PCB Manufacturing

4-Layer Rigid-Flex PCBs: A Guide for Manufacturing Techniques

4-Layer Rigid-Flex PCB

PCBs are the most significant parts of today’s electronics. There are different types of PCBs, such as rigid PCB, flex PCB, rigid-flex PCB, and rigid-flex PCB is the most advanced circuit board. Of all the rigid-flex PCB types, 4-layer-rigid-flex PCBs are the most commonly used type because 4-layer-rigid-flex PCBs can meet most of the functional requirements with the lowest possible cost. That is why 4-layer-rigid-flex PCBs are very important.

In this article, we will talk about the definition, material and techniques of this type of advanced printed circuit board.

What are 4-layer-rigid-flex PCBs?

PCBs with both rigid and flexible parts are called rigid-flex PCBs. Rigid-flex PCBs simultaneously have the characteristics of rigid and flexible PCBs and put them together in one package, giving them the advantage of both rigid and flexible PCBs. Usually, components will be soldered on the rigid part, and flexible parts are used as connecting wires.

4-layer-rigid-flex PCBs are rigid-flex PCBs with a total of four copper layers. Usually, the top and bottom layers are rigid layers, and two inner layers are the flexible ones. They are circuits with four layers. It is made up of several layers of copper wires that are glued to a printed circuit board with plating through holes connecting the layers. Such PCBs bring greater economy in space, better dependability, and improved flexibility to electrical functions.

Materials Used in Manufacturing 4-layer-rigid-flex PCB

  • Rigid substrates: In almost all cases rigid substrates are made from a fiberglass-reinforced epoxy (FR4) composite.
  • Flexible substrates: Substrates using Polyimide (PI) or polyester are dielectric (insulating) and flexible.
  • Copper foils: Copper foils are used in the trace of the circuit and the links between them. For the most of time, its width is 17-35um.
  • Adhesive materials: It is a compatible connection between different materials, such as rigid and flexible materials.

Let’s explore these things of 4-layer-rigid-flex PCB in more detail:

  • Rigid Substrates:

This is the case when the boards being made require rigid substrates. The rigid substrates give the base of the PCB that makes the whole assembly stable and strong. The most common material for rigid surfaces is fibreglass-reinforced epoxy laminated, which is called “FR4” as well. FR4 is beneficial because of its ideal features including high electric insulation capacity, high mechanical strength and cost-effectiveness. It is characterised by layers which are primitively made up of woven fibreglass fabrics and have epoxy resin put in them, thus resulting in a rigid structure, making it able to handle the electronic components.

  • Flexible Substrates

Flexible Substrates in 4-layer-rigid-flex PCB

Flexible substrates become the most useful tool to recreate the shape of PCB in line with the border shape of the board- irregular and curved. The polyimide and polyester are the most common bendable substrate raw materials. Concerning polyimide, what we should most mention is its better high-temperature protection, chemical stability, and adaptability in a range of temperatures from high to low. Additionally, the material is strong and durable allowing the components to survive the challenges of PCB manufacturing processes such as soldering and reflow without warping or degrading.

  • Copper Foils:

Copper foils are utilized to make crisscross designs, as well as traces, on the circuit boards. These conductive pathways help in the transfer of electrical information. These lines provide a way to carry electrical signals within components or transmit electrical signals from one layer to another in the PCB. Copper is an ideal material due to its great electrical conductivity, resistance to corrosion, and ease of shape and form. In circuit design, copper foil has different thicknesses and processing quality requirements so such covers may be made differently to meet specific design needs.  Wider or larger lines are used in circuits requiring higher current carrying capabilities, but relatively finer and smaller lines are for better fitting or compact design. Surface covering with processes such as electroplating or chemical surface deposition may be used to provide better solderability and keep away from oxidation.

  • Adhesive Materials:

The adhesive material plays a vital role in fully connecting the rigid and flexible layers of the PCB board. The stated adhesives allow joint strength, thermal stability, and insulation function which are important to the building process of PCB. Such stick sorts used in PCB production are epoxy resins, acrylic adhesive and pressure-sensitive adhesive (PSA). The selection of adhesive materials is affected by many factors, like the type of substrate material, production methods needed and end mechanical characteristics wanted. Niche adhesives may be chosen in some cases based on the type of application, for instance, working in high-temperature settings or the chemicals showing aggressiveness.

The difficulty generated from 4-layer-rigid-flex PCB includes a careful selection of materials from the rigid and flexible substrate to copper foils and adhesive materials. Particularly, any component of material gives to the assembly the usefulness, the reliability, and the adaptability, therefore, such assemblies are used in the electronics devices. Engineers gain information about how these materials work based on their properties and qualities. With this information, engineers can build and make wholesome and effective 4-layer-rigid-flex PCBs that meet the standards.

Manufacturing Techniques of 4-layer-rigid-flex PCB

4-layer-rigid-flex PCB Manufacturing

4-layer-rigid-flex PCB manufacturing is a very complex task, from design to material preparation, drilling, plating, copper etching and surface finishes. Please refer to the following details.

Design Phase in 4-layer-rigid-flex PCB:

  • Design software:

Engineers with specific software programming abilities outlay the 4-layer-rigid-flex PCB design, ensuring correct electrical links, component placing, and mechanical limits.

  • Layer arrangement: 

Develop a framework that combines rigid and flexible layers to generate maximum space and margin.

  • Trace routing: 

Here we trace our signals carefully to ensure purity of the signals and avoid interferences in 4-layer-rigid-flex PCB.

Material Preparation in 4-layer-rigid-flex PCB:

  • Cutting: 

Rigid and flexible substrates are cut into desired sizes using the cutting tools which have the maximum accuracy for manufacturing 4-layer-rigid-flex PCB.

  • Cleaning: 

The 4-layer-rigid-flex PCB surface is cleaned using different solvents to get rid of any current contaminants. This process makes surface bonding and conductivity possible.

  • Copper foil application: 

With the help of the thin copper foils energy is moved on the substrate surfaces to make conductive paths.

  • Alignment: 

The aligning of the rigid and the flexible layers, using registration lines, and optical systems, is done exactly.

  • Bonding: 

Heat and pressure are used concurrently to glue the layers into place, normally employing a mix of sealing presses as well as adhesives.

  • Curing: 

The cured bonded assembly is put through a curing process to check the usefulness of the glue and the assembly’s stability.

Drilling and Plating in 4-layer-rigid-flex PCB:

  • Drilling: 

Smaller holes, named vias, are drilled through layers to be used as the conductive paths building the link between the layers.

  • Plating: 

Vias are copper, like other conductors, plated to keep electrical links relevant through the whole 4-layer-rigid-flex PCB.

Circuit Patterning in 4-layer-rigid-flex PCB:

  • Photolithography: 

The copper surfaces will receive a photosensitive layer followed by being exposed to UV light via a photomask, hence, this formation would be through development into circuit patterns.

  • Etching: 

Engineers use etchants for copper exposure areas to etch the cathode along underwater line tracks leaving the copper line trace behind.

Surface Finish in 4-layer-rigid-flex PCB:

  • Surface coating: 

There are multiple surface finishes in 4-layer-rigid-flex PCB, such as gold immersion, HASL (hot air solder levelling), and ENIG (electroless Nickel Immersion Gold), which are used to protect the copper from contact and ease soldering.


The making of 4-layer-rigid-flex PCB is completed via a successful execution of a number of demanding processes which include; preparing the material, design, and circuit patterning and surface finishing. Through mastery of such skills, engineers are capable of making miniature and reliable Microchip Printed Circuit Boards (PCB) that will be fit for a variety of electronic applications. With the growth of technology, revolutionary improvements in the manufacturing process of 4-layer-rigid-flex PCB develop indefinitely, which is essential in the development of the electronics sector.

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