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An Overview of the Rigid-Flex Fabrication Process

Key Takeaways

  • Flex boards are classified into single-sided flex boards, double-sided flex boards, multilayer flex boards, and rigid-flex boards under IPC 6013 standards. 

  • The rigid-flex fabrication process involves steps such as material preparation, pattern generation, etching, drilling, plating, flex-board cutting, and electrical testing.

  • The rigid-flex fabrication process is ideal for the construction of circuits in the medical, aerospace, military, and telecommunication industries.

 Aerospace industry

Rigid-flex boards are commonly used in the automotive, medical, military, and aerospace industries

The demand for flexible PCBs is increasing due to advantages such as their light weight, high assembly density, and miniaturization capabilities. Flexible PCBs also offer advanced mechanical design and excellent electrothermal performance. The popularity of flexible PCBs has led to the development of other variants such as HDI flex PCBs, embedded flexible PCBs, and rigid-flex PCBs.

Among the variants of flex boards, rigid-flex PCBs are commonly employed in the automotive, medical, military, and aerospace industries. Rigid-flex fabrication is a time-consuming process, with steps including material preparation, etching, drilling, and plating before the final product fabrication. In this article, we will explore flexible PCBs and take a look at the rigid-flex fabrication process. 

Flexible PCBs 

A circuit board made of flexible substrate material is called a flexible PCB. Flexible PCBs, or flex boards, provide flexibility and conformity to take the desired shape demanded by the application. Flex boards offer numerous advantages over traditional rigid boards, including:

  • Reduction in size and weight
  • Reduction in thickness
  • Circuit miniaturization and high-density assembly
  • Reduction in wiring errors
  • Flexibility and the possibility of bendable circuit formation 
  • No mechanical connectors are required
  • Compatible with 3D interconnections
  • Higher level of freedom in mechanical and electronic design
  • Improved signal integrity and circuit reliability
  • Enhanced impedance control
  • Suitable for higher operating temperature ranges
  • Improved vibration withstanding capabilities
  • Applicable in harsh environments

Flex PCBs offer all of the above benefits at the expense of more material costs compared to rigid or rigid-flex boards. Rigid-flex boards bring the merits of both rigid and flexible PCBs into circuit manufacturing. We will explore rigid-flex boards and the rigid-flex fabrication process in the upcoming sections. 

Rigid-Flex Boards 

Flex boards are classified into different types under IPC 6013 standards. The classifications are single-sided flex boards, double-sided flex boards, multilayer flex boards, and rigid-flex boards. Rigid-flex boards are specified under IPC 6013 type 4, which combines the advantages of flexible and rigid boards into one. It can be described as a hybrid or rigid and flex board. Rigid-flex boards are bifurcated into:

  1. Flex to install types - This type can be bent only once during installation or repair. 
  2. Dynamic flex types - This type supports continuous bending while in use. 

Rigid-flex boards are suitable for applications that require size reduction–incorporating more components in smaller footprint areas. Rigid-flex boards are designed in the 3D space, which gives them enhanced spatial efficiency. Rigid-flex boards can be dynamically bendable or foldable, which helps them to take the desired shape in the final product package. There are not many interconnections needed in rigid-flex boards. They are lower-maintenance circuit boards with a proven track record of reliability. Rigid-flex boards are most often used in the medical, telecommunication, and aerospace industries. 

The Rigid-Flex Fabrication Process

Prior to the commencement of the rigid-flex fabrication process, there is a PCB design layout stage. Once the layout is fixed, the rigid-flex fabrication process begins.

The rigid-flex fabrication process combines rigid and flexible board manufacturing technologies. These boards stack up layers of rigid and flexible PCBs. The components are assembled in rigid areas and the interconnection to neighboring rigid boards is made via flexible areas. The connection between the layers is introduced through plated through holes.

Rigid-flex fabrication involves the following steps:

  1. Preparation of base material: The rigid-flex fabrication process starts with the preparation or cleaning of laminate. The laminate containing the copper layer with or without adhesive coating is pre-cleaned before other fabrication processes. 

  2. Pattern generation: Accomplished through either screen printing or photo imaging.

  3. Etching process: Both sides of the lamination containing the circuit pattern are etched by either dipping in an etch bath or by spraying an etchant solution. 

  4. Mechanical drilling process: The circuit holes, pads, and via patterns required in the production panels are drilled using precision drilling systems or techniques. For example, the laser drilling technique. 

  5. Copper plating process: The copper plating process focuses on depositing required copper inside the plated through holes to establish the electrical interconnection between the rigid-flex board layers. 

  6. Cover lay application: The cover lay material, usually polyimide film, and the adhesive are imprinted on the surface of the rigid-flex board using screen printing. 

  7. Cover lay lamination: The proper adhesion of the cover lay is ensured by laminating it under specified limits of heat, pressure, and vacuum. 

  8. Application of stiffener: If required as per the design of the rigid-flex boards, localized additional stiffeners are applied prior to the additional lamination process.

  9. Flex board cutting: Hydraulic punching methods or specialized blanking knives are used for cutting the flex board from the production panel.

  10. Electrical testing and verification: Rigid-flex boards are electrically tested as per IPC-ET-652 guidelines to confirm that the isolation, continuity, quality, and performance of the boards meet the design specifications. Testing methods include flying probes and grid testing systems.

The rigid-flex fabrication process is ideal for the construction of circuits in the medical, aerospace, military, and telecommunication industries, as these boards offer great performance and precise functioning, especially in harsh environments. To design sturdy rigid-flex boards for extreme conditions, Cadence’s suite of design and analysis tools can be used.

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