Flexible Printed Circuits (FPC), also written as Flexible Circuit, Flexible PCB, Flex Circuit, or Flex PCB, which is built with flexible base substrate PI (polyimide) or PET (polyester). And polyimide is the preferred material. Unlike rigid PCBs, flexible circuits have ability to enable the circuitry to be conformed to a desired shape to fit the electronic device or product, or to flex during its use. Flexible circuits are thin, light-weight electrical circuits that conform to small spaces and contoured shapes. We are the reliable quick-turn flexible circuit manufacturer and provider in Shenzhen, China.

A flex circuit board is a patterned arrangement of printed circuits and components that utilizes flexible based material with or without flexible coverlay. Sometimes, the FPCBs contain a stiffener with FR-4, PI or stainless-steel material to rigidize particular areas. Rigidizing is recommended under SMT components, or on the back of the gold jack fingers, or for some use of thermal dissipation. If the flex circuits have EMI (electromagnetic interface) requirement, the PCB hardware designers will ask to add EMI shielding film in particular areas. The flexible PCB outline processing can not be done by V-cut or routing, but by unique laser cutting or punching.

Flexible printed circuits were originally designed as a replacement for traditional wire harness. A flexible circuit in its purest form is a vast array of conductors bonded to a thin dielectric film. Nowadays, flexible circuit technologies are widely used in all walks of life from the simple applications. It’s said that flexible PCBs constitute about 10%-15% of all PCBs production globally. Electric and electronic hardware developers bring FPCBs to automotive, medical, telecom, industrial or commercial applications.

What’s Advantages of Flex Circuits?

By far the biggest drives for flexible-circuit technology are the range of benefits and capabilities that the technology offers. Some of the advantages of flexible printed circuits are highlighted in below table.

Cost Reduction	Performance	Others
Elimination of wiring errors associated with manual wiring harnesses Simplified PCB assembly Reduction in component numbers Reduced PCB assembly effort and time Increase reliability	Dynamic flexing 3D packaging Excellent control over transmission Impedance control Lower inductance than normal wiring Improved heat dissipation Improved airflow capabilities Compliant substrate for minimising thermal mismatch	Reduced weight Reduced PCB assembly size Convenience of roll-to-roll processing Increased package density Improved product appearance More integrated design

In the face of increasing industry challenges and consumer demands such as miniaturization, lightweight products, lower cost, greater product design freedom, high reliability and more environmentally extreme applications, flexible circuits are proving their worth.

There are many advantages to using FPCs. They utilize the thinnest dielectric substrates available for electronic interconnection, down to 2mil, and are known for their ability to reduce package size as well as package weight. FPCs can reduce the weight of an electronic package significantly -by up to 75%. This weight reduction makes flexible circuits extremely popular in the aerospace industry. Another advantage of FPCs is assembly costs. Reduction of assembly costs is achieved by reducing the number of assembly operations and having the capability to test the circuit prior to committing it to PCB assembly. A properly designed flexible circuit is an excellent means of reducing the number of levels of interconnection required in an electronic package. Flexible printed circuits can eliminate wiring erros associated with hand-built wire harness, since it is not possible to route flexible printed circuits to points other than those designated in the artwork.

Development within the field of electronic components are also positive developments for flexible circuits. The growth in surface mount technology (SMT) and the development of conductive adhesives, used to attach such components to circuit boards at relatively low temperatures, has favored the use of flexible substrates. Such components are highly sensitive to thermal mismatch between substrate, mounting and component materials. It is recognized that flexible circuits offer a highly compliant material that is able to counteract the effects of thermal stress with more success than rigid laminates, making their use in environmentally conditions highly appealing.

Flexible Circuit Services

• Type I: Single-Sided Flexible Printed Circuit
• Type II: Double-Sided Flex Circuit
• Type III: Multilayer Flex PCB
• Type IV: Rigid-Flex PCB board
• Type V: Flexible Circuit Assembly
• Type VI: Rigid-Flex PCB Assembly

Flexible Circuit Constructions

Despite the variability of flexible circuit materials there is a topology to flexible circuit construction that follows a number of generic variations. Many of the flexible circuits found in the vast proportion of interconnection and flexible packaging applications follow basic FPC designs.

•  Single-Sided Flexible Circuit: Usingle-sided flexible circuit is the most common type of flexible circuit available. It consists of a single conductor layer on a flexible dielectric film with access to circuit termination features accessible from one side only. Single-sided flexible circuit can be fabricated with or without cover lays and protective coatings, and their relatively simple design makes them highly cost effective. The conductors used can be conventional metal foil, or, for low-cost, polymer thick-film (PTF) ink can be used. This is simply printable conductive ink, loaded with carbon or silver particles, which is directly applied to the flexible substrate in the circuit pattern required by a variety of printing and stenciling techniques. Single-sided circuit can offer the lowest cost and relative ease of production. Because of their thin and lightweight construction such circuits are best suitable to dynamic-flexing of wiring-replacement applications such as computer printers and disk drives. Nearly all of the world’s calculators consist of PTF flexible circuits on polyester fil, a combination that offers and exceptionally low flexible circuit cost.
• Dual Access Flexible Circuit: Also known as Back-Bared Flexible Circuit, has been developed to meet the demand for low cost circuitry that can handle an increase in component real-estate demand. Such circuits allow PCB designers to place components on both sides of the flexible dielectric film. To access both sides of the film requires careful FPC design to allow exposed areas of the conductor to be available for underside component attachment. This often means punching through-holes in the dielectric film prior to its lamination with the conductor. Other methods involve after-lamination machining – laser or chemical milling for example – to provide rear access to the conductive layer. Because of the process steps required to produce dual access it is not widely used.
• Double-sided Flex Circuits: The double-sided flexible circuits are also very popular. With the demand to place more components on a circuit board and increasing circuit density and power-handling capabilities comes the need for greater conductor numbers. This ca be met by incorporating more than a single conductive layer on the same base film. Double-sided circuits can be constructed by various means such as separate conductors on both sides of the base film and printed conductors separated by printed insulating coverlays. With double-sided flexible circuits an issue is ensuring reliable connectivity paths between components mounted on the top and the bottom of the board. Various techniques have been developed to provide connectivity through such multilayered laminates. Early examples include conductive metal staples, pins and rivets. The most popular flexible circuit through-board interconnectivity technique is the plated through hole (PTH), which is also the most popular approach in the rigid-circuit world, from which it has successfully transferred. In PTH, holes are drilled or laser cut in or through the conductors and base film laminate. These holes are then primed and plated with conductive materials to produce a reliable interconnect feature.
• Sculptured Flex Circuits: Sculptured flexible circuit is a derivative of flexible circuit architectures in which a specialized, patented technique is used to yield a conductor layer of varying thickness. The conductive layer is selectively etched back in places to provide thin layers where flexibility is required, and thicker layers for joining and circuit interconnection. It is typical that the thicker layer forms leads that protrude from the circuit, to provide plug in connectors or greater lands for improved solder joint formation. Typically, such leads also provide the circuit with improved mechanical strength and rigidity.
• Multilayer Flex Circuits: Flexible circuits that have three or more layers of conductors are referred to as multilayer flex. These flex circuits are complex to construct and have high costs, but they meet the demands of PCB designer, FPC manufacturer and consumer for even greater circuit density. A multilayer flex circuit consists of bonded conductive layers that are interconnected by means of plated through-holes. Unlike their rigid multilayer counterparts, the individual circuit layers in multilayer flex PCB may or may not be continuously laminated together, depending upon the flexing and dynamic characteristics required. Multilayer flexible circuits are popular where they provide dynamic high-density circuits. Their drawback is that with current substrate and conductor materials they are often restricted to a maximum 25 layers. Even with flexible circuit there is a degree of mismatch between the coefficient of thermal expansion of various materials used in their construction, particularly the adhesives. This means that over multiple layers laminates stress can cause through-hole interconnects to barrel and stretch, restricting their reliability.
• Rigid-Flex Circuits: Rigid-flex circuits are hybrid constructions consisting of rigid and flexible substrates laminated together. Predominantly, the rigid circuits are used to house the components, whilst the flexible circuitry provides the necessary interconnects between them. Like double-sided and multilayer circuits, they make use of PTH interconnects where required. These types of printed circuit boards have found particular favor where the combination of reliability, strength and flexibility has not been lost on equipment designers. They are used in a wide variety of commercial microelectronics applications such as laptop computers and notebooks and extensively in the construction of hearing aids. Rigid-flex circuits are a combination of rigid circuitry and flexible interconnects. The interface between the flexible and rigid elements requires careful design, particularly if it is to be subjected to repeated flexing. If this is the case, compliant materials are often applied to the join to reduce the direct flexing of the interconnects as the rigid board. There are a lot of variations of rigid-flex PCB available. Among them is rigidized flex which is in effect a flexible PCB which has a stiffening material attached, to support the weight of mounted components and to provide the circuit with some rigidity to aid assembly. Suitable stiffener materials depend upon the application at hand but polyimide, FR4 and stainless-steel materials are commonly used.

Flexible Circuit Material Configurations

With a typical flexible circuit, four distinct classes of materials are used:

• Flexible Base material: A suitable base material has to perform a variety of important functions. It must electrically insulate the conductivity circuit tracks from one another and it must be compatible with any adhesives used for conductor or coverlay bonding. Under normal circumstances the base material will also provide the circuit with much of its mechanical characteristics, such as its flexing strength and durability. In the case of adhesiveless laminates, which will be discussed later, the base substrate provides all of the circuit’s strength.
• Copper: Copper is the material of choice for flexible circuit conductors. In practice, of the wide variety of possible conductor materials, only a selected few have found use within volume applications. As well as providing the electrical connectivity and electrical performance features of flexible circuits, conductor properties greatly influence the fatigue lift, stability, and mechanical performance of FPC assemblies. In many static applications bending is limited to installation and general servicing. In dynamic applications, conductors should be of the minimum acceptable thickness and their material of construction must be carefully chosen, along with their grain orientation and deposition technique, to match the performance levels required. The relatively low cost of copper, its high workability, good plating and good electrical characteristics make it an excellent material for flex PCB conductors. It is also the case that there are several different kinds of copper available, which can be matched by the circuit designer to specific applications.
• Adhesive (optional): Adhesive play an important role in flexible circuits. They are used to provide a secure join between the substrate and the chosen conductor material, to join circuits together where multilayer or rigid-flex constructions are required, and to provide a protective coverlay over exposed conductors once they formed. It is also important that adhesives act as part of the dielectric packaging of the signal, power, and ground circuit traces. They determine a fundamental part of the circuit’s electrical behavior. Adhesives are typically available in a range of thicknesses from 0.5mil to 5mils in 0.1mil increments.

1. What’s the adhesive materials in flexible circuits manufacturing? The flexible PCB adhesives have polyester, acrylic, modified epoxy and polyimide. The chart below shows some main properties of them.

Property	Polyester	Acrylic	Modified Epoxy	Polyimide
Peel Strength (Lb./in)	3-5	8-12	5-7	2.0-5.5
Adhesive Flow	10mil	5mil	5mil	<1mil
Chemical Resistance	fair	good	fair	good
Moisture Absorption	1-2%	4-6%	4-5%	1-2.5%
CET (PPM)	100-200	400-600	100-200	220-260
Tg (?)	90-110	30-40	90-165	220-260
Dielectric Constant @ 1KHz	3.1	2.5-3.5	3.5-4.5	3.4
Dielectric Constant @ 1MHz	3	2.2-3.2	3.3-4.0	3.4
Dielectric Strength (volt/mil X 1000)	1.0-1.5	1.0-3.2	0.5-1.0	2.0-3.0

2. How to apply adhesive to flexible circuits? There are many methods for applying adhesives but generally they are coated onto the dielectric substrate and then laminated to the conductor foil. Depending upon the nature of the base this can be done via a heated press for sheet processed materials such as polyamides, or through heated rollers for roll-to-roll materials such as polyester. Some form of post curing at elevated temperature is usually required after roller lamination due to the relatively short contact time between roller and laminate.
3. How to choose flexible circuit adhesive? Adhesive must be carefully chosen to offer compatibility with both substrate and conductor materials. They must be able to provide adequate mechanical strength, have good chemical resistance, and be able to withstand the conditions used in FPC manufacturing without delamination. It is often the case that the chosen dielectric material determines the type of adhesive used. For example, polyester adhesives are typically used with polyester laminates, and new formulations of high-temperature polyimide adhesives are increasingly used for polyimide substrates.

• Adhesiveless Laminates: As discussed previously, adhesiveless laminates represent a new emerging class of base material for flexible circuits. Today many flexible circuits consist of a base film and copper foil bonded together with an adhesive. Industry experience, research and product histories have shown that it is often the adhesive used in flexible circuits that is the principal limiting factor in many of the circuit’s capabilities. The performance of the adhesive chosen binders high-temperature performance in particular but also chemical resistance and multilayer flexible circuit and rigid-flex PCB build capability. For example, the relatively poor thermal expansion charateristics of adhesive causes excessive expansion problems in multilayer flexible PCB, which causes stress in conductors and delamination of fine circuit features if unchecked. Adhesiveless materials avoid all of these pitfalls by doing aways with the adhesive layer. Adhesiveless laminates are manufactured in a number of different ways. A thin coating of seed conductor material, typically copper pf less than 1?m, may be placed directly onto the base laminate via techniques such as sputtering and electroless plating. These thin surface layers can then be selectively electrodeposited up to the required circuit thickness, and finally the whole circuit is given a flash etch back to remove the web of copper connecting the circuit traces to produce the final flex PCB. This flexible circuit manufacturing process is termed semi-additive. Alternatively, the base laminate material, such as polyimide, may be cast in resin form onto a carrier foil, such copper, and when processed through heated rollers and allowed to cure, forming a continuous adhesiveless laminate suitable for further processing. Driving the market uptake of adhesiveless laminates is demand for complex high-speed flexible PCB that are capable of operating at higher temperatures and frequencies. These must make use of multilayer construction techniques. In this regard an adhesiveless copper-polyimide, for example, will be more suited to the application, and has the added advantage of being thinner. Traditional foils adhesively bonded to base films are proving too thick for thigh-density circuitry. Also the extra copper thickness associated with adhesively bonded laminates required extra processing time to etch back and is less environmentally friendly. Adhesiveless copper-polyamide laminates are now offered with thicknesses ranging from 0.3?m to about 35?m, with the thinner material proving much more suitable for fine line circuitry.
• Coverlay: A coverlayer, also known as a “cover layer”, is usually a combination of a flexible film and a suitable pressure-sensitive or thermosetting adhesive. The most commonly used materials are polyester film coated with polyester adhesive, polyimide film with acrylic adhesive, and polyimide film with epoxy adhesive. As stated, in flex PCB design the usual practice is to match the coverlay film to the material of the base substrate. The purpose of a coverlay is threefold: to prvide circuit and conductor protection; to allow access to flexible PCB pad and contact areas for further processing such as soldering and conductive adhesive bonding of components; and to enhance circuit flexibility and reliability. To enable access to required conductor features beneath the coverlay, such as pads and contact points, registration holes are drilled, punched, or laser machined into the film. The coverlay is then registered over the conductor pattern and laminated using heat and/or pressure accoding to the adhesive’s requirements. To reduce conductor damage from frequent bending, the thickness of the coverlay should be the same as the thickness of the dielectric layer. This arrangement places the conductor traces near the neutral axis of the finished flexible circuit assembly in effect in the center of the layered construct, which significantly reduces conductor stress during flexing. An increasingly popular alternative to pre-punched and drilled adhesive films is the photo-imageable coverlay. A layer of ligh-sensitive material, either in film or liquid form, is placed over the top surface of the conductor trace layer. The layer is exposed to light through a photographic negative coating cures in the exposed areas and subsequent processing strips uncured materials to leave a patterned covering which provides access to contact pads and soldering lands.
• Protective Coating: Protective films or coatings may be selectively applied to the surface of FPC board to protect it from moisture, contamination and abrasion, and to reduce conductor stress during bending. The protective layer is placed over the circuit once the conductor pattern has been established.

What Are Flex Circuit Stifferners?

Stiffeners can be FR4, Polyimide (PI) and stainless steel in our flex circuit manufacturing.

• Using Stiffeners To Increase Thickness. Polyimide Stiffener are the most common method to achieve the thickness requirement, at the contact fingers, as specified by the ZIF connector that the flexible circuit plugs into. But we do not recommend to design a “thicker” flexible PCB in an attempt to eliminate the demand for a ZIF stiffener. This will result in an excessively thick part that will not have the required flexibility or bend reliability.
• FR4 Stiffeners To Support Components. In many flex PCB designs, a rigid stiffener is applied to create a localized rigid area in the flex circuit where SMD components and/or connectors are placed and soldered. In these instances, the stiffeners can prevent the circuit from being bent in or adhacent to component area(s) which can potentially compromize the part’s solder joint integrity. Besides, the general rule is to use the thickest FR4 stiffener that the finished thickness of FPC plus stiffener will allow for up to 1.6mm which replicates a traditional rigid PCB board.
• Metal (Stainless Steel) Stiffeners To Heat Sinking or Increase Rigidity. In some flexible circuits manufacturing requirements, metal stiffener, like stainless steel is available. The stainless steel stiffener is typically used for applications requiring heat sinking or added rigidity. Metal stiffener need to be customized and the lead time will be a little longer, so it’s better to use stainless steel stiffener only when required.
• Adding Stiffeners For Bend Constraints. FPC stiffeners are also be used to restrict the bend areas to specific locations within a flexible design to either facilite the final assembly process or to achieve a specific bend requirement or end use function. These stiffeners are typically an added thin layer of polyimide material, but thicker layers of FR4 can be used if required. The flexible PCB manufacturer should always be done to ensure that the stiffener(s) do not create any mechanical bend stress concentrator that may result in a kink in the part rather than a bend in a preferred smooth arc.

PCB Blog: Flexible Printed Circuit Board Stiffeners

Different Stiffeners and Stiffener Thicknesses in Same FPC Design

In some applications, an flexible circuit require one or two or three different stiffeners, and these stiffeners have multiple thicknesses in the same flex circuit design. The FPC supplier will review and find out the manufacturing limitations before flex circuit fabrication. Stiffeners can be attached to either one or both sides of the FPC. Before manufacturing, the flexible PCB manufacturer should carefully to design the assembly panel array to decrease the assembly complications. Flexible circuit assembly with unflat FPC surface, it always require special FPC SMT pallet, or it is recommended to add FR4 stiffener throughout the panel array border (/ waste tabs), and which will eliminate the need for any additional tooling plates etc. But it will increase the FPC cost since using more FR4 material.

How To Add Flexible Circuit Stiffeners?

There are two flex circuit stiffener attachment methods. (1)The prefered attachment method is to thermally bond the stiffener to the flex circuit under high heat and pressure. This method utilizes the same flexible adhesive (AD) as used to attach the coverlays, and will result in a very strong permanent bond. Type of flexible adhesive will depend on the configuration and/or location of the stiffener(s). (2)Another is an alternative attachment method, which is done with a pressure sensitive adhesive (PSA), if the design prevents the use of a flexible adhesive. The specific PSA (e.g.: 3M / Tesa tape) will depend on whether the flex PCB will be subject to an automated reflow cycle assembly or what material it is to be adhered to.

Fuchuangke Technology is a flexible circuits manufacturer that has consistently delivered high quality flex circuits since 2008, and we have significant experience in the flexible printed circuit (FPC) and flexible PCB industry. The flexible PCBs are built to the exacting specifications of our customers, meeting the most demanding of applications in smart phone, watch, UAV, wearable device, medical device, car, industrial equipment and etc. Relied on our experienced team and advanced flexible PCB manufacturing equipment and excellent Flex Circuit and Rigid-Flex PCB Capabilities, we are the perfect choice for all of your flexible circuit demands from prototype flex boards to mass production.

As a leading flexible circuits provider in China, Fuchuangke Technology offer quick-turn flex PCB prototypes and high-volume production and assembly services -single-sided, double-sided, multilayer and rigid-flex printed circuit board, and aim to be a highly reliable flexible printed circuits provider and supplier in the world.

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