Rigid Flex PCB

Rigid Flex PCB Design Considerations

Designing a successful rigid-flex PCB(R-FPCB) requires careful consideration of various factors to ensure optimal performance, reliability, and manufacturability. Here are some key considerations to keep in mind:

a. Bend Radius: The bend radius is the minimum radius at which the flexible portion of the PCB can bend without damaging the traces or the substrate. It is essential to define the bend radius carefully based on the chosen substrate material and application requirements.

b. Layer Stack-up: The layer stack-up determines the arrangement of flexible and rigid layers in the PCB. It should be designed to accommodate the required connections while maintaining mechanical integrity. Proper stack-up design helps prevent issues like cracking, delamination, or impedance mismatches.

c. Component Placement: Careful component placement is crucial to ensure that components fit within the rigid portions of the PCB and do not interfere with the flexible sections. Consider the 3D spatial constraints of the enclosure and the flex-to-installation ratio.

d. Trace Routing: Routing traces on a rigid-flex PCB requires careful planning to avoid overstretching or compressing the flexible sections during bending. Maintain adequate clearance between traces and vias to prevent short circuits during bending.

e. Copper Weight and Plating: Select the appropriate copper weight and plating thickness for the PCB based on the current-carrying capacity, signal integrity, and mechanical requirements of the design.

f. Thermal Management: Effective thermal management is essential, especially in designs with components that generate heat. Heat sinks, thermal vias, and appropriate trace widths can help dissipate heat efficiently.

g. Reliability Testing: Rigorous testing and simulation are essential to ensure that the rigid-flex PCB can withstand environmental conditions, repeated bending, and other stresses it may encounter during its lifecycle.

h. Design for Manufacturability (DFM): Work closely with QFPCB Rigid-Flex PCBs Manufacturer to ensure that the design is manufacturable. Consider factors like panelization, fiducial markers, and assembly processes.

i. Environmental Factors: Rigid-flex PCBs may be exposed to harsh environments in applications like automotive or aerospace. Designers must consider factors like moisture resistance, corrosion protection, and thermal cycling.

j. Material Selection: Selecting the right materials for both the rigid and flexible portions of the PCB is crucial. Different materials have varying thermal expansion coefficients, which can lead to reliability problems during temperature variations.

k. Signal Integrity and EMI: Maintaining signal integrity and controlling electromagnetic interference (EMI) in the flexible sections can be challenging. The flexibility of the substrate and the proximity of signals to the bend region can affect signal quality.

l. Connector Placement: Deciding where to place connectors to transition between the rigid and flexible sections is a critical design decision. Poor connector placement can lead to mechanical stress and reliability issues.

QFPCB Rigid Flex Boards Manufacturing Process

After the production of FPC flexible boards is completed, the production of QFPCB rigid flex Printed Circuit Boards can be completed through the following processes.

1. Punching

Drill holes on FR4 and PP film, and the design on the alignment hole is not the same as the general through hole. After the punching is completed, browning is required.

2. Riveting

Laminate copper clad laminates, PP adhesives, and FPC circuit boards and place them neatly. The original old process is to laminate and pressing step by step, but it is a waste of time. After many attempts, we found that the stacking process can be completed once.

3. Laminate

This is a relatively complete step in the production of rigid-flex pcb board. Most of the materials are integrated for the first time. First, the bottom layer is copper clad laminate and PP film, above are the FPC flexible board produced in the previous process, and a PP film is placed on the FPC flexible board, then place the last layer of copper clad laminate. All materials to be laminated are placed in order and pressed together.

4. Trimming

That is to remove the part of the circuit board where there is no circuit at the edge of the circuit board currently and in the future. Afterwards, it is necessary to measure whether the material has excessive expansion and contraction. Because the PI used in the production of flexible boards is also expansion and contraction, this has a very large impact on the production of circuit boards.

5. Drilling

This step is the first step to turn on the entire circuit board, and the production parameters should be produced according to the design parameters.

6. Desmear

First, remove the scum generated by the drilling of the circuit board, and then use plasma cleaning to clean the through holes and the board surface.

7. Immersion copper

This step is the process of electroplating through holes, also known as hole metallization. Realize through-hole power conduction.

8. Plate plating

Partially electroplating copper on the upper surface of the electroplating hole makes the copper thickness above the through hole exceed a certain height of the copper clad board surface.

9. Outer dry film positive film production

The same as the production process of the anti-corrosion dry film of the FPC board, the circuit to be etched on the copper clad board is made. After the development is completed, check the circuit.

10. Graphic plating

After the initial copper sinking, pattern electroplating is performed, and the current time and copper plating wire are used according to the design requirements to reach a certain electroplating area.

11. Alkaline etching

12. Print solder mask

This step has the same effect as the FPC board protective film. We see that the PCB rigid board is generally green. This step is generally called green oil printing. After the printing is completed, the inspection is carried out.

13. Open the cover

Cover opening, which is the area where the FPC board is located, but the area not needed by the rigid board is laser cut to expose the FPC board.

14. Curing

It is also a baking process.

15. Surface treatment

Generally, at this time, a rigid-flex PCB board has been manufactured, and only the metallization treatment is required on the surface of the circuit board, which can play a role in preventing wear and oxidation. Generally, this process is to soak the circuit board in a chemical solution, and the metal elements in the solution are densely distributed on the circuit board circuit.

16. Characters printing

The positions of the parts to be assembled and some basic product information are printed on the rigid-flex board in the form of characters.

17. Test

This is a process of checking whether the circuit board is qualified. The test items are tested for electrical properties according to customer requirements. The tests generally include impedance test, open and short circuit test and so on.

18. Final inspection

19. Packaging and shipping

There are many ways to package circuit boards. Generally, QFPCB use packaging bags to separate them, and then use a vacuum packaging machine to vacuum package the rigid-flex PCB boards .

Rigid Flex PCB Cost Factors

Rigid-flex PCBs tend to cost more than standard rigid PCBs due to the specialized materials, processes, and lower fabrication volumes. Here are some of the key factors that influence rigid-flex PCB pricing:

1. Layer Count

Adding more conductive layers increases material costs, lamination complexity, and fabrication difficulty. High layer count rigid-flex PCBs cost exponentially more than 2-4 layer versions.

2. Panel Utilization

Rigid-flex PCB panels often have lower utilization due to complex board geometries. Less PCB area per panel drives up cost. Tight panel layout Optimization is critical.

3. Finishing and Coatings

The specialized solder mask, coverlay, and surface finish add cost compared to baseline FR-4 finishing. Thick copper, buried vias, and other techniques also increase cost.

4. Flexible Material Type

The flexible dielectric material choices like polyimide, LCP, PEN drive cost. More durable and heat-resistant flex materials are more expensive.

5. Registration Accuracy

The precision alignment of layers and drilling/routing accuracy requirements affect cost. Tighter tolerances require advanced equipment and processes.

6. Design Complexity

Dense routing, high component counts, HDI features, and impedance control requirements increase fabrication difficulty and cost.

7. Low Volume

The overall smaller market for rigid-flex PCBs prevents economies of scale. Shorter fabrication runs increase cost per board.

8. Testing

Rigid-flex PCBs require extensive inspection and electrical testing to validate quality. This adds cost compared to basic PCB qualification.

In addition to fabrication costs, there are engineering costs associated with specialized rigid-flex design, simulation, prototyping, documentation, and qualification.

Commonly Used Materials in Rigid-Flex PCB Manufacturing

Rigid-flex PCBs are constructed using a combination of rigid and flexible materials, each playing a critical role in achieving the desired performance, reliability, and durability. The following are some of the commonly used materials in the manufacturing of rigid-flex PCBs:

1. Rigid PCB Materials

FR-4 (Flame Retardant-4):

• A widely used material for the rigid sections of rigid-flex PCBs.
• Made from woven fiberglass and epoxy resin, it offers high mechanical strength, excellent electrical insulation, and cost-effectiveness.
• Suitable for multi-layer designs due to its excellent heat resistance and dimensional stability.

Polyimide (PI):

• Used for enhanced thermal and mechanical properties in high-performance applications.
• Offers superior heat resistance compared to standard FR-4, making it ideal for demanding environments.

2. Flexible PCB Materials

Polyimide (PI):

• The most common substrate material for the flexible sections.
• Known for its flexibility, thermal stability, and excellent electrical properties.
• Thin and lightweight, making it suitable for compact and foldable designs.

Glued material: copper foil + adhesive layer + substrate:

Adhesiveless Copper-Clad Laminates:

• A flexible copper layer directly laminated onto the polyimide substrate without using adhesive.
• Provides better flexibility, reduces overall thickness, and improves reliability in high-frequency applications.

Glueless material: copper foil + base material:

SF202 Double Side Adhesiveless FCCL:

3. Adhesive Systems

Acrylic Adhesive:

• Used for bonding rigid and flexible layers.
• Offers good bonding strength and flexibility, suitable for medium-performance requirements.

Epoxy Adhesive:

• Provides higher heat resistance and dimensional stability compared to acrylic.
• Often used in high-performance or high-temperature applications.

Adhesiveless Construction:

• Some advanced rigid-flex PCBs use adhesive-free bonding for improved flexibility, thinner profiles, and enhanced electrical performance.

4. Conductive Materials

Copper is the most commonly used and readily available conductor material for rigid-flex circuit assembly. The material is preferred due to its benefits such as high workability and good electrical characteristics. For circuitry applications, two forms of copper foils are typically used – electro-deposited (ED) and rolled annealed (RA) cooper foil. Both these foil forms are available in various thickness and weights. They are subjected to surface treatment before they are used for the rigid-flex PCB assembly. Sometimes, constantan foil is adopted for constantan FPC fabrication. The foils are chemically treated to reduce bond degradation, increase adhesion, augment bond strength, and protect from oxidation.

Electro Deposited (ED) copper is manufactured by depositing copper foil using the electrolysis method. Electricity is passed through solutions containing copper compounds through cathode and anode. The copper from the anode gets oxidized and pure copper metal gets collected on the cathode. This pure copper from the cathode is then placed onto a rolling cylindrical metallic rod to form a thin layer of copper foil. This Thin layer of copper is brittle and has a rough surface which makes it rigid. Moreover, the rough surface of ED copper also causes a relatively high insertion loss when compared with RA copper, ED copper is most often used in rigid PCB boards.

Rolled Annealed (RA) copper foil is made by subjecting a copper strip through a rolling mill until it reaches its desired thickness. RA copper has a smoother surface which makes them ideal for use in flexible circuitry. The smoothness of the surface will cause low insertion loss of high-frequency signals. Insertion loss is kept to a minimum, to obtain the best performance.

RA Copper or Rolled Annealed copper is a popular choice for rigid-flex PCBs, flex circuit manufacturing, and bendable boards. RA Copper foil has excellent extensibility/flexibility up to 20% to 45% making it ideal for flexible PCBs. However, they are considerably more expensive than ED Copper.

5.Rigid-Flex PCB Surface: Coverlay vs. Solder Mask

The surface of rigid-flex PCBs is comprehensively coated using protective films, which is called protective coating. This helps rigid-flex PCB resisting chemicals, oils, hydrocarbon solutions, dust, and other contaminations. The protective coating is selected after understanding the types of materials used in the rigid-flex PCB assembly, compatibility of PCB components with the coating material, and most importantly the application areas. The most commonly used forms of coating are:

Coverlay: When a flexible film like polyester or polyimide is combined with a suitable adhesive, the resulting product is a cover layer (coverlay). Coverlay has three major roles to play in a rigid-flex PCB assembly: (1) to provide comprehensive protection to the entire assembly. (2) to access circuitry areas like circuit pads for further processing. (3) to augment the reliability and resilience of the circuitry.

Solder Mask: Unlike coverlay method, a thin coating of liquid acrylated epoxy and acrylated polyurethane solder mask ink is applied onto the circuitry surface. The liquid coating is applied using several methods, one of such is screen printing. The coating is then thermally cured. In some complex rigid-flex boards, the coverlay openings on flex parts are so dense, we suggest to use flexible solder mask.

6. Protective Coatings

Surface Finishes:

• Common finishes include ENIG (Electroless Nickel Immersion Gold), OSP (Organic Solderability Preservative), and Immersion Silver.
• These finishes enhance solderability, protect against oxidation, and improve signal performance.

Material Selection Factors:

When selecting materials for rigid-flex PCBs, several factors need to be considered:

• Thermal Requirements: Materials should withstand high temperatures during assembly and operation.
• Mechanical Durability: Especially in flexible sections, the material should tolerate bending, folding, and dynamic stress.
• Electrical Performance: Low dielectric constant (Dk) and low loss factor (Df) are crucial for high-frequency applications.
• Environmental Resistance: The materials should resist moisture, chemicals, and extreme environmental conditions.
• Cost Considerations: Balancing performance with cost efficiency for the intended application.

The material used greatly determines the quality and overall functioning of the rigid-flex boards. As mentioned earlier, PCB materials must be carefully chosen after analyzing several criterial including cost, shelf life, and electrical requirements of the printed circuit board, among others. This helps produce rigid-flex PCBs that provide many years of reliable and trouble-free service.

IPC Standards for Rigid and Flexible PCBs

The list of IPC standards below applies to rigid PCBs and flex circuits. Take note that this list is not exhaustive, and additional IPC standards may need to be considered. You should consult the ipc.org website for a full list of available IPC standards.

• IPC-2221A, Generic Standard on Printed Board Design
• IPC-2223, Sectional Design Standard for Flexible Printed Boards
• IPC-4101, Specification for Base Materials for Rigid and Multilayer Printed Boards
• IPC-4202, Flexible Base Dielectrics for Use in Flexible Printed Circuitry
• IPC-4203, Adhesive Coated Dielectric Films for Use as Cover Sheets for Flexible Printed Circuitry and Flexible Adhesive Bonding Films
• IPC-4204, Flexible Metal-Clad Dielectrics for Use in Fabrication of Flexible Printed Circuitry
• IPC-6013, Qualification and Performance Specification for Flexible Printed Wiring

Find a Rigid-flex PCB manufacturer in China? You can count on QFPCB, which supports rigid-flex PCB prototypes and mass production. For meeting quick-turn PCB requirement nowadays, our professionals can offer quickly after receipt of your inquiry. What’s more, we can attach the stack-up proposal and panelization in the offer no matter it’s simple or complex rigid-flex boards. This makes us different from most rigid-flex PCB manufacturers and trading companies.