Flexible Printed Circuit

Flexible Printed Circuits (FPCs), also called Flexible Circuits, or Flex Circuits, by IPC definition, a flexible printed circuit is a patterned arrangement of printed circuitry and components that utilizes flexible based material with or without flexible cover lay. This definition is accurate, and conveys some of the potential given the available variations in base materials, conductor materials, and protective cover materials. But sometimes, flexible circuits are also called as Flexible PCB or Flex PCB, that’s because the inherent concept of most people is that flexible circuit is a bendable printed circuit board (PCB) consisting of a flexible film with a pattern of copper conductors on it.

In reality, a flexible printed circuit consists of a metallic layer of traces, usually copper (rarely constantan), bonded to a dielectric layer, usually polyimide (rarely polyester). Of course, a multilayer flex circuit can contain many metallic layers. As a flex circuit manufacturer, MADPCB can fabricate 8-layer flex PCB. Thickness of the conductive layer can be very thin (0.47mil, 12μ, 1/3oz) to very thick (2.8mil, 70μ, 2oz) and the dielectric thickness can vary from 0.5mil (13μ) to 5mil (125μ). Flexible copper clad laminates (FCCL) can be single-sided and double-sided with or without a layer of adhesive to bond the metal to the substrate.

 

Click to check our advanced PCB Capabilities and PCB Fabrication Process in flex circuits section.

 

FPC Applications

 

Flexible circuits (FPC) are used in a multitude of applications, ranging from the lowest end consumer products to the highest end military and commercial systems. It is no coincidence that the ranges of materials used to fabricate these circuits are as diverse in performance as the range of products in which they are used.

Flexible PCB Services

 

Flexible PCBs are widely used in electronic devices as an indispensable component offering the benefits of being compact, thin, and highly pliable. 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, 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 Dual-Access Flex Circuits: 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.

 

How to make Sculptured Flex PCB?

 

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.

In traditional 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 3oz. 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:

• Eliminating the need for ZIF connectors
• Reducing assembly labor costs
• Increasing flexibility
• 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 coverlays. 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.

 

All about 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, also called bonding sheet, plays an important role in flexible circuits. They are used to provide a secure join between the substrate and the chosen conductor material (generally copper), 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.

 

• Coverlay: Coverlay, also known as Cover Lay, or Cover Layer, consists of a layer of solid polyimide or polyester sheet and the other  layer of acrylic,  modified epoxy, or polyester based flexible adhesive. The most commonly used coverlay is composited of polyimde and acrylic adhesive. Coverlay can be laminated under heat and pressurized the circuit surface as a protective film on outer layers, or as a insulator in inner layers of a multilayer flexible printed circuits. When choose coverlay, we need take coverlay thickness into consideration, and there are two main rules: (1) Thinner coverlay may be required to meet tight bend requirements; (2)Min. of 1 mil of adhesive is required per OZ of copper to ensure complete encapsulation (e.g. 1oz = 1mil coverlay adhesive).

  Coverlay Thickness
  Polyimide Thickness μm (mil)	Adhesive Thickness μm (mil)
  12.5 (0.5)	15 (0.6)
  12.5 (0.5)	25 (1.0)
  13 (0.5)	25 (1.0)
  25 (1.0)	25 (1.0)
  25 (1.0)	51 (2.0)
  51 (2.0)	25 (1.0)
  51 (2.0)	51 (2.0)
  76 (3.0)	25 (1.0)
  76 (3.0)	51 (2.0)
  typical coverlay: 25μm (1.0mil) polyimide + 25μm (1.0mil) adhesive

 

• Stiffener: Stiffeners can be Polyimide (PI), FR4, Stainless Steel and Aluminum in our flex circuit manufacturing.

1. PI Stiffeners are 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.

 

Constantan Flex PCB

 

The conductive circuit patterns of flexible circuit (FPC) are etched and built by constantan (a copper-nickel alloy that consists of 55% copper and 45% nickel). Constantan foil is a a desirable conductive material for flex PCBs designed for hostile operating environments and rugged electronics, like strain gauges, thermocouples and infrared imaging.

Some flexible PCBs need to work in extreme cold circumstances, like in 0 Kelvin (-273.15oC) and 373.15 Kelvin (100oC). You may have experience of applying flex circuits for that range of temperatures, but you are aware that copper conductors cannot get rid of the leakage of coldness and warm. For solving these problems, why not try to use constantan to manufacturing your flexible PCB?

The good electrical and thermal conductivity of copper makes it an ideal choice as conductor in printed circuit board manufacturing. But in many applications, constantan flex PCBs have proven effective for higher resistance and lower thermal conductivity. So, constantan can be far more suitable than copper to be conductor, especially when work in extreme heat or cold environments.

Constantan’s resistance remains stable across an extreme range of temperatures as well as in high pressure and other stressful situations. From the constantan property chart below, you will have a better understanding.

Similar to other printed boards, constantan flex PCB cost is also affected by layer count, size, thickness, surface finish and etc. But it is much more expensive than standard flex circuit with copper as conductors. The reasons are:

• No constantan clad laminate available in the market, need to laminate base material by PCB manufacturer.
• Constantan foil itself is cheap, but high-class thin constantan foil is expensive.
• Constantan is hard, increases processing difficulties of dry film bonding, coverlay bonding, developing and etching.
• Constantan FPC manufacturing need more extra processes. Any not good handling will cause de-lamination and failure.

We are proud of ourselves being a flex PCB manufacturer. MADPCB supports copper and constantan flexible PCB prototyping and production. Contact us for more information about PCB design, manufacturing and assembly, or to request a quote. We are able to provide quick stack-up and panelization in the offer.