Flexible Printed Circuit Boards or also called ( FPCB ) are initially designed to replace the original wire connections in an innovative way. The difference is in the end design, fabrication, and functionality. The unique set of rules is of core importance in flexible circuits. Designed from flexible material it doesn’t affect anything in the workflow and it is efficient enough. The FPCB contains a very thin layer of copper that is usually appearing metallic on the first look.
In 1950 with this innovation in design FPCB the way we built electronics. Now flexible circuits are used in many industries such as automotive, cell phones, laptops, LCD televisions, and antennas. There are many different kinds of flexible circuits such as one metal layer, multilayer, double-sided and rigid-flex circuits. To illustrate further, some of them do not have the components attached. But even when there are, the components are considered to be from a flexible family. Flexible printed circuit ( FPC) is always referring to the same flexible design and photolithographic technology is at the core of this innovation.
The rigid circuit boards are the common serviceability applied to projects that are requiring space-saving and flexibility? A quick example is keyboards, designed from a flexible circuit. Most of the flexible circuits are used to interconnect electronic components such as resistors, integrated circuits, and other electronic assemblies. Another great example would be the application of FPC or FPCB is the application in automotive. They are located in an instrumental panel on top of the hood-controls designed to be hidden, concealed with the ABS system and the headliner of the cabin.
These flexible circuits are also found on medical devices where interconnections are required. This revolutionary design even finds applications in satellites which the majority considers the highest-end technology. Furthermore, flexible circuits can be sent underwater. There are a few basic structures, even though the types can vary when referring to terms and conditions.
Rigid-flex circuits - Also called Rigid-flex is a hybrid combination between the rigid and flexible substrates that are used together in a single structure. Rigid-flex circuits have layers that are electrically connected as normal circuits. The most common rigid-flex circuit application is in the military and aerospace field of work for more than 20 years. In the most popular designs, the consistency of multiple flexible inner layers is selected. In general rigid-flex circuits are the best of both types of boards combined in one circuit. This design’s interconnection is done with plated holes providing higher quality and better component density. This is the design that fits in the tight spaces or in projects that require space and flexibility, we mentioned earlier. There are many and many different rigid combinations possible and to illustrate some of them, here are the most commonly utilized. One of the main benefits is that when compared to a traditional rigid board, the rigid-flex circuits require fewer interconnection and parts to assemble. The rigid-flex circuit can be designed to solve highly complex challenges including the following:
Three to eight-layer configurations
Highly Complex configurations
- Controlled Impedance
Often the rigid components are utilized for a higher density of the device populations. Additionally, flexible circuits can allow narrow lines giving way to denser device populations and the construction of the product is always light and dense. The package and weight are also improved with the innovation of rigid-flexible circuits. This allows the product to fit in tight spaces and the space-saving aspect has found many applications in different industries. Here are some of the basic materials used in the assembly of a Rigid-flex circuit:
Adhesives - The selection of adhesives usually depends on the customer’s need in the creation of a specific, sometimes new end design. The adhesive matter is usually epoxy, pre-preg, P pressure S sensitive A adhesive ( PSA ), or adhesiveness base material.
Conductors - If we take a look at the thicknesses in the conductor assembly, again we have the customer’s requirements. The most commonly used types are Electro Deposited copper ( ED ), Rolled Annealed copper ( RA ).
Insulators - This is the flexible base (substrate) and it is available once again in a variety of thicknesses depending on the different projects. Some of the common insulators are solder mask , fr-4 , photo image-able cover lay (PIC), polyimide , and flexible solder mask .
- The foil is basically used as a conductive element in the flexible design. From this material, the circuit paths are normally etched providing great efficiency. As we mentioned several times the thickness here varies as well providing a wide range of choices and order flexibility to the customers. The metal foil design can fit perfectly in many applications and this is the reason for the high market demand. There is a perfect balance in price and efficiency. For example copper is a great combination of performance and cost, allowing this matter choice to find a variety of applications making it an excellent choice. Speaking of copper foil, we should add that there are many different types of it. For example,
has identified eight completely different types of copper foil designed for printed circuits that are divided into small categories. Each of these categories has its own sub-types and as a result, there are a number of different copper foils that are available on the market for constructing different circuits in order to meet the requirements of any project. There are, however, some certain cases in which the manufacturer can be required to create a specific laminate assembled by an alternative metal foil. This metal foil can be a special copper foil or other construction using a foil. The whole operation is accomplished by laminating the foil without using adhesive considering the natural properties of the base.
End Designs - As we already mentioned every single end design depends on the customer’s requirements, recommendations, and guidelines. Here is a shortlist of the most common finishers or end designs delivered.
Solder (Tin/Lead or RoHS compliant)
Wire bondable gold
- Organic: Entek or Organic Solderability Preservative ( OSP )
Flexible Circuit Board Benefits
The innovative rigid flexible circuits design many advantages that have been discovered and applied in the industrial field of work. With the market growing some of these advantages include:
Component Assembly - The service mount capability and trough hole that this design offers is a remarkable electrostatic and protective packaging.
Air Gap - The technology of flexible circuits rapidly increased the flexibility that is mainly achieved by layers with one main advantage “unbinding”. This is an essential aspect of the flexible circuit assembly, allowing the layer to freely flex.
- The engineers finally understood to control the impedance of traces which is responsible for increasing the signal switching speeds and with that in mind now, instead of simple interconnections in the modern digital circuitry are used transmission lines. Today’s end - designs are manufactured in a way that is increasing the speed and minimizing the electrical reflections. This way they ensure no errors between the transitions and the track interconnections. On the other hand, a perfectly designed and optimized controlled impedance can allow great control of the material of the cable and its physical dimensions. With the controlled impedance the requirement of flexible circuit materials is now not a barrier for the industry in order to be able to uniform in both thickness and electrical properties. It is essential to be perfectly designed by the manufacturers and optimized as per its application.
- This feature allows the circuit to be durable in components assembly and more specifically during the wave soldering process and in a popular manufacturing segment “pick & place”. Once the panel is completed the circuit is ready for the assembly of the end product.
Pressure Sensitive Adhesives (PSAs) - These types are used in projects and applications that a portion of the circuit has to be extra secured to a specific condition or location in the final product. The assembly process includes a releasing of the liner that is peeled away and the adhesive that remains exposed is allowing the manufacturer to press the circuit on its place and keep it there as long as needed.
Shielding - Securing is applied when there is a demand for it in the limits of electrostatic and electromagnetic interferences. The shielding can be solid or electromagnetic. Both of the materials are insulating signal lines and any noise.
Flexible Printed Circuit Board Production
In general, the flexible printed circuit boards are created by a
one sheet at a time. There is, however, a more efficient way that manufacturers are adopting called
). Basically, the photolithographic technology is the whole production process, step by step. We will briefly review each step in order to get a basic understanding.
- The first step is material utilization. The material is being a cutter and adjusted as per the desired end-design.
Computer Numerical Control (CNC)
- Basically, CNC is the drilling process. In order for the FPCB to fit the customer's desires, specific holes are being drilled on the board's surface.
Plating through hole
- In order for the pattern connection between the layers to be realized, a plated copper on the hole-wall is placed after the drilling. It is done this way because both the top and bottom latter are not conductive.
Dry film lamination
- It is laminated to create an etching layer of resistance.
- The laminated film is driven onto the foul in order to create an exposure area.
- Ultraviolet UV light is cast directly on the dry film. This way the exposure area will be polymerized and hardened.
- After the development step, the manufacturers remove the non-exposure area by chemical reaction.
- The dry film covering is removed which leaves the circuit pattern exposed.
Automated Optical Inspection (AOI)
- AOI tested checks the quality of the printed board.
- After the AOI check, the board is covered with an insulation layer called “overlay”.
- Then the board passes through a hot press machine which forms adhesion between the overlay and the copper.
- The bonding area gets coated in order to protect the terminator’s function.
13. Test - Functional test of the finished circuit design.
Flexible Printed Circuit Board Layering
With all the benefits and capabilities in mind along with rigid-flex, we will now take a look at the other different layering types in flexible circuits.
Single-Sided Flexible Circuit
This is the general single-layer design for flex circuits that are laminated in a thin surface of copper. A chemical etching is applied to the copper layer, in order to produce a specific pattern according to the requested design. For environmental protection and insolation, a polyimide overlay is later added to the circuit.
Double-Sided Flexible Circuit
This layering type is called double-sided because the flex circuit has two conductive layers. The interesting thing here is that traces can be created on both sides, specifically for customer requirements. Sides are connected wherever needed by placing a-holes designed in the copper plate. They are famous for the connection between the layers.
Multi-Layer Flexible Circuit
This is the circuit where the combination of several single and double-sided circuits happens. They are a great solution where manufacturers are facing major issues with the circuits with fewer layers. Many individual requirements can be covered and handled flawlessly by the multi-layer circuit design in combination with the high-end manufacturers.
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