Typically, a flexible printed circuit board material will bend, but not snap like an FR4 board will. However, the copper foil on the flex board will likely crack if the bend creates a crease. It is easy to confuse a flex board with a very thin FR4 board, which may be semi-flexible. Although a thin FR4 board of less than 10 mil thickness can flex, it will eventually snap, as FR4 material is brittle.

Rigid and Flex Boards Differences

There are several differences between rigid and flex boards:

Material

While rigid boards are typically made of glass-epoxy compounds like FR4, flexible circuits are made from Polyimide. This gives flexible circuits the ability to bend without breaking.

Coverlay

While rigid boards only have a solder mask for protection, flexible boards can have either a flexible mask or a coverlay. With a coverlay, the fabricator typically laser cuts the necessary openings. The coverlay has an adhesive layer of 1 to 2 mils thickness to allow it to adhere to the flex board surface.

Stiffeners

Manufacturers use stiffeners on flex boards to stiffen their non-flexing regions. The stiffeners can be either FR4, polyimide, aluminum, or steel. They either laminate the stiffener to the flex or attach it using a pressure sensitive adhesive. Rigid boards, as their name suggests, do not require stiffeners.

Permittivity

Rigid boards, being made from a variety of materials, can have a wide range of relative permittivity or dielectric constants. However, flex boards do not have a range, because they are made mostly of polyimide material with a dielectric constant of 3.4.

Types of Flex PCBs

Flex PCBs are classified into three types:

Flexible PCBs

These boards are fully flexible, but can have stiffeners for areas with ZIF sockets.

Flex PCBs with Stiffeners

When it is necessary to stiffen the flex board in certain areas, manufacturers add stiffeners. These are suitable for flex circuits that have components mounted only on one side.

Rigid-Flex PCBs

These are a combination of rigid and flex circuits. Manufacturers sandwich the flexible layers between the rigid ones, typically using no-flow prepreg. These are suitable for flex circuits with components on both sides.

Why Use Flex Circuits?

There are several advantages of using flex circuits:

Weight—Flex circuits are much lighter than FR4 boards. This is due to two factors. One, flex boards are typically thinner than FR4 boards are, and second, polyimide material, of which flex boards are made, is much lighter than the glass-epoxy material of FR4.

Thin—Flex circuits are much thinner than regular FR4 boards. For a typical 2-layer flex board, the thickness can range from 4 to 10 mils.

Flexible—As the name suggests, flex boards are meant to be able to bend without breaking. On the other hand, rigid boards will snap when bent, as they are brittle.

High Temperature—As compared to the glass-epoxy material of rigid FR4 boards, the polyimide material of flex boards can withstand much higher temperatures. In addition, the flex material is resistant to many acids, gases, and oils.

Flex PCB Assembly Process

Although both use the same surface mount technology for assembly, the process for assembly of a flex board is very different from that of a rigid FR4 board. This is due to the flexible nature of the flex board, which requires the use of a dedicated carrier board to support it and give it the necessary stiffness.

Baking

Most flex boards are relatively soft, and manufacturers usually do not vacuum pack them when dispatching the boards. Therefore, they can easily absorb moisture during transportation and storage, and require baking before assembly. Baking is necessary to slowly force the moisture out from the boards. If this is not done, the high temperature impact within the reflow oven will quickly vaporize the water vapor in the flex board, which will emerge forming defects like blisters and delamination.

Baking conditions generally require a temperature of 80-100 °C, for about 4 to 8 hours. If necessary to bake for shorter periods, the temperature may be raised to 125 °C. Prior to baking, a sample test is recommended to ascertain if the flex board can withstand the baking temperature. The manufacturer may also be consulted for suitable baking conditions.

Assembly Carrier Fixture

As stated earlier, flex boards require a carrier fixture to give them the necessary stiffness. To make the carrier fixture, it is necessary to manufacture a high-precision positioning template based on the drilling hole layer file of the flex board. It is essential that the diameter of the pin on the positioning template be the same as the diameter of the positioning hole on the carrier board, and matches with the hole diameter on the flex board.

The flex board may not be the same thickness throughout. Some part of the board may be thicker than the others by design, while other areas may have stiffeners making them thicker. The carrier board must follow the thickness difference to allow the flex board to remain flat during the printing and placement process. The material of the carrier board will have to be light and thin, of high strength, capable of low heat absorption, high heat dissipation. It must also not warp and deform after several thermal shocks. Manufacturers typically use special high-temperature materials like magnetized steel, silica-gel plate, aluminum, and synthetic stone for making the assembly carrier fixtures.

Positioning the Flex Board

Before starting the surface mount process, it is necessary to accurately fix the flex board to the carrier board. There can be two types of carrier boards, one with positioning pins, and others without. Carrier boards without positioning pins need a positioning template with positioning pins.

The carrier board must be placed on the template to expose the positioning pins through the holes on the carrier board. The flex board must now be placed aligning it on the positioning pins. After fixing the exposed positioning pins with tape, the carrier board may now be separated from the positioning template. The carrier board is now ready for further processing—printing, mounting, and soldering.

Solder Paste Printing

Most of the solder paste printing process for flex boards is the same as that for rigid boards. The solder paste composition can also remain the same, easy to print and release. It should adhere firmly to the surface of the flex board. The stencil design should allow easy release, with not blocking, no leakage or collapse after the stencil has been lifted after printing.

The squeegee or scraper for moving the solder paste on the flex board should not be made of metal. As the flex board is mounted on the carrier board, its plane is not perfectly flat, and the thickness and hardness may be inconsistent. It is recommended to use a polyurethane scraper with a hardness of 80-90 degree. This allows the scraper to adjust to the tiny gaps between the flex board and the carrier board surfaces. It may be necessary to experiment with the degree of the scraper for the best printing effect.

Surface Mount Component Assembly

The process for mounting SMCs on flex boards is not much different from that for rigid boards. However, due to partial air gaps between the flex board and the carrier board, it may be necessary to adjust the suction nozzle drop height and the blowing pressure. It may also be necessary to reduce the moving speed of the nozzle, and this may reduce the yield of the assembly line for flex boards.

Reflow Soldering Process

Unlike the reflow process for soldering rigid boards, the soldering process for flex boards requires infrared reflow ovens to use forced hot air convection. This is necessary to maintain a uniform temperature over the entire flex board. Typically, four corners of the flex board are fixed to the carrier with tape, and the center of the board can rise or fall to deform under hot air. Therefore, it may be necessary to adust the velocity of the hot air appropriately.

Thermal Profile Setting

Different heat absorption properties of the components on the flex board and the carrier board material may cause the temperature to rise at different rates during the reflow soldering process. This requires a carefull setting of the temperature curve in the reflow oven for improving the quality of soldering.

Inspection and Testing

After soldering, the assembled flex board can be visually inspected under a lighted magnifying glass for solder defects, resident glue, solder beads, no solders, shorts, and other problems.

As the surface of flex PCBs is not smooth, it is difficult to use Automated Optical Inspection methods. Also the soft nature of the boards requires special test fixtures for ICT or In-circuit testing, and FCT or function circuit testing.

Separating

Most flex boards are connected for ease of assembly, and it is necessary to separate them before testing. Although it is possible to use manual tools for separating the flex boards, the scrap rate is likely to be high. It is recommended to use stamping and splitting dies for separating the flex boards, as they improve the efficiency significantly. They also make the edges of the flex boards clean and well-cut, while protecting the inner surfaces from undue stress.

Conclusion

Rush Flex PCB recommends using strict production process management for assembling flex boards. Operators must implement all requirements of the standard operating procedures. In-process quality controllers and engineers must inspect appropriately, and analyze the causes for failures and abnormal assembly conditions. They must also take necessary action for controlling the defect rates of the production line within the specified PPM.

The post All About the Flex PCB Assembly Process appeared first on RUSH Flex PCB.

When designing flex PCBs, designers must consider many aspects. These include the board outline, its bending requirements, the stackup, placement of copper features, and most importantly, the material selection and the cost factor.

Any well-designed flex PCB will be easy to install. It will also be robust, reliable, durable, and lightweight. That makes flex PCBs eminently suitable for applications in various industries, including medical, IoT, wearables, automobiles, and aerospace. Flex boards are highly resistant to harsh environmental conditions and can endure vibrations and high temperatures much better than their regular counterparts can.

With a proper design, flex boards can be lightweight and reduce consumption of space significantly. They can also provide major savings in manufacturing costs. For that, the designer must optimize their design for the specific use case, along with selection of suitable materials. The designer must consider many factors, especially bendability, during the design.

What is Flex PCB Bendability?

One of the special features of flex PCBs is their flexibility, which enables installing them in restricted spaces. To make the PCB flex, the designer must understand bendability—the extent to which the board can flex without damage, and how many times it must flex in its lifetime.

For static flex PCBs, it is enough if the board flexes only about a hundred times in its lifetime. The design of static PCBs allows it to bend a few times during install at assembly. On the other hand, a dynamic flex PCB must flex regularly during its lifetime. Therefore, dynamic flex boards must be more robust, as they must bend tens of thousands of times.

To put it very simply, the thinner the board, the more easily it can flex. In other words, thicker boards will not bend easily. The stiffness of a flex board depends on many factors like:

• Material of the board
• Number of copper layers
• Thickness of the copper layers
• Adhesive thickness.

Two factors govern bendability. These are the bend radius, and the bend ratio. The bend radius defines the minimum radius through which the board can flex without damage to its internal or external layers. The designer must define this minimum radius early on in the design of the flex board. This will ensure that the board will have the necessary flexibility without causing damage to the copper circuits when executing the bend. The bend radius depends on the number of layers in the board, increasing with the number of layers. A smaller or tighter bend radius increases the probability of failure.

Bend ratio is a number based on the ratio of the bend radius to the board thickness. While static boards can do with a smaller bend ratio, dynamic boards require a bend ratio at least ten times more. For double layer boards, this number can be as high as 15, while bending is not recommended for multilayer flex boards.

Factors Affecting Flex PCB Bendability

Sharp bends at 90° can cause high strain on the layers of a flex board. During installation, it is recommended to use only gradual and large curves to prevent damage.

Plated through holes and components in the bend area. Being stiff parts, these affect bendability. It is recommended to avoid placing PTH and components in bend areas of a flex PCB.

Copper conductors running parallel to the bend axis can compromise the reliability of the board. It is recommended to place copper conductors only perpendicular to the bend axis.

If it is unavoidable to place copper conductors parallel to the bend axis, they must be reduced to smaller than 10 mils in width. Additionally, they must be placed within the neutral bend axis, to face the least compression or tension during bending.

Copper conductors can face high strain during bending if placed adjacent to each other in multi-layered flex boards. Designers should preferably stagger them to avoid the strain.

Factors Affecting PCB Flexibility

Several factors influence the flexibility of a flex board.

• The thickness of a flex circuit is the largest contributor to its flexibility. The thinner the board, the more it can flex.
• The thickness of the copper traces influences the flexibility. Thicker copper layers can reduce the flexibility significantly. Multilayer flex PCBs with high layer count are difficult to bend. One method designers use to reduce copper thickness is to use cross-hatching, especially on ground planes on either side of signal layers.
• If the board is designed for smaller or tighter bend radius, it will have high flexibility. However, this also increases the chances of damage during flexing. The designer must, therefore, exercise an optimum tradeoff between a small bend radius and the extent of bending of the board.
• Introduction of slots or cutouts can help minimize the bend radius, provided the flex circuit has no copper traces in the bend region. This also reduces the amount of material in the bend region. It is possible to improve flexibility substantially by judiciously removing sections of the flex where there is no circuitry.

Flex PCB Materials

Designers must be aware of and have adequate knowledge of materials they would like to use for their flex PCBs. They can find a prescription of different materials and their specifications in IPC standards 4202, 4203, and 4204. The choice of PCB materials and their characteristics depends on the application and the design criteria of the flex PCB. Several properties are important here, including:

• Resistance to fire
• Resistance to moisture absorption
• Glass Transition Temperature – Tg
• Dielectric Constant – Dk
• Coefficient of Thermal Expansion – CTE

Constituents of Flex PCBs

In comparison with regular rigid boards, designers choose materials for flex boards to achieve better material properties. There are several constituents of flex PCBs:

Copper-Clad Laminate

This is the core component in a flex board, comprising layers of copper foil and polyimide. Flex boards typically use rolled annealed copper, as it offers higher ductility, making it more suitable for dynamic applications, and allowing achievement of tighter bend radius.

There are two major types of copper-clad laminates in popular use—adhesive-based and adhesive-less. Typically, fabricators use adhesives to laminate the copper layer on the core (polyimide). However, using adhesives often causes cracks to appear in the copper layer within the hole wall of plated through holes. This is due to the acrylic adhesive becoming soft when heated. Therefore, it is necessary to incorporate teardrops and anchors in the design for adhesive-based materials. Moreover, adhesive-based materials absorb moisture from the environment. Therefore, it is not advisable to use adhesive-based copper clad laminates in systems that will be exposed to the outside environment.

The introduction of adhesive-less copper-clad laminates has helped to avoid the above problems of adhesive squeeze-out, cracks, and dimensional errors.

Conductors

Conductors in a flex board are always made of copper, as it has high electrical conductivity. Typically, the copper is very thin, 0.5 oz, making it suitable for both static and dynamic applications. Depending on their current carrying requirement, flex boards may also use copper weights of 1 and 2 oz.

Bondply

These are composites made of polyimide films coated on both sides with an acrylic adhesive. Mostly used for encapsulating heavy copper in multi-layered flex boards, fabricators use bondply between two adjacent conductive layers of copper-clad laminates.

Adhesives

Manufacturers use various types of adhesives while fabricating flex boards. These include PSA or pressure sensitive adhesives, epoxy adhesives, and acrylic adhesives. Among them, PSA are the most popular, being flexible and offering superior bond strength. Another advantage is they adhere to the surfaces of substrates directly. Other adhesives, like epoxy-based and acrylic are available as flexible tapes, and are mostly thermosetting types.

The flexible tapes become tacky on application of heat and pressure, allowing securing of components in their places. To finalize the bond, it is necessary to cure it by applying additional heat and pressure.

For flex boards meant for dynamic applications, acrylic adhesives are more suitable, as they stay malleable even after curing. On the other hand, epoxy adhesive is not suitable for dynamic applications, as it becomes hard after curing. Therefore, when the PCB is a combination of both rigid and flex regions, the manufacturer uses epoxy adhesives for the rigid regions, and acrylic adhesives for the flex regions.

Stiffeners

Adding localized rigid materials in specific areas can help stiffen flex boards, if the application so demands. This is applicable for all types of flex boards, whether single-, double-, or multi-layered. Stiffeners can add mechanical support necessary when mounting components, increasing the strength, rigidity, and thickness of the flex part. Stiffeners may be made of stainless steel or aluminum, and fabricators use either pressure-sensitive adhesives or thermally cured acrylic adhesives to attach them. Stiffeners typically offer strain relief, heat dissipation, and weight balancing. In addition, they increase the abrasion resistance of the flex board, while helping to reinforce solder joints.

Surface Finish

Manufacturers prevent copper oxidation while providing a solderable surface on the flex PCB by applying a surface finish. There are several types of surface finishes to choose from, depending on the application. Fabricators protect the non-solderable parts of the flex board by using a coverlay, a liquid photo-imaged polymer, a photo-imaged dry film, or a covercoat.

Importance of Flex PCB Thickness

The application defines the constituents of the flex board, and ultimately, its thickness. For instance, the number of layers of circuitry on the board determines its thickness, the choice of materials determines its toughness and its capabilities in withstanding environmental stresses like vibration and high temperature. Most electronic equipment use multiple PCBs, which makes the decision of PCB thickness a design consideration. Manufacturers offer flex boards in standard thicknesses, of which 1.6 mm is a popular choice.

Factors Affecting Flex PCB Thickness

Layer Count

One of the major factors governing the thickness is the number of layers in the flex PCB. Moreover, the thickness of each layer is material-dependent, and based on whether the PCB is meant for dynamic or static flexing. While it is for the designer to decide the thickness of each layer, it is understandable that thicker layers will reduce flexibility, while allowing higher packaging density on a single PCB.

Copper Weight

The copper weight on each layer also significantly affects the ultimate board thickness. Copper weight is typically measured in ounce per square foot, and typically this is 1 oz/ft2 for flex boards. This copper weight allows for nine metal layers to be present on a board. While it is possible to get more metal layers by using thinner or different material, increasing the layer count can increase the board stiffness.

The thickness of the copper layer influences the appearance, size, cost, and flexibility of the board. However, as copper is essential to the board’s functioning, its thickness is essential.

Trace Width

Trace width influences the overall size of a flex board. Wider traces add to the PCB area necessary for components and shielding. For applications requiring very few traces on the flex part, traces of lower width may be an option for reducing the overall thickness of the board. Although 10-mil trace widths are typical for general purpose applications, designers often use 5-mil taces.

Conclusion

The size of the board determines its thickness and flexibility. Although flexibility is necessary for an application, it costs money. Therefore, Rush Flex PCB recommends experimenting with prototype flex boards of different thicknesses to determine the one performing best for the specific application.

The post Expert Guide to Flex PCB Thickness appeared first on RUSH Flex PCB.

• Compatibility for thickness
• Proper pin pitch to match the width and pitch of the flex PCB
• Proper selection for the type of connector to suit the PCB type
• Proper selection for insertion into via, mechanical crimp, or ZIF socket
• Provide proper surface treatment for matching with connected sections.

Here, we are offering a comprehensive guide for selection and use of flex PCB connectors in the industry.

Expert Guide to Flex PCB Connectors

For transmitting or exchanging data and control signals, most PCBs need to connect to other PCBs or devices. Connectors facilitate these interconnections. The industry often uses flex PCBs to interconnect to rigid PCBs, thereby avoiding connectors. However, there are many interconnections that need the presence of a connector. Among these, flex PCB connectors are a subgroup of a larger group of PCB connectors that ease the interconnection process in the industry.

With electronic devices trending towards lower profiles, lighter connectivity, and smaller form factors, connector manufacturers have also taken up the challenge to make matching connectors, especially flex PCB connectors.

As a result, we have flex PCB connectors capable of fitting anywhere necessary in the system. Let us look at some types of flex PCB connectors available.

What Are Flex PCB Connectors

Flex PCBs use flex PCB connectors to provide complicated interconnections. Most flex PCB connectors are capable of handling clock intervals of about one second. They typically offer heights of less than one inch, while providing centerline and lightweight connections.

Catering to industry standards, flex PCB connectors focus primarily on compactness. In a majority of cases, installing FPCs require no tools. Users can choose from different pitches of these FPCs, starting from 0.25 mm, to 0.3 mm, 0.5 mm, 1.0 mm, and 1.25 mm.

It is easy to distinguish FPCs, as most have a low-profile design and miniscule dimensions. However, FPCs provide reliable interconnections, usually employing an actuator for clamping the cable terminations, thereby providing a secure connection.

Uses of Flex PCB Connectors

An FPC is not just another connector, but it is a PCB as well. Most FPCs are ultra-thin and lightweight. Although they primarily use flexible polymer film, they can be made from other flexible materials also. Their primary characteristic is flexibility, which makes them fit for innumerable applications.

In the electronic industry, FPCs are replacing not only typical rigid PCBs, but also conventional flex PCBs. Many applications today use FPCs to connect flexible PCBs with a flex PCB edge connector. This arrangement offers a suitable choice for requirements of lightweight features and enhanced flexibility.

For instance, the automobile industry is a very big user of FPCs. They use it in automobiles for providing lighting, autopilot facilities, navigation, entertainment, and for vehicular safety.

Another industry using FPCs in a big way is the wearables sector. Primarily in the medical equipment sector, applications include diagnostic equipment, hearing aids, internal organ monitoring, health monitoring, and many more.

The development of modern communication technologies has also increased the use of FPCs. Next generation electronics like 5G applications prefer using FPCs. The impressive range of flex PCB connectors make them eminently suitable for applications in sectors like automotive, automation, medical, commercial, and industrial.

Many other electronic devices use FPCs. Among them are cellphones, GPS tracking devices, hard drives, set-top boxes, desktop computers, Televisions, tablet devices, cameras, gaming consoles, Liquid crystal displays, and many more.

Being rugged and tough, FPCs can work undeterred in rough and challenging environments, along with extreme vibration and temperatures.

Special Flex PCB Connectors

An FFC or flexible flat cable is a connector that resembles a ribbon cable. The name flat cable is due to the wide and flat structure of the ribbon cable.

FFCs typically provide straight connections not needing any external parts. FFCs mostly use flexible plastic and polymer films for their construction. The end of the FFC typically has a metallic connector. The wire plane attatches to an integrated circuit, making the arrangement a flexible printed circuit board. FFCs are available in five pitch sizes of 0.5 mm, 1.0 mm, 1.25 mm, 2.49 mm, and 2.54 mm.

Their compact design gives the FFCs a relatively small footprint overall. Because of their thin and compact form factor, FFCs typically occupy less space compared to what traditional straight cables do. Additionally, FFCs are more adaptable, and improve the mitigation of electromagnetic interference.

The industry uses FFCs as the most popular cable connection solutions for many applications. As FFCs offer very high density along with a flexible structure, the industry uses them for many sophisticated applications, such as in laser printers, 3D printers, test equipment, and industrial displays. They are also applicable in membrane switches, medical equipment, household appliances, data systems, hard drives, and many more.

Various Types of Flex PCB Connectors

The industry uses various types of FPCs. Common types are:

Single-Layer FPC

The single-layer FPC has a single conducting layer. Depending on the application, these FPC may be made of plastic film or rust polymer. Single-layer FPCs are the cheapest connectors in the market.

Two-Layer FPC

The two-layer FPC has two conductive layers. In specific cases, the conductive layers may include driver ICs.

Multi-Layer FPC

As the name suggests, multi-layer FPCs have at least three conductive layers. Multi-layer FPC connectors are expensive and may include driver ICs in some of the conductive layers.

Important Parts of Flex PCB Connectors

Although one of the thinnest parts in the PCB industry, the construction of FPCs requires some important parts. Primarily, FPCs have three parts:

Body

Almost all FPC connectors have a plastic body. This is adequately sturdy for withstanding harsh conditions that the connector might face. The body provides the connections with the necessary mechanical stability.

Terminals

Terminals form the significant part providing the necessary electrical connections. Each contact has two functions—one to make contact with the circuit on the PCB, and the other to connect to the external circuit. For both functions, it is necessary for the terminals to be mechanically and electrically compatible.

Locking Arrangement

The locking arrangement mechanically locks the external circuit to the connector once it is firmly pushed in. The locking arrangement is strong enough to withstand repeated use. Typically, it is made of polyphenylene sulphide for providing the necessary stiffness.

Connecting Flex PCB Connectors

With almost all electronic equipment becoming smaller and thinner, it is necessary that the connectors that they use are not only simple to assemble, but also are space-saving.

Such electronic equipment, like cameras, tablets, computers, and smartphones typically use FPC connectors with a 0.2 mm pitch. Not only do these FPCs offer small pitch, but they are also of very small height.

Ear tabs on the connector housing of the FPC help in the alignment during cable insertion. The terminal casing is typically white, which makes it easier to locate and position the cable.

Some FPCs also provide Zero Insertion Force. They have terminal contact gaps that are larger than the cable thickness. The actuator opens and rotates to about 130 degrees, allowing for easy cable insertion. Once the cable is properly in place, the actuator can be rotated back and locked into position. Internally, the terminals move to make proper contact with the cable, before it is locked into place.

Features of Flex PCB Connectors

Manufacturers of FPCs offer several features with different types. Primarily, these are:

• Flex PCB connectors use flexible FPC and FFC cables
• Both ZIF or Zero Insertion Force and Non-ZIF or Non-Zero Insertion Force connectors are available
• It is possible to have top, bottom, and dual contact versions of FPCs
• Installing FPCs does not require any tools
• FPCs are very thin and lightweight
• FPCs are specially designed for low profiles
• 25-pitch series are popular in FPCs

Why Use Flex PCB Connectors

FPCs have brought about a revolution in the electronic industry. This is due to their compact size, low profile, and sturdy construction. The major benefit of these connectors is the high density of interconnectivity they can provide with their simple construction. Other benefits of FPCs are:

• Flexibility: This is a major advantage, especially when the connectors are used with flex PCBs. The use of flexible materials for making these connectors also make them more reliable. Being foldable and bendable, FPCs provide a secure connection between the FPC edge connector and the PCB header.
• Lightweight: FPCs are designed to be lightweight. Being small in dimension, the FPC connector saves a lot of space, especially in complicated and tightly fitting assemblies.
• Long Life: FPCs typically have a long service life. On a modest scale, they can survive at leat 200 thousand insertions.
• Reliability: Because they are made of materials that can bend and flex easily, FPCs can withstand higher levels of vibration as compared to what other types of connectors can.
• High Speed: FPC connectors are available for interconnecting high-speed and high-frequency circuits.
• SMD Compatibility: FPCs are compatible with SMD technology.
• Good Thermal Properties: The materials used for making FPCs are highly temperature resistant—they can withstand high temperatures.
• Corrosion Resistant: FPCs are made from materials that can not only resist corrosion, but is also water-resistant, resistant to moisture, and shock-resistant. This allows FPCs to be highly functional in hostile environments.
• Cost-Effective: The small size and high functionality of FPCs makes them very cost-effective.

Different Connector Styles for Flex PCB Connectors

Zero Insertion Force Types

These are IC socket type of connector that can be easily connected as they require very little insertion force. Although more expensive compared to standard connectors, ZIF connectors use an actuator that opens to receive the flex cable and then locks it into place.

Eminently suitable for applications subject to high vibrations, ZIF connectors reduce the stress and wear on contacts, thereby improving their working life.

Non-Zero Insertion Force Types

This type of connector makes use of frictional force to ensure a proper connection of the flex cable. However, this reduces the mating cycle, and is suitable for static applications. The non-ZIF connector is smaller and lighter than its ZIF connector counterpart, and also less expensive.

Conclusion

The flex PCB connector has been a boon to the electronic industry, allowing OEMs to manufacture devices with thinner and smaller form factors. Rush Flex PCB recommends using FPCs for all upcoming future electronic projects. The high quality of manufacturing that goes into manufacturing flex PCB connectors make them worth the investment.

The post An Expert Guide to Flex PCB Connectors appeared first on RUSH Flex PCB.

For combining the best of both worlds—the standard rigid printed circuits with flexible circuits on a single board—Rush Flex PCB offers their rigid-flex PCB design. Typically, these boards primarily consist of flexible polyimide strips connecting two or more rigid areas with copper cladding substrates. Through-holes on the rigid areas of the board connect their circuit to those on the flexible polyimide board.

The complexity of the rigid-flex boards requires a lot of attention while designing them. By using our design methodologies and tips it is possible to fabricate not only highly reliable rigid-flex boards, but which are also capable of withstanding several hundreds of flex cycles without failure.

Reasons for Flex PCB Use

When it is necessary to interconnect multiple subcircuits, one of the alternatives is the use of connectors, especially for effective transport of power and signal through individual subcircuits. However, connectors add cost and also provide additional opportunities for failure. Moreover, connectors being physically large, require more space in the system.

This is where the rigid-flex circuit technology comes in. Rigid-flex boards use built-in interconnects in the form of flexible circuits. There are several advantages to this design:

Physically smaller size — Absence of connectors makes the entire circuit light and small, taking up substantially smaller space.

Higher reliability — Without connectors, it is much easier to assemble rigid-flex circuits, as flexible circuits already interconnect the different subcircuits right after the manufacturing of the board. In high-vibration environments, where connectors are more likely to fail, the rigid-flex circuit structure is more reliable, as it can absorb vibrations in a better way.

Space-Saving — As the flex circuit can bend and fold into a smaller profile, it offers significant space-saving opportunities for using a small enclosure.

The rigid-flex PCB design has some disadvantages also that designers need to look into while verifying whether this design-type really suits the application. As the production process of rigid-flex circuits is significantly more complex compared to regular rigid boards, the production yields are likely to be lower. Fabrication costs for rigid-flex boards are also higher on account of the manufacturing cycle being longer.

Basics of Rigid-Flex PCB Design

Once the designer has decided the suitability of rigid-flex circuit for their application, they must consider:

• The intended environment for the board
• Whether the board is meant for a dynamic bend or a stable bend
• The bend radius
• Placement of vias and traces
• Design of power and ground planes

Operating Environment

It is necessary to know the intended environment in which the board will operate. The rigid-flex design typically operates within an enclosure. The designer must consider the mechanical stresses and forces that may affect the board. They must ensure the design of the board within the enclosure must be capable of dealing with those forces and stresses.

Type of Bend

If the board is meant for a stable bend, the operator will bend it while installation, and it is meant retain this shape without change. Designers make sure the bend radius is about ten times the thickness of the flexible substrate.

However, when the board is meant for a dynamic bend, it will be constantly subjected to bending and unbending stresses throughout its working life. Designers typically use only one or two layers for flex boards meant for dynamic bending operations. They ensure that the bending radius is at least a hundred times that of the thickness of the flexible material.

Placement of Holes and Vias

The area of the flexible circuit that undergoes bending is its most delicate part. Designers typically avoid placing holes, pads, and vias in the bend areas. Areas close to the bending lines can stress the structure of a plated hole close to it. Designers prefer to keep all pads and vias in areas that are not subject to bending, that is, on the rigid, hard section of the board.

However, sometimes it is inevitable that holes and vias appear on the flexible section. In such situations, designers prefer to use anchors for strengthening them, while adding teardrops to connect them to traces. In such cases, the rule of thumb is to use 10 mil holes with at least a 10 mil annular ring around them. The larger size anchors the pads, thereby preventing any peeling during the flexing. Designers typically place holes and vias at least 15 mil away from the edge of the stackup.

Placement of Traces

In regard to traces and routing, designers prefer to position the traces as perpendicular, straight lines. For boards that bend or fold along a horizontal line, designers prefer to place the traces vertically. Ideally, all traces should go in one direction, but in case a change of direction is necessary, designers prefer to curve the traces rather than use sharp corners of 45 or 90 degrees. They do this to eliminate high-stresses on the trances. Another way designers decrease areas of high stress is by using narrow traces spread out across the flexible area.

It is a standard practice to add in dummy traces and redundant traces to increase the mechanical sturdiness of the flexible areas. For boards subject to dynamic bending, adding dummy/redundant traces prevents traces from breaking the signal path totally.

In case a flex circuit has traces on both, the top as well as the bottom sides of the flexible section, it is customary for the designer to alternate them such that no bottom-layer trace has any top-layer trace immediately above it, and no top layer trace has a bottom-layer trace immediately below it.

Design of Power and Ground Planes

In conventional PCB design, it is customary to use a continuous solid area of copper pour for power and ground planes. However, doing the same for flex PCB design will add a significantly large amount of stress on the board, while reducing its flexibility. Rather than use solid areas of copper, for flex circuits, designers prefer to use hatched-polygon patterns for planes. To gain maximum flexibility, they minimize the trace widths on the fill. However, cross-hatch patterns are not suitable for high-speed signal integrity.

High-Speed Design

Because if layer stackup, impedance control can be somewhat of a challenge on rigid-flex PCB designs. Designers typically use a co-planar stripline construction on single-layer flex circuits, where the signal layer alternates with ground strips. However, this is susceptible to EMI.

Better results are possible with two layer flex stackups. Here, designers can use a microstrip structure that is suitable for 50 ohm circuits. Although higher layer counts offer the ability to create a standard construction like stripline, the extra layers will tend to reduce the flexibility of the circuit.

Design Standards for Rigid-Flex PCBs

Among the numerous standards available for rigid-flex board design, IPC 2223 specifies the coverlay construction, air gaps, adhesive flex cores, strain relief filets, and the pre-bake requirements. It also has numerous tips for locating plated vias or holes near the transition zone where the rigid part changes over to the flexible part.

The transition zone is especially unique, as the polyimide coverlay encapsulating the flexible area overlaps the rigid areas by a small distance. During the lamination process, this ensures the rigid area encapsulates the flexible area through the adhesive. The coverlays attach to the flex surface using acrylic or epoxy adhesives. If vias are present in this area, the expansion, or contraction of the adhesive may subject them to stress, as the adhesives typically have a high thermal coefficient of expansion during reflow.

Board Design and Stackup

For proper flex behavior, it is necessary for the designer to communicate with board manufacturers for deciding on usage of materials. This is necessary as several factors can affect the board’s ability to bend. Among these are:

• Mask types
• Dielectrics
• Stiffeners
• Thickness of copper layers

In addition, it is necessary to pay attention to other factors also. These are:

• Flammability rating
• Mechanical requirements
• Impedance control

While deciding the stackup design, it is customary to split it into three separate zones:

• Primary stackup or Zone 1 — for the rigid area of the board
• Zone 2 — for the flexible area of the board
• Zone 3 — also for the flexible area, but with stiffener applied, thus limiting its bending ability

In general, designers tend to sandwich the flex layer in between the rigid board. This reduces the stress on the system, as against placing the polyimide board in the top or the bottom layer, which can result in greater stress and adhesive peel off.

The outline of the board should not have sharp corners, as this can result in tearing. Designers typically use the filet tool in their PCB design software to round off any sharp corners.

Image Files for Flex Circuits

While getting the design to the manufacturer, it is necessary to use image files just as in regular PCBs. However, the organization and content of these files can be more complex in the case of flex circuits. Designers prefer the use of more intelligent data formats such as IPC 2581, rather than traditional fab and assembly files.

Conclusion

Most 2½-D design tools are adequate for designing conventional rigid boards, and they are suitable for flex circuits as well. However, as flex circuits have a unique bending nature, Rush Flex PCB recommends use of tools that can operate in 3D for visual verification and automated checking. Moreover, it is also necessary to configure flex design tools to meet the unique requirements of the above guidelines with respect to flex specific features and functionality.

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