and performance. It's essential to select cover materials that are well-suited to the specific application of the PCB. Quality is a key factor, whether the flexible printed circuit boards are used for personal or commercial purposes. The manufacturer should ensure that cover materials are selected based on their performance, specifications, and the operating environment of the board.
In the routing process of printed circuit board (PCB) design, "wire routing" refers to the path that electrical connections take. This step is crucial in PCB design, particularly for flexible PCBs, where installing circuitry can be challenging due to the increased strain on the board as the bend radius grows. To ensure safety and reliability in PCB design, it is important to route traces through low-stress areas. Proper trace routing helps prevent short circuits and reduces the risk of electrostatic discharge on the PCB.
Consideration must be given to features that could cause discontinuities, particularly in bending areas. When a flexible circuit is twisted or stretched, internal stress can lead to breaks and cracks in the insulation or conductors. To mitigate this risk, it's advisable to avoid features like beams and minimize the chances of thinning copper circuits.
Layer stacking is crucial in flexible PCBs due to the multiple layers that are stacked together. To ensure flexibility and prevent shifting, the most pliable layers should be positioned in the center of the stack.
Delamination, also known as "layer detachment," is a frequent problem in PCB manufacturing and usage. This issue can lead to sudden damage to the circuitry and, in some cases, even result in PCB failure. To mitigate delamination, it is essential to use appropriate solder paste. When selecting solder paste, it's important to consider the flexibility requirements of the PCB. Additionally, the design should include features that help prevent delamination.
When designing flexible printed circuit boards (PCBs), it's advisable not to base the adhesive width solely on drawings. Instead, the design should focus on specifying the overall width of the PCB and including key dielectric materials that affect impedance.
Designers should prepare detailed drawing packages for their flexible PCB designs. While it's important to include critical features, avoid over-specifying non-essential details. Striking the right balance ensures that the design is accurate while keeping costs as low as possible.
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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