What Are Flexible Circuits?

FPC is the abbreviation of Flexible Printed Circuit, which also called as Flexible Circuit, or Flex Circuit. What’s flexible circuit? According to IPC definition, flexible printed circuit is a patterned arrangement of printed circuitry and components that utilizes flexible base material with or without flexible coverlay. This definition is accurate, and convey some of the potential given the available variations in base materials, conductor materials, and protective finishes. But sometime, flexible circuit is also called as Flexible PCB or Flex PCB, that’s because the inherent concept of most people is that flexible circuit is a bendable PCB consisting of a flexible film with a pattern of copper conductors on it.

Flexible Circuit Services

As a reliable flexible circuit manufacturer, we support all kinds of 1-8 layers flexible circuits manufacturing, including flex circuits with thru-hole interconnection, buried and/or blind via interconnection, buried and blind microvia interconnection. Moreover, MADPCB supports carbon ink, silver ink, constantan, Nickel-Copper, and hatch impedance controlled flexible circuits.

  • Single-sided Flex Circuits

    Single-sided flexible circuits have a single conductor layer made of copper, constantan or silver ink on a flexible polyimide or polyester base material. Component termination features are accessible only from one side (top or bottom layer). Holes may be formed in the base material to allow component leads to pass through for interconnect, normally by soldering on the conductive layer annular rings. Single sided flex circuits can be fabricated with or without such protective coating as cover layers (coverlay), however the use of a protective coating over circuits is the most common practice. The development of surface mounted devices (SMDs) on sputtered conductive films has enabled the production of transparent LED films, which is used in LED strip light but also in flexible automotive lighting composites.

  • Double Access or Back Bared Flex Circuits

    Double access flex PCB, also known as back bared flex circuit, are flexible circuits having a single conductor layer but which is processed so as to allow access to selected features of the conductor pattern from both sides. While this type of flex circuit has certain benefits, the specialized processing requirements for accessing the features limits its use. Double access flexible PCB sometimes is used to directly solder onto the surface of a rigid board. In this application, the bottom conductors contact the pads of the rigid board, and put solder paste on the top of the conductors to solder.

  • Sculptured Flex Circuits

    Sculptured flex circuits are novel subset of normal flexible circuits structures. The manufacturing process involves a special flex circuit multi-step etching method which yields a flexible circuit having finished copper conductors wherein the thickness of the conductor differs at various places along their length. The conductors are thin in flexible area and thick at interconnection points.

    Sculptured flex circuit is a modification of the basic flexible printed circuit board, where the unsupported copper traces protrude beyond the edge of the flexible circuit and act as replacement for the male pins of a connector. The copper traces are of variable thickness, being thicker and sturdier in areas that will act as the connector part, while the rest of the circuit remains thin enough to facilitate bending and flexing. Sculptured flex circuit is growing in popularity because of it can reduce the width of a PCB by eliminating connectors, while at the same time allowing for more efficient use of space.

    Intraditional flex circuit boards, assembling a connector requires the board to have plated through holes (PTH) suitable for accepting the pins of the connector or solderable pads of suitable pitch matching the SMT ZIF (Zero Insert Force) socket. Additional reinforcement is necessary at the junction of the board and the connector fore stress relief once the connector is soldered onto the PCB. However, sculptured flex PCB can eliminate all the above.

    Sculptured flex printed circuit boards start with thick copper clad, as thick as 5oz. This forms the copper traces that will protrude from the board to make the pins of the connector. After the unwanted copper is etched away, more selective etching reduces the rest of the traces to thicknesses that are conductive to good flexibility, while plating with metals such as gold, tin, and copper build up the thickness of the traces at the edges. This selective sculpturing yields thicker traces at the edges, while keeping the rest of the circuit flexible.

    The next step involves removing the base dielectric from under the thicker traces, using back-baring technology. Rather than using lasers for ablating or removing the dielectric, flexible PCB manufacturers prefer to drill or pre-punch the base dielectric to expose the traces that will eventually become the connector pins. Here are the Sculptured flex circuit benefits: (1) Eliminating the need for ZIF connectors. (2) Reducing assembly labor costs. (3) Increasing flexibility. (4) Improving reliability and performance.

  • Double-sided Flex Circuits

    The double-sided flexible circuits are also very popular. With the demand to place more components on a circuit and increasing circuit density and power-handling capabilities comes the need for greater conductor numbers. This can 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 cover lays. With double-sided 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.

  • Multilayer Flexible Circuits

    Flexible circuits that have three or more layers of conductors are referred to as multilayer flex. These circuits are complex to construct and have high costs, but they meet designers’, manufacturers’ and consumers’ demands for even greater circuit density.

    A multilayer 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 flexible multilayer circuits may or may not be continuously laminated together, depending upon the flexing and dynamic characteristics required.

    Flexible multilayer circuits are popular within the defense and aerospace sectors where they provide dynamic high-density circuits. Their drawback is that with current substrate and conductor materials they are often restricted to a maximum of twenty-five 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 laminate 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 rigid-flex circuits, they make use of PTH or micro-via interconnects where required. These types of boards have found particular favor in the defense sector 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, notebooks, drones, and extensively in the construction of hearing aids and ear phones.

    There are a number of variations of rigid-flex available. Amongst them is rigidized flex which is in effect a flexible circuit which has a stiffener 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 plastic, composite and metal backing materials are commonly used.

    Beyond the generic variations of flexible circuit constructions there are a number of alternatives. One such major variation is moulded circuits. These are typically 3D module plastic components with mechanical capabilities, into which electrical circuitry is incorporated. For some types of moulded circuits the electrical functionality is provided via a flexible circuit that is introduced into the mould at the time of manufacture. Other variations utilize complex moulding and selective plating techniques to form suitable conductor patterns in and on the component.

What’s Flexible Circuit Materials?

There are a number of basic material elements that manufacture a flexible circuit: a dielectric substrate file (base material, or FCCL), circuit traces, a protective finish (coverlay or cover coating), and, not least, adhesives to bond the various materials together. In MADPCB, the FPC materials include:

  • FCCL: FCCL is the abbreviation of Flexible Copper Clad Laminate, which is the base material to build flex circuit and rigid-flex PCB. Single- and double-sided FCCL are made of either Kapton polyimide (PI) film, or polyester (PET) film as dielectric substrate material and thin copper foil as conductor with flexibility on surface. With adhesive in-between substrate and copper, it calls Adhesive FCCL, otherwise, it calls Adhesiveless FCCL. FCCL material has to perform a variety of important functions. It must electrically insulate the conductivity circuit traces from one another and it must be compatible with any adhesives used for conductor or coverlay bonding. Under normal circumstances the FCCL will also provide the circuit with much of its mechanical characteristics, such as its flexing strength and durability.

  • Copper Foil: Copper foil can be ED (Electrodeposited) and RA (Rolled-Annealed) for forming FCCL or extra copper layer(s). 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: Adhesive plays 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 flexible circuit or rigid-flex PCB 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 thickness from 0.5mil to 5mils in 0.1mm increments. The flexible circuit 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

2-5.5

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

  • Coverlay: Coverlay also known as Cover Lay, or Cover Layer, which 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 light-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.

  • Stiffener: Stiffeners can be Polyimide (PI), FR4, Stainless Steel and Aluminum in our flex circuit manufacturing. (1)PI Stiffener is the most common used 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” FPC 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. (2)FR4 Stiffeners to support components. In many flex PCB designs, a rigid-flex stiffener is applied to create a localized rigid area in the FPC where SMD components and/or connectors are places and soldered. In these instances, the stiffener can prevent the circuit from being bent in or adjacent to component area(s) which can potentially compromise 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. (3)Metal Stiffeners to heat sinking or increase rigidity. Metal stiffeners include Aluminum stiffener and stainless-steel stiffener. 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.

Type

Common Thickness

Unusual Thickness

FR-4 Stiffener

0.2mm(8mil), 0.3mm(12mil), 0.4mm(16mil) 0.5mm(20mil), 0.6mm(24mil), 0.7mm(28mil)
0.8mm(32mil), 1.0mm(39mil), 1.2mm(47mil) 1.5mm(59mil)

0.1mm(4mil), 0.9mm(35mil)
1.1mm(43mil), 1.3mm(51mil)
1.4mm(55mil), 1.6mm(63mil)

PI Stiffener

0.1mm(4mil), 0.15mm(6mil)
0.2mm(8mil), 0.25mm(10mil)

0.075mm(3mil), 0.125mm(5mil)
0.175mm(7mil), 0.225mm(9mil)
0.275mm(11mil)

Metal Stiffener

0.1mm(4mil), 0.15mm(6mil), 0.2mm(8mil)
0.25mm(10mil), 0.3mm(12mil), 0.35mm(mil)
0.4mm(16mil), 0.5mm(20mil), 1.5mm(59mil)

0.45mm(18mil)


Hatch Plane Grounds on Flex PCB Impedance

As a flex and rigid-flex PCB manufacturer, we always need to pre-calculate the impedance of stripline and microstrip PCB traces when corsshatching (or meshed) return paths are deploayed rather than the solid copper return paths of conventional rigid PCB. The use on crosshatched planes on flex circuit and rigid-flex PCB has proved a practical method of keeping controlled impedance traces at wider, more manufacturable deimensions while also ensuring the desired flexibility of the PCB assembly. Crosshatching is also deployed to keep impedance controlled line widths -for example, on interposer PCBs. How to calculate the impedance on mess ground or hatch flexible circuit (FPC) PCB? This is a nightmare for most flex PCB manufacturer or even for some designers. At MADPCB, we are capable of calculating the flex circuit controlled impedance. 

It’s nightmare for most flex PCB manufacturers or even for some designers. But at Fuchuangke Technology, we are capable of calculate flex circuit impedance on Polar Field Solver. When it comes to calculate, just follow to below steps:

  1. Set the stack-up as per thicknesses of each dielectric layers and copper layers. i.e. fining out the thicknesses of H1, H2 (or H3, or H4), and W1(lower trace), W2(upper trace) and S1(trace spacing).

  2. Checking the hatch plane grounds (or mesh ground) copper to get HP (Hatch Pitch) and HW (Hatch Width), and fill them in Hatch Configuration diagram.

  3. Calculating the thicknesses and filling in each cell, then start FPC impedance calculation. The dielectric consistant (Dk or Er) of fex PCB materials is always 3.2.

  4. Upon the required controlled impedances, adjust the thicknesses or HP and HW till to meet impedance requirements.

Hatch Configuration of Flex PCB Hatch Impedance

Flex PCB Hatch Impedance Calculation