What Is a Flexible Printed Circuit?
Originally designed to replace traditional wiring harnesses, flexible printed circuits have grown exponentially from the early days of World War II to today. Flexible circuits offer great versatility and are ideal for simple to complex applications and can handle almost any circuit board job.
A flexible printed circuit is a lightweight circuit that easily fits into smaller spaces and contoured shapes. However, they are not just bent PCBs. Flexible circuits have unique differences and advantages that require unique designs from innovative PCB manufacturers.
A flexible printed circuit board consists of conductive metal strips, usually made of copper, and insulated by a dielectric material or solder mask. A key function of a circuit board is to provide mechanical flex while passing electronic signals, enabling the use of smaller connectors and shielding from radiated noise from EMI.
A PCB is a substrate or non-conductive plate used to connect electronic components to form a circuit.
Basic Types of Flexible Printed Circuits
The variety of available configurations, sizes, and features highlights the versatility of flexible circuit boards. The basic types of flex circuits include single-sided circuits, double-sided circuits, and multi-layer circuits.
one-sided
Single-sided PCB circuits have only a single layer of metal on one side of the dielectric layer.
duality
In contrast, double-sided traces have metal layers on both sides of a single dielectric layer.
multilayer
In double-sided circuits, the metal layers are often connected by metallized vias. The same vias can be found on multilayer boards containing several individual copper layers encapsulated by dielectric layers.
Rigid and soft
A rigid-flex PCB combines components from rigid and flexible circuits to form a hybrid circuit substrate.
Benefits of Flexible Printed Circuits
Flexible electronics offer several advantages over traditional rigid boards and cables. These include more precise routing, elimination of mechanical connectors, complete design flexibility, higher circuit density, wider operating temperature range, stronger signal quality, improved impedance control and reliability, and size reduction.
Automated production eliminates human errors common in manual wiring harnesses. The flex circuit can only reach the point indicated by the design plan. Less labor is involved in the custom PCB printing process, and flexible PCBs are expensive and time-saving to produce.
Since the freedom of the design process is not limited by two dimensions, flexible circuit boards can be customized in various ways. This flexibility extends to the installation, allowing a third dimension of connecting channels between two or more planes.
The functionality of flexible circuit boards is improved by increased airflow, heat dissipation, and the application of high-density devices in point-to-point connections using simplified circuit geometries.
infrastructure application
Before designing a circuit, two basic structural applications will determine the choice of conductors. Static applications are applications that can be flexibly installed with only a flex circuit. Electrodeposited (ED) copper is typically used for this application, which is a less expensive option.
Dynamic flex applications involve dynamic flexing of flexible circuits during the daily use of the product. Clamshell phones, laptops and computer arms all require active applications using Rolled Rolled (RA) copper.
Flexible Printed Circuit Features
Single-sided and double-sided flex circuits can have reverse bar, through-hole, floating finger, and ZIF terminations. Multilayer flexible PCBs can contain up to 20 layers and handle high circuit density with multiple conductive layers, through-hole components, embedded resistors, controlled impedance and EMI shielding.
When via diameter needs to be minimized, flexible printed circuit boards use the smallest pitch and component size to improve signal integrity, electrical performance, and thermal performance.
Flexible designs can also use the same high-density components in a rigid PCB by using blind or buried vias to route signal lines out of high-density areas of the device. Controlled impedance is significantly improved with fast signal switching, fast transitions and high clock rates. Signal transmission in all PCBs requires laminates with uniform thickness and electrical signals.
Films provide protective shielding to reduce noise and control signal line impedance, while stiffeners enhance flexible circuits where components require durability and mounting support.
Flexible Printed Circuit Board Materials
Flexible printed circuit boards are composed of materials similar to other PCBs. All circuits require metal conductors for power, and copper is the most commonly used material in PCBs. Copper comes in a variety of thicknesses to suit different preferences. Other conductor options include aluminum, silver ink, carbon and constantine.
Adhesives bond the layers of the printed circuit board together. The specific material depends on the user and conductor thickness requirements, but all bonds must remain strong over the operating temperature range. Standard adhesives used in flexible PCBs include epoxy, acrylic or pressure sensitive adhesives.
The insulator encapsulates and separates the circuit with a flexible substrate and cover material. Polyamides, polyesters, solder masks, and dielectric materials all often act as insulators for flexible printed circuit boards.
A complete PCB has the finishing touch at the end of the assembly process and can be customized to consumer preferences and circuit functionality. Standard finishes applied to the final product include solder, tin, nickel, gold, silver or carbon.
But are printed circuit boards toxic? Burning plastics and metals in PCBs releases toxins such as furans and dioxins, which means circuit boards in landfills can end up polluting groundwater if not recycled properly.
However, you can quickly repair a damaged PCB to restore its original function or recycle it in a completely different device.
Advanced Features
Since its invention, flexible circuit boards have come a long way. As the flexible PCB market expands, advances in technology allow advanced features to be included in circuit designs. Advanced features include heat sinks to dissipate heat away from sensitive areas and additional components for extended mounting capabilities and protection.
As signal switching speeds increase, better impedance control is required. Advanced impedance traces minimize electrical reflections to prevent errors during transitions between interconnects and control cables when optimizing.
Other features are also available, including crimp pins, graphic overlays, laser cut slots, engraved flex circuits, shields, flex heaters and stiffeners. With so many possibilities, there is a flexible circuit board to suit every need.
Flexible Printed Circuit Boards: Functions and Applications
The main features of flexible printed circuit boards:
Many electronics manufacturers prefer to use flexible circuit boards because of their unique features:
Flexible PCBs are inherently highly adaptable, and as such, they can be easily used for careful design and placement of electronic components.
Flexible PCBs are lightweight, so they can be efficiently integrated into electronic assemblies without increasing their bulk;
It is well known that flexible circuit boards can effectively manage the heat dissipation process.
Flexible PCBs are easy to assemble, requiring less time and cost;
Flexible circuit boards have flexible substrates, making them ideal for tiny electronic parts.
Where can I use a flexible printed circuit board?
Flexible PCBs are universal in nature. They can be used in different electronic devices where saving space is the main concern.
Consumer electronics products
Military (Defense) Accessories
medical equipment
car parts
Rigid Flexible Printed Circuit Board Technology Goes Further
Substrates and Protective Films
First, let’s consider an ordinary rigid printed circuit board, usually made of fiberglass and epoxy. In fact, these materials are fibers, and although we call them “rigid”, if you take out a layer, you can feel its elasticity.
The plies can be stiffer due to the cured epoxy. Because it is not flexible enough, it cannot be applied to some products. But for many simple assemblies where the board doesn’t continue to move, electronics are appropriate.
In more applications, we need more flexible plastic films than epoxy resins. Our most commonly used material is polyimide (PI), which is very soft and strong and won’t tear or stretch easily.
It’s also incredibly thermally stable and can easily withstand the temperature changes during reflow soldering, and during temperature fluctuations, we can barely find it stretching out.
Polyester (PET) is another commonly used flexible circuit material. Compared with only polyimide (PI) films, its heat resistance and temperature deformation are inferior to PI films. This material is commonly used in low-cost electronic devices with printed wires wrapped in flexible films.
Since PET cannot withstand high temperatures, let alone soldering, flexible circuit boards are made through a cold pressing process. I remember that the display part of the clock radio uses this kind of flexible connection circuit, so this kind of radio usually doesn’t work properly, the root cause is this poor quality connector.
Therefore, we recommend using soft and rigid bonding boards or opting for PI films, other materials can be used but are not often used.
PI films, PET films, thin epoxy resins and fiberglass cores are common materials for flexible circuits. In addition, the circuit requires the use of other protective films, usually PI or PET films, and sometimes shielding resistance soldering inks.
Protective films can insulate conductors from corrosion and damage, and PI and PET films are available in thicknesses ranging from 1/3 mil to 3 mils, with 1 mil or 2 mil thicknesses commonly used. Glass and epoxy materials are thicker, typically 2 to 4 mils.
electrical conductor
Printed conductors, usually carbon films or silver-based inks, are used in the economical electronics described above, but copper conductors are a common choice. According to different applications, we have to choose different forms of copper foil.
If you’re just replacing wires and connectors, reducing manufacturing time and cost, your best bet is electrolytic copper foil for responsive circuit boards. Electrolytic copper foil can also increase the current carrying capacity by increasing the weight of the copper to obtain the possible width of the copper plate, such as a planar inductor.
Copper is known to be relatively poor at work hardening and stress fatigue. Advanced rolled tempered copper foil (RA) is a better choice if the flex circuit needs to be folded or displaced repeatedly in the final application.
Clearly, more roll toughening processes are bound to increase the cost, but the rolled toughened copper foil can be bent and folded many more times before fatigue fracture.
It increases the Z-deflection resiliency we need, and in applications that bend and roll a lot, it gives us longer life. Because the rolling toughening process elongates the grain structure in the plane direction.
Exaggerated version of rolling toughening process diagram, non-proportional components. After the copper foil passes through the high pressure roller, its grain structure can be extended in the plane direction, making the copper softer and increasing the elasticity of the z-axis. A typical example is the connection between the bracket and the knife head, or the laser head in a Blu-ray drive (shown below).
In a Blu-ray machine, a flex circuit is used to connect the laser to the main circuit board. Note that the flex circuit on the circuit board on the laser head needs to be bent at right angles. Glue balls are used here to enhance the connection of the flex circuit.
A adhesive
Usually, we need adhesives to bond copper foil and PI film (or other films) because, unlike traditional FR-4 rigid board, there are not many surface burrs on the rolled toughened copper foil, so high temperature and high pressure cannot achieve good of adhesion.
It uses acrylic or epoxy based adhesives with millet or 1 mil thickness. This adhesive was specially developed for flexible circuit boards.
“Glueless” laminates are becoming more common due to the introduction of new processes such as direct coating and copper coating on PI films. Films can be useful in HDI circuits that require finer pitch and smaller vias.
When added protection is required, we use silicone, hotmelt or epoxy beads to soft and hard joints. This enhances the mechanical strength of soft and hard joints and ensures no stress fatigue or tearing during repeated use.
About Wonderful
Shenzhen Wonderful Technology Co., Ltd. is China’s top model and small batch circuit board manufacturer.
The company targets precision circuit board samples and small batches as the main target market. Since its establishment, it has adhered to the development concept of good quality and important reputation.
Invest huge sums of money to introduce advanced equipment, strictly regulate the operating standards and QC standards for incoming and outgoing materials, to ensure that the shipped products reach Seiko quality.
The self-developed ERP management system can realize the whole process of paperless operations such as online ordering, online payment, online production progress query, online logistics tracking, and online statistical report production, which greatly improves work efficiency.
It ensures that each order can be shipped quickly. After more than ten years of development, the company’s products and services have been well received by the market.
As well as the opening of an electronic component mall, integrated circuits, semiconductors, resistors, capacitors and other products are sold.
The company’s scale is developing rapidly, and the sample and small batch shipment capacity reaches 500 models/day.
The products are widely used in communications, medical equipment, industrial control products, aviation, military products, testing instruments,Automotive, computers and other peripheral products.
We will continue to pay attention to customers, with efficient delivery speed and perfect after-sales service, to become the most trusted brand supplier in the PCB industry.
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