all about flexible pcb

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A contemporary and the increasingly sophisticated electronic world demands a high-performing and flexible PCB. Electronic designers have to understand the nuances and intricacies of flexible PCB, including design consideration, fabrication, and application areas. It becomes essential in addressing the typical electronic challenges and needs. So what does it all entail?

About Flexible PCB

flexible pcb

A flex circuit, also inferred as a flexible PCB, refers to a component arrangement and patterned printed circuitry using flex-based material. It comes thin and lightweight to conform to contoured shapes ad small spaces. While flexible PCBs may possess or lack flexible coverlays, they often get fabricated using similar components as those of rigid PCBs. But in such instances, the only caveat is that they need to conform to the anticipated flex shape when deployed.

The flexible PCB technology is continuously growing. However, it has revolutionized the legacy approach of electrical interconnections. In the past, circuit connections entailed the end-to-end connection of components within the same circuit or connections of numerous electronic devices. Currently, the flexible PCB model allows for a considerable reduction of weight, space, and cost than the rigid counterpart. It accomplishes all these while ensuring compactness and a high density of connections get realized.

A vital attribute of the flexible printed circuit entails the flexible thin conductive layers and films. It replaces the old-fashioned wires and permits electronic components to get attached directly via conductive adhesive or soldering. As a result, the need to wire by hand reduces significantly, limiting the total electrical wiring cost by close to 70%.

Therefore, understanding a flex PCB and what it entails makes it possible to come up with appropriate designs for your application. Applications areas can span consumer electronics, medical, automotive, aerospace, transport sector, military, and communication.

 

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Flexible PCB Structure

The structure of a flexible circuit board is always instrumental for engineers when designing flexible PCBs. Like rigid printed circuit boards, we can classify the flexible PCB as multilayer, double layer, or single-layer circuits. While the number of layers can distinguish one flexible PCB from the other, the core structure mostly always remains the same save for the additional layers.

A single layer flex PCB contains the dielectric film as its base material. The material is primarily composed of polyimide that comes highly resistant to temperature and traction. Another critical element of the structure entails the electrical conductors consisting of copper. It represents the circuit traces and is instrumental in signal transmission. Protective finish is also a vital element of the flex PCB structure and comprises of a coverlay or coat. The adhesive material concludes the last aspect of the flex PCB structure. It gets consisting of epoxy resin or polyethylene.

The copper gets etched to get traces, while the protective coating gets drilled to permit the solder pads. Such components joined together with the dielectric substrate from the single-layer flex PCB. However, as the complexity and components increase, the need for more layers becomes apparent. Here, based on the number of components and complexity of the circuits, a designer either picks a double-layered flex PCB or a multilayer flex PCB.

One structural difference between a single layer and multilayer flex PCBs involves the PTH (plated through-hole). The PTH connects the different conductive layers of the flexible circuit board. Additionally, the adhesive material joins the dielectric substrate with the conductive tracks.

 

Types of Flexible Printed Circuit Boards

Different types of flexible PCB systems

Four types of flex PCBs exist, including single layer FPC, double layer FPC, multiple layer FPC, and rigid-flex PCB.

Single-layer FPC PCB. It only contains a single conductive layer, and this sits on the flexible dielectric layer or film. Additionally, the electrical parts only lay on one of the PCB board’s sides.

Double layer FPC PCB. It provides a conductive layer on either side of the printed circuit board. As a result, you can connect different electrical components on both sides of the dielectric copper or conductive layer.

Multiple layer FPC PCB. It contains three or three plus conductive layers, with the presence of the dielectric material to separate them. An irregular lamination increases and ensures higher board flexibility. However, the lamination thickness often proves less in the bonding spots.

Rigid-flex printed circuit board. It combines both flexible and rigid circuit board aspects and provides a better component density than other PCB types.

HDI FPC PCB. The HDI is an acronym for High-Density Interconnect and represents a reliable, efficient circuit with an enhanced design, layout, and construction. It provides a reduced package size besides offering a better EP or electrical performance.

 

Benefits of Flexible PCB

The extra flexibility that an FPC has infers to the ability to get flexed and bent. It allows for greater freedom, especially in the operation or design of the flexible PCB. For instance, to make the FPC adapted into cramped and small spaces or amorphously shaped spaces.

Flexible PCBs tend to occupy less space and thus reduces the overall weight in the device’s motherboard. It also becomes a beneficial aspect of improved thermal management as it reduces the heat getting dissipated.

FPCs are more reliable besides proving their longevity when compared to their rigid equivalents. It is especially true in applications that subject them to constant mechanical and vibration stress.  It arises from the attributes accompanying flex circuits such as ow thickness and weight, high mechanical and temperature resistance, etc.

Flexible PCBs also limit the occurrence of human errors that result from manual wiring. As a result, it enhances the quality and reduces the costs of the PCB and electronic product. It is also a significant contributor to reduced weight and size in electronic applications and thus leads to compact and reliable, highly integrated electronic devices.

 

Drawbacks

While the benefits of flexible FPCs are aplenty, the flexible PCB technology features some drawbacks. Firstly, FPCs have an expensive original cost compared to rigid PCBs. It results from the highly distinctive design for specific applications, which raises the prototyping costs. Therefore, you could do well by opting for an FPC PCB when indulging in large volumes of circuit boards.

FPC PCBs are also difficult to repair or modify when it requires reworking. In most instances, you have to remove the protective wrap around the circuit and fix it before restoring the protection.

The flexible FPC is also relatively new as a technology, and not every manufacturer has the capacity and equipment to fabricate it. Additionally, the assembly process requires much care to reduce the risk of damaging the flex circuit board.

FPC printed circuit boards also need proper storage. It is preferable to use sulfur-free plastic.

It is also sensitive to any scratching and has a low-temperature capability.

 

The Manufacturing Process of Flexible PCB

Flexible PCB bending

The fabrication of a flexible PCB takes a slightly different approach from that of standard rigid printed circuit boards. Polyimide (PI) as the base substrate material heavily features in the fabrication process. It is more expensive when compared to FR-4 material and thus calls for care when handling. An essential technique used to manage the PI appropriately entails the deployment of the nesting approach. It keeps the circuit boards close to one another.

Steps

Looping. It is vital to ensure that the service loop services the circuit assembly and length. It represents the extra material added beyond the limit set by the circuit designer.

Sizing conductor. A thin copper layer gets deployed as the board’s conductive material. It is responsible for the flexibility of the circuit, which makes it ideal for dynamic applications.

Etching. It compensates for isotropic losses in the manufacturing process. The thickness of the copper foil has to prove half of the line width loss. Some factors that influence line width entail etch mask, conductor, and the diverse conductor types used.

Routing. It is a simple process and reduces stress besides enhancing the folding and bending of the FPC.

Ground planes. It is instrumental in reducing the board’s weight and results in better flexibility of the circuit board. The process involves creating holes and applying filleting to expand the pad, which divides stress. An adhesive-backed film gets incorporated after that, especially for dynamic application areas of the circuit. After this, the application of the screen-printable overcoat (liquid) gets incorporated before adding the film polymers and the photo imaginable liquid.  It functions by preventing the circuit from damages.

 

Flex circuit Materials  

The unique nature of a flexible printed circuit board requires a specific set of materials during its fabrication process. Some of the typical flexible PCB material used in the manufacturing process of flex circuits include the following.

Conductors. It is instrumental in allowing the flow of electrical current to proceed freely. Copper features predominantly as the flex circuit conductor. However, other materials such as carbon, aluminum, silver ink, etc., also apply for some circuits.

Adhesives. It is crucial for binding surface mount components. Adhesives get used based on the thickness of the conductor besides customer needs. Some of the typical adhesives used include acrylic, epoxy, or pressure-sensitive adhesives. However, it is instrumental to note that not all flex circuits use adhesives. Some come built adhesive-less.

Insulators. An insulator is instrumental in separating electrical conductors in a circuit without carrying the current. It is also ideal for providing support. Examples include solder masks, polyethylene terephthalate, polyester, and polyimide.

Finishes. A finish protects the metal surface from exposure to air and possible oxidation. However, the ultimate finish always depends on the circuit’s application. Some of the renowned and standard finishes comprise tin, solder, gold or nickel immersion, carbon, silver, etc.

 

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Design Factors to Consider for Flexible PCB

The design process of flex PCBs goes beyond understanding the flexible circuit board materials. The approach also has to differ from that of a rigid printed circuit board slightly. It is essential to go beyond the significant flex benefits of PCBs like manufacturing cost reduction, reduced consumption of space, and weight. It entails optimizing the design for flexible PCB material besides the use cases.

A perfectly designed flexible PCB should prove durable, lightweight, simple to install, and ideal for diverse and demanding applications like satellites and wearable devices. But what should you consider to design a top-rate flex PCB?

 

Bendability or Flexibility of the Flexible PCB

Two key aspects of flexibility in circuit boards include the number of times the circuit board needs to flex and the extent of flexing extent.

Firstly, the number of times it can bend determines if the board will become dynamic or static. A static board can get considered bend-to-install. It flexes below a hundred times within its lifetime. On the other hand, a dynamic board allows for more bendability and flex more than ten thousand times. It is ideal for application in harsh conditions like military and spacecraft sectors.

The flexing extent gets influenced by the bend radius. It infers to the minimum amount of bend for the flex area. Identify it early and adequately during the design process. The bend radius in your design allows for the required number of bends without any copper damage. It often depends on the flex PCB layers. Other considerations should encompass avoiding 90-degree bends, avoiding having plated through-holes within the bend area, and staggering conductors in multilayer circuits.

Smaller conductors (smaller than ten mills) need placing in the neutral bend axis. However, for your flex PCB fabrication needs, you can rely on RayMing PCB and Assembly. You will get expert advice, among other crucial flex circuit services.

 

Heat-forming Flex Circuit Boards

It needs a steel jig because it forces a board to orient in a specific way. The steel jig then gets inserted into an oven. A key benefit derived from this heat-forming exercise entails getting a tighter bend radius. It is, however, prudent to remember that heat-forming primarily gets deployed to ease the installation process and that the tighter bend radius accrues as a surplus capability.

 

Slots and Cut-outs within the Bend Area

It is possible to minimize the bend radius whenever traces lack within the bend area. We can achieve such through the insertion of slots or cut-outs. The deployment of cut-outs reduces the number of materials needed to bend. An alternative entails removing the elimination of the flex sections that have no circuitry. However, you can carry out the exercise lengthwise and needs routing post-completion.

 

PCB Materials

Materials play a crucial role in the design and fabrication process of the flexible PCB. Here, PI or polyimide gets used for the coverlay and core layers. It provides superior material properties compared to rigid circuit boards. The flex material thickness always comes uniform, complete with a superior dielectric constant of 3.2-3.4. Further, the absence of the woven glass reinforcement reduces Dk variations.

Polyimide cores in flex PCBs get cladded with rolled annealed or electro-deposited copper. It is thin and ideal for static and dynamic applications. A standard copper cladding weight entails 0.5 ounces and one ounce, while the maximum weight is 2 ounces. It provides an incredible blend of the thinnest construction possible. It can either be adhesive-less or adhesive-based copper cladding.

Adhesives can cause cracks, primarily when used for lamination on the copper lying on the PI-based core. It is thus crucial to incorporate teardrops and anchors into the design. Some of the drawbacks of using adhesive materials include the emergence of cracks, especially in via holes of the copper plating. It also enhances the thickness of the copper-clad laminate, besides proving prone to the absorption of moisture. Lastly, the thickness of the core of the adhesive material gets reduced post the manufacturing process.

However, such drawbacks get addressed by the adhesive-less fabrication. It increases flexibility, reduces flex thickness, and enhances impedance control. Additionally, it leads to a better temperature rating and makes the flex ideal for harsh environmental applications.

Finally, flexible PCB board needs a looser tolerance on the outline compared to other boards. It arises because the materials used have lesser dimensional stability. Based on the tolerance profile, a laser cutter or hard tool may also become necessary for cutting purposes, and this may prove too expensive.

 

Flex Trace Routing

It is always vital to watch out for the routing of the flex trace because the circuitry layout can make or break a printed circuit board. For instance, the bend radius needs to be sufficiently large to shorten the board’s lifespan. Additionally, avoiding the I-beaming becomes vital in minimizing the stress that thin out the copper circuits. It is important to note that curved traces leads to lower stress compared to angled traces. Further, traces need a perpendicular orientation to the general bend. However, when placed on a flexible PCB containing over two layers, staggering on the bottom and top becomes crucial.

 

Basic Testing and IPC Guidelines for Flex PCBs

The quality of FPCs is integral in ensuring seamless application in diverse and applicable areas. Therefore, it is instrumental for a designer to ensure the flex PCB manufacturer they choose can guarantee the required quality standards required by the industry. It represents a key consideration for picking an FPC PCB manufacturer.

FPC PCB manufacturers need to test the flex PCB to ensure it possesses the required integrity and quality for application. Plenty of approaches exist for the quality verification process for both the finished FPC product and the raw materials.

Some of the IPC guidelines from ACEI (Association Connecting Electronics Industries) to consider for your FPC PCB include the following.

IPC-6013

It initially got published in 2013 and offer the performance and qualification specification for FPC PCBs. It is a standard that supersedes numerous earlier IPC standards, including that of 1998. The IPC-6013 of 2013 specifies numerous diverse test methods, including bend, impedance, and thermal testing. It also encompasses quality assurance elements like sample test guidelines and coupons for quality conformance and acceptance tests.

IPC-2223

It applies as a sectional design quality standard for FPCs. IPC-2223C offers guidance on choosing rigid-flex interfaces and adhesive materials. It also gives guidelines on the flex vias and plated-through holes.

IPC-FC-234

It is instrumental in providing information on the use of pressure-sensitive adhesives in the assembly process of FPC PCBs. The guide gives valuable information on the types of adhesives available besides their suggested corresponding application, weakness, strengths, and limitations. It also contains highlights of how to begin their implementation and sources of extra information about them.

IPC-2221

It is a quality standard that establishes the typical PCB interconnecting and component mounting structures. IPC-2221 also offers design standard coupons for testing quality conformance. It is a generic PCB design standard.

IPC-2223

It is a sectional design standard applicable for rigid-flex and flexible PCBs.  The quality standard gets utilized in combination with the IPC-2221 standard. IPC-2223 establishes the specification of the flex printed circuit board design. It also offers information on the interconnecting component mounting structures.

 

The IPC Testing Approach for Flexible PCBs

Testing of circuit boards often represents an environment and application-specific process. An excellent example includes the space and aerospace applications need a rigorous and formidable testing process compared to consumer-oriented applications.

The IPC guidelines always prove an excellent foundation for designing reliable, flexible circuit boards. It is crucial to start with IP-FC-234 and IPC-223 when testing flexible PCBs. These testing standards offer an excellent framework for designing a diverse range of flexible PCBs beginning from the single metal flexible PCB to the advanced multilayer boards like 6-layer flexible printed circuit boards.

 

Applications of Flexible PCBs

The application of FPCs can often get confused with the application areas of the FPCs. However, flex circuits applications can only get classified into two: static application and dynamic flex application. Static application entails a situation where the flexible PCB gets installed as a tight fit for the specific application. On the other hand, dynamic flex application entails a case or application where the flexible PCB gets bent numerous times as the application of the flex PCB.

Consequently, application areas can range from consumer electronics like calculators, printers, cell phones, LCDs, cameras, and televisions to the medical field. The medical sector includes heart monitors, hearing aids, pacemakers, etc.

 

Final Thoughts

A flexible PCB has a lot to offer in diverse sectors, especially when designed and fabricated well. But to ensure a proper design and fabrication of the FPC, comprehension of the different design elements of flex PCBs, their distinct structure, materials, and manufacturers become essential. Hopefully, the article has plugged in some of the information holes or queries you had about Flex PCBs.