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    Definition of Flexible Printed Circuit Boards

    A flexible printed circuit board also called a soft board or a flex PCB, is made of a flexible insulating material circuit board. Its biggest feature is flexibility, able to bend and rotate freely.

    Flexible printed circuit board selection of insulation substrate commonly used there are two: polyimide(PI) or polyester(PET) film. Viasion is a trusted flex PCB manufacturer with 16 years of experience for all these flexible circuit material.. 

    Types of Flexible Printed Circuit Boards

    There are many ways to classify flex PCBs. If we talked about the layer counts, single-sided flexible PCBs, double-sided flexible PCBs, and multi-layer flexible PCBs are common types of flex PCBs.

    Flexible circuit boards can also be classified from the flex PCB design perspective, then there are two types of flexible PCBs that can be designed as dynamic or static PCBs.

    Here we would like to take more about static and dynamic flexible PCB design. It may be helpful to you which kinds of boards should be used in your devices when doing your flexible circuit designs. 

    Static design refers to the product encountering the bending or folding only in the assembly process, or the bending or folding which rarely occurs during use. Single-sided and double-sided PCBs like multi-layer printed circuit boards can also be successfully implemented in the folded, static flex PCB design. Typically, for most double-sided and multi-substrate designs, the minimum bend radius for folding should be ten times the thickness of the entire circuit. Circuits with more layers (eight or more) can become so stiff that it is difficult to bend them. Therefore, for double-sided circuits requiring a tight bend radius, all copper alignments are set on the same side of the substrate film in the folded area. The folded area is approximated to a single-sided circuit by removing the laminate on the opposite side.

    Dynamic flex PCB design are used for repeated bending over the entire life cycle of the product. Taking the example of cables for printers and disk drives, as an illustration, to achieve the longest bending life cycle for a dynamic circuit, the relevant part should be designed as a single-sided circuit with copper on the central axis. A central axis is a theoretical plane that is the central layer of the material that makes up the circuit. By using a substrate film and laminate with the same thickness on both sides of the copper, the copper foil is accurately placed in the center and is subjected to minimal stress during bending or flexing.

    The Benefits of Flexible Printed Circuit Boards

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    Advantages of Flexible Printed Circuit Boards

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    Disadvantages of Flexible Printed Circuit Boards

    Factors Affecting Flexible PCB Welding Quality

    The flexible printed circuit board is composed of the plate and the above components. The components of the welding process are critical. Any improper operation, or poor connection, is likely to lead to the malfunction of the components of the flexible circuit board, even leading to factory production of a whole bunch of PCB scrap, which is a big loss to the company and customers. So in the production process, we must pay attention to welding this key step. The following is about the impact of welding on several factors:


    Design of the flex PCB

    The design data of the flex printed circuit board is very important in determining the quality of the flex PCB from the source. What’s more, the design size can not be too large, and also can not be too small. If the size is too large, the impedance increases, and the cost increases. If the size is too small, the distance between components is too short, which is easy to cause interference of adjacent lines, and the components are too close together, which will also cause an increase in the difficulty of welding, and it is inevitable to make mistakes.


    The solderability of the flexible printed circuit board hole

    Solderability refers to the degree of difficulty of welding. If the solderability of the flex PCB is not good, it will cause virtual welding and fake welding, failing in the whole flex printed circuit board information transmission.


    The warping of the flexible printed circuit board

    The flexible printed circuit board warping is also called circuit board deformation, if the temperature of the upper and lower parts of the circuit board is not the same, it leads to thermal expansion and cold contraction which will cause warping. Once there is warping, the welding between the flex PCB and components will appear as virtual welding. Moreover, when the temperature is different, warping will occur not only on the circuit board but also on the components. So the flex printed circuit board production has to control the consistency of temperature.


    The welding method of the flexible printed circuit board

    There are many different methods of welding the flex printed circuit board. The factory should choose the corresponding welding method according to the specific situation, and the difference between the welding point and melting point.


    Welding materials

    The main welding materials used in flex printed circuit board welding are electrodes, solder paste, etc. Electrode and solder paste will affect the quality of welding.


    Welding temperature

    The temperature of the flexible circuit board welding will also affect the quality of welding. The welding temperature should be kept within a reasonable range. It is not acceptable that the temperature is too high or too low.


    The cleanliness of the flexible printed circuit board surface

    If the flexible circuit board surface is not clean enough, some impurities will directly affect the quality of welding.

    Reasons and Improvement of Flexible PCB Deformation

    As mentioned before, the deformation of the flex printed circuit board will seriously affect the quality of component welding, and the normal use of the flex printed circuit board. What are the factors leading to the deformation of the flex PCB?

    flex pcb and assembly
    Given the above causes of circuit board deformation, what measures should we take?
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    flexible pcb

    Circuit Board V-cut and Its Residual Thickness and Angle Requirements

    V-cut is the dividing line cut by a knife on the edge of the flex printed circuit board by the circuit board company for future assembling. Its shape is similar to the letter V, so it is called V-cut. V-cut is rarely used in flex PCBs, but possible to use in rigid-flex PCBs.
    V-cut thickness refers to the remaining thickness of the flex printed circuit board after it is Cut. This index is very critical and determines whether the flex PCB is easy to be broken or deformed.

    Conventional requirements for Chinese circuit boards are that the residual thickness is one-third of the total thickness of circuit boards, but greater than 0.35 mm, not greater than 0.8 mm, meet these three parameters, the residual thickness is too small is not good, because if the residual thickness is too small, easy to break; The residual thickness is too large, too strong, and then it's harder to break.
    The Angle of V-cut is the Angle of the slot Cut on the rigid-flex PCBs, and there are three kinds: 30 degrees, 45 degrees, and 60 degrees. The most common is 45 degrees.

    The Angle size has advantages and disadvantages, if the Angle is larger, it means that the rigid flex printed circuit board edge is cut off the circuit board by the cutting knife, for normal connection of the line, the line must move a little, so as not to be cut off the line by the cutting knife. If the Angle is smaller, on the one hand, the line does not have to move inside, on the other hand, this requires that the blade of the cutting knife is thinner, wears faster, and even the blade is broken. Therefore, the Angle of the V-cut should be moderate. Not too big, not too small.

    Thermal Conductivity of the Flex Printed Circuit Boards

    When the temperature of the card rises to an exact extent, the solid-state is reborn into a rubber state, and therefore the temperature at this point is termed the thermal physical phenomenon of the card. brought up as TG, it is far-famed that the thermal physical phenomenon is the highest temperature within the rigid state of the card. higher than this temperature, the card becomes soft and its mechanical and different properties decline apace.

    There square measure 3 levels of TG, the final grade is a hundred thirty degrees, the medium grade is a hundred and fifty degrees, and therefore the advanced grade is a hundred and seventy degrees. The thermal physical phenomenon of the card is improved, which suggests that prime temperature resistance, stability, and different properties square measure higher.

    With the rapid development of electronic products, especially consumer goods such as mobile phones and computers, the volume of data transmission is increasing and the transmission speed is getting faster and faster, which leads to the long-term high-load operation of flex printed circuit boards and the generation of a large amount of heat. Objectively, circuit boards are required to have good heat dissipation and high-temperature resistance, which leads to the increasing demand for high thermal conductivity circuit boards.

    According to the experimental data, the general circuit board and high thermal conductivity circuit board, in a high-temperature environment, the mechanical performance of the circuit board, stability, heat dissipation, and other indicators are very different, the high thermal conductivity circuit board is better than the general circuit board.

    In recent years, the quantity of shoppers buying high Tg boards has magnified year by year.

    Application of Flexible Printed Circuit Boards

    There are three main application fields of flexible printed circuit boards in China: automotive electronics, wearable devices, and smartphones etc.

    Manufacturing Capabilities of Flexible Printed Circuit Boards in Viasion

    Viasion is one of the most trusted flex PCB manufacturers in China, please refer to our manufacturing capabilities as following:

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    Frequently Asked Questions

    As mentioned above, a flex circuit is made of a flexible insulating material circuit board. Its biggest feature is flexibility, able to bend and rotate freely.

    Flex circuit has the advantages of high wiring density, lightweight, thin thickness, bendability, flexibility, etc. Firstly, it can withstand millions of dynamic bending without damaging the wire, move and expand at will according to the requirements of space layout, realize three-dimensional assembly, and achieve the effect of integration of component assembly and wire connection. Next, flexible PCBs have excellent electrical properties. The lower dielectric constant allows for fast electrical signal transmission; the good thermal properties make it easy to cool down the components; the higher glass transition temperature or melting point allows the components to operate well at higher temperatures. Furthermore, the assembly reliability and yield of flexible PCBs are higher.

    Manufacturing flexible PCBs is challenging for the following reasons.
    1) The costs for the design and wiring of flexible PCBs are high.
    2) The production process of flexible PCBs is complex.
    3) The maintenance of flexible PCB is difficult and the maintenance cost is expensive.
    4) The base material used in flexible PCBs are more expensive than FR4 material;
    5) Drill hole treatment and copper plating in flexible PCBs fabrication is more difficult.

    The mot commonly used flexible PCB material are Polyimide (PI) and Polyester (PET).

    Yes, we supply single layer flex PCB, double layer flex boards and multilayer flex printed circuits up to 10 layers. But normally flex boards only have 1-4 layers, since flex circuits with higher layer boards are not flexible enough. So high layer count flex boards are designed as rigid flex PCBs.

    Flexible PCBs are widely used in mechanically sensitive equipment because of their flexibility to resist vibration. Their application areas are described below.
    1) Computers and accessories
    2) Aircraft, drones, cameras
    3) Automobiles
    4) Cell phones
    5) Medical electronics
    6) Wearable devices
    8) Industrial applications: For example, flexible PCBs are also used in industrial machine and power distribution control systems
    8) Military equipment
    9) Aerospace

    With the continuous promotion of information technology and intelligent construction, the market demand for flexible printed circuit boards will also become greater and greater. It is already difficult to find slightly complex electronic products that do not use flexible PCBs as well as flexible PCB assembly. As China will further increase its investment in various fields of electronic information construction, the pace of electronic information construction in downstream areas is bound to drive the development of the flexible PCB & PCB assembly industry. In addition, with the emergence of new electronic products, the market and applications of flexible printed circuit boards will also be expanded, and the renewal of different categories of electronic products will bring more considerable development prospects than the traditional market. Particularly, flexible PCBs have a promising market in satellite and avionics devices, high-end sensors, flexible heat-generating components, medical devices, remote control devices, and security devices.

    Flex PCBs is produced with only flexible PCB materials, such as PI and PET, while rigid flexible PCB combine both rigid material (normally FR4 material) and flexible material together. So on flex boards, the whole flexible circuit boards can be bent; but on a rigid flex PCB, only a certain part where these is flexible material can be bent.

    As we know that every flexible printed circuit board should have a custom flex PCB design, here is a typical stack up for a normal 2 layer flex PCB:
    –cover layer 25um
    –adhesive: 25 um
    –top copper layer: 35 um
    –Polyimide (PI) base material: 25 um (or 50 um)
    –bottom copper layer: 35 um
    –adhesive: 25 um
    –cover layer 25 um