RAYMING 4 Layer Rigid Flex Board with ENIG process and Pi Stiffener

Rigid Board: 4 Layer PCB

Flex:  2 Layer Flexible PCB

Surface Process: Enig

Number of layers: 4 layers
Board material:FR4+PI+NFPP
Plate surface copper thickness: ≥35um
Copper thickness in the hole: 20um
Line width and spacing: 0.075mm
Minimum hole diameter: 0.2mm
Surface treatment: immersion gold ≥ 2u”
Product use: automobile inverter

Estimate PCB Cost, Pls send PCB File and Bom List to Sales@raypcb.com with Requirement.


4 Layer Rigid Flex Board with ENIG Process and Pi Stiffener

In the ever-evolving world of electronics, printed circuit boards (PCBs) play a crucial role in connecting and supporting various components. As devices become more compact and complex, the demand for flexible and reliable PCB solutions has increased. One such solution is the 4 Layer Rigid Flex Board with ENIG process and Pi Stiffener. This article will explore the intricacies of this advanced PCB technology, its benefits, and applications.

Understanding Rigid Flex Boards

What are Rigid Flex Boards?

Rigid flex boards are a hybrid PCB technology that combines the best features of rigid and flexible circuit boards. These boards consist of rigid sections for component mounting and flexible sections that allow the board to bend and fold as needed.

Advantages of Rigid Flex Boards

  1. Space-saving design
  2. Reduced weight
  3. Improved reliability
  4. Elimination of connectors and cables
  5. Enhanced flexibility in product design

The 4 Layer Structure

Layer Stack-up

A 4 layer rigid flex board typically consists of the following layers:

  1. Top copper layer
  2. Inner copper layer 1
  3. Inner copper layer 2
  4. Bottom copper layer

Between these copper layers are insulating materials, such as polyimide or FR-4, depending on the rigid or flexible nature of each section.

Characteristics of Each Layer

Layer Function Typical Material
Top Layer Component mounting, signal routing Copper
Inner Layer 1 Power distribution, ground plane Copper
Inner Layer 2 Signal routing, ground plane Copper
Bottom Layer Component mounting, signal routing Copper

ENIG Process

What is ENIG?

ENIG stands for Electroless Nickel Immersion Gold. It is a surface finish applied to the exposed copper pads and traces on a PCB.

The ENIG Process Steps

  1. Cleaning and preparation of the copper surface
  2. Catalyzation of the copper surface
  3. Electroless nickel plating
  4. Immersion gold plating

Benefits of ENIG

  1. Excellent surface planarity
  2. Good solderability
  3. Long shelf life
  4. Suitable for fine-pitch components
  5. Resistant to oxidation

Pi Stiffener

Purpose of Pi Stiffener

The Pi Stiffener, named for its resemblance to the Greek letter π, is a reinforcement structure added to flexible or rigid-flex PCBs to provide additional support and stability in specific areas.

Characteristics of Pi Stiffener

  1. Typically made of FR-4 or polyimide material
  2. Applied to the back of the flexible portion
  3. Shaped like a “π” to allow for bending while providing rigidity

Benefits of Pi Stiffener

  1. Prevents excessive flexing in critical areas
  2. Protects components and solder joints from stress
  3. Improves overall reliability of the board
  4. Allows for easier handling during assembly

Design Considerations

Layout and Routing

  1. Keep critical components on rigid sections
  2. Minimize the number of layers transitioning from rigid to flex
  3. Use tear-drop pads for improved copper adhesion
  4. Avoid 90-degree angles in flex areas

Material Selection

  1. Choose appropriate flex materials (e.g., polyimide)
  2. Select adhesives compatible with flex requirements
  3. Consider thermal management needs

Bend Radius

  1. Calculate minimum bend radius based on material thickness
  2. Design flex areas to accommodate required bend radius
  3. Use bend relief features to reduce stress

Manufacturing Process

Step-by-Step Manufacturing

  1. Material preparation and cutting
  2. Drilling of holes and vias
  3. Copper plating and etching
  4. Lamination of rigid and flex sections
  5. Application of solder mask and silkscreen
  6. ENIG surface finish application
  7. Pi Stiffener attachment
  8. Final inspection and testing

Quality Control Measures

  1. X-ray inspection for internal layer alignment
  2. Electrical testing for continuity and shorts
  3. Bend testing for flex areas
  4. Microsection analysis for layer integrity


Industries Utilizing 4 Layer Rigid Flex Boards

  1. Aerospace and defense
  2. Medical devices
  3. Automotive electronics
  4. Consumer electronics
  5. Telecommunications

Specific Use Cases

  1. Wearable devices
  2. Implantable medical devices
  3. Satellite and space applications
  4. Foldable smartphones and tablets
  5. Automotive dashboard displays

Advantages and Challenges


  1. Reduced assembly time and costs
  2. Improved reliability due to fewer interconnects
  3. Design flexibility for complex geometries
  4. Weight reduction in final products
  5. Enhanced performance in high-frequency applications


  1. Higher initial manufacturing costs
  2. More complex design process
  3. Potential for delamination in flex areas
  4. Limited availability of specialized manufacturers

Future Trends

Emerging Technologies

  1. Integration with 3D printing
  2. Incorporation of stretchable electronics
  3. Development of biodegradable flex materials
  4. Advancements in nanomaterials for improved flexibility

Potential Improvements

  1. Increased layer count in flex areas
  2. Enhanced thermal management solutions
  3. Improved manufacturing processes for cost reduction
  4. Development of new surface finishes for specific applications


The 4 Layer Rigid Flex Board with ENIG process and Pi Stiffener represents a significant advancement in PCB technology. By combining the benefits of rigid and flexible circuits, along with the durability of ENIG finish and the support of Pi Stiffeners, these boards offer a versatile solution for modern electronic devices. As technology continues to evolve, we can expect further innovations in this field, pushing the boundaries of what’s possible in electronic design and manufacturing.


Q1: How does the cost of a 4 Layer Rigid Flex Board compare to traditional rigid PCBs?

A1: 4 Layer Rigid Flex Boards are generally more expensive than traditional rigid PCBs due to the complexity of the manufacturing process and the specialized materials used. However, they can lead to overall cost savings in product assembly by eliminating the need for multiple boards, connectors, and cables.

Q2: What is the typical lifespan of a 4 Layer Rigid Flex Board with ENIG finish?

A2: The lifespan of a 4 Layer Rigid Flex Board with ENIG finish can vary depending on the application and environmental conditions. However, ENIG finish typically offers a shelf life of 12-18 months before soldering, and the flex portions can withstand hundreds of thousands of flex cycles when properly designed.

Q3: Can 4 Layer Rigid Flex Boards be repaired if damaged?

A3: Repairing 4 Layer Rigid Flex Boards can be challenging, especially in the flex areas. Minor repairs on the rigid sections may be possible, but damage to the flex portions often requires replacement of the entire board. It’s crucial to handle these boards with care during assembly and use.

Q4: How does the Pi Stiffener affect the flexibility of the board?

A4: The Pi Stiffener is designed to provide localized rigidity while still allowing for flexibility in designated areas. It reinforces specific regions of the flex portion, typically near connector interfaces or component mounting areas, without significantly impacting the overall flexibility of the board.

Q5: Are there any special storage requirements for 4 Layer Rigid Flex Boards with ENIG finish?

A5: 4 Layer Rigid Flex Boards with ENIG finish should be stored in a clean, dry environment with controlled temperature and humidity. They should be kept in antistatic packaging to prevent ESD damage. While ENIG finish provides good protection against oxidation, it’s best to use the boards within the manufacturer’s recommended shelf life for optimal solderability.