Rigid-flex printed circuit board as the name suggests, combines properties of both rigid board and flexible circuits assimilated into one circuit. Based on the application, flex boards can consist of multiple layers. State of flex usually kept constant and formed in curved shape during manufacturing or at the time of installation.
Rigid-flex boards are usually made from a combination of PI flex material sandwiched between FR4 epoxy and polyamide layers. The integrated circuits are connected through plated direct holes. Flex PCB provides a higher density. Usually, in the rigid-flex PCB, the circuit consists of multiple circuits inner layers selectively attached together using an adhesive bonding film. However, a multilayer flex PCB circuit incorporates a board both sides as the requirement of design arise. This also offers dynamically and mechanically robust system.
The proper application of rigid circuit flex boards offers best solutions for difficult, limited space conditions. This technology offers a possibility of secure connections of device components with the assurance of polarity and tight connections, as well as the reduction in plug and connector components.
a) Conductors: - Conductors are usually made up of copper. It could be rolled annealed copper or electrodeposited copper (depends upon the requirement).
b) Adhesives: - The selection of adhesive depends upon the conductor’s thickness. Adhesives offer chemical resistance, ability to withstand with high temperature, controlled impedance and high flexibility. Commonly used adhesives are enlisted below
- ④Pressure-sensitive adhesive
c) Insulators: - Commonly used insulators are
- ①Polyester, Polyethylene Terephthalate,
- ②Solder Mask
- ④FR-4 (Composite)
d) Finishes: - The finishing material depends upon the application of the flex board. Commonly used materials are
- ①Solder (Lead-tin)
- ②Immersion (gold/nickel)
- ③Wire bondable gold
The manufacturing process involved in the development of rigid-flex PCB is a little more complex and time-consuming compared to the manufacturing of rigid board. Usually, the steps involved in the design process. Usually, the steps involved in the manufacturing process of the flex board are somewhat similar to the rigid board manufacturing like printing the design, creation of substrate, the printing of inner layers, removal of unwanted copper, etc. But certain issues come up while we talk about previously mentioned steps of the flex board. Since it is required to maintain the flexibility of the board to achieve the required degree of freedom. Depending on the application, the complete Rigid-flex board can be manufactured symmetric with inner flexible board, or asymmetrically with outward flexible board. There are various circuit designs available
- Board to board
- Board to chip
- Chip to chip connections
But in order to get all those requisite properties there are certain issues enlisted below:
1) When the copper foil is etched the polyamide core shrinks. Therefore this variation must be encountered during the design process by providing the compensating material.
2) As it was mentioned the flex boards are required to bent into shape at the time of installation or during the forming process and during the bending process, the stress fracture could occur.
3) Copper foil requires a surface treatment prior to bonding prepreg resins.
4) Some resins can require different treatment.
Since the design of the flex board is done in a 3D frame, therefore, this process offers more challenges to the designer. 3D also offers few advantages like a designer could bend, fold the flex board according to the requirement. Appropriate bending parameters are required to be considered, and this solely depends upon the material used.
Compare to rigid boards it requires fewer parts and interconnections. Rigid-flex can be designed to meet highly complex configurations while utilizing a rigid substrate. Additional advantages of rigid-flex circuit boards are dynamic and mechanical stability, the resulting 3-dimensional freedom of design, simplified installation and maintenance of uniform electrical characteristics. Some other advantages of the flex PCB are enlisted below
1) Unlike the rigid board, it has less weight, doesn’t have space limitations.
2) Connections in such boards are more reliable since there are less soldered joints.
3) Compare to rigid boards the handling of flex board is easier during assembly.
4) The need for cables and connectors is completely eliminated.
5) Logistic cost reduces significantly.
6) Comparatively it is a excellent and portable device.
7) Ensure a wide range of material and design –layout options to meet the unique requirements of the applications.
8) Appreciable increase in circuit density.
9) This system reduces the cost significantly. Flex cost is less than the rigid board cost.
10) It eliminates the parasitic inductance and capacitance. And also helps in preservation of signal, speed power and clarity. All these things helps in reduction in error.
The above discussion concludes that rigid-flex circuits are sandwiched, consisting of rigid and flexible substrates laminated into a single structure. Rigid board consists of advantages of flexible printed circuit technology and PCB.
Rigid-flex PCB offers multiple applications in various technologies. Advances in rigid board technology have broadened the applications on electronic products from a computer, communication, and consumer electronics, to automobile, medical and military electronics. Increasing demand for more powerful and small size products drive the need for multiple layers to accommodate dense, fine line width and spacing. Most of the electrical testing types of equipment are equipped with rigid-flex PCB, since the reliability because of its simple assembly and the absence of solder joints and the fragile wiring which has connection issues at a certain point of time has also been eliminated completely. Tools and Automobiles have also taken flex PCB in use. Certain control systems that are in use nowadays are also using flex boards, because of its reliability. There two fundamental applications of rigid-flex PCB’s
1) Static Application:- An application where flex circuits are needed only to install the circuit and fit it into its application.
2) Dynamic Application: - A situation where the flex circuit area itself is dynamically flexed for multiple reasons for and otherwise simple installation.
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