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A Guide for Using PCB Stiffeners to Flex & Rigid-Flex PCBs

Views: 1288 Author: Site Editor Publish Time: 2023-08-02 Origin: Site

In the PCB assembly, flexible circuit designs have gained immense popularity due to their ability to fit into spaces where rigid boards pose challenges. Flex and rigid-flex PCBs offer a solution for compact devices, but they come with a drawback - the flexible parts lack sturdiness, making them susceptible to damage during component addition and soldering. However, there is a solution to this challenge - the introduction of PCB stiffeners. These essential components act as supporting mechanisms, ensuring the stability of flexible sections during assembly and soldering processes.

What is stiffener in Flexible PCB

A stiffener is a rigid material added to certain areas of a flexible PCB to strengthen it. Flexible PCBs are made of thin and flexible materials, which can bend excessively in some places.

The stiffener, usually made of materials like FR4 or polyimide, is attached to the flexible PCB with adhesives. It adds rigidity to specific regions, helping the PCB maintain its shape, support components, and prevent excessive bending during use.

Stiffeners are not electrical components but provide mechanical support to the PCB during assembly. They are used to make certain parts of the flexible PCB rigid when needed.

Advantages of Using PCB Stiffeners

Using PCB stiffeners offers several advantages, which enhance the overall performance and reliability of electronic assemblies. Here are the key advantages:

  • Mechanical Support: PCB stiffeners provide mechanical reinforcement to the circuit board, preventing excessive bending and flexing. This support is especially crucial in applications where the PCB may experience vibrations, shocks, or physical stress.

  • Component Protection: Stiffeners protect delicate or heavy components, such as connectors, ICs, or surface-mounted devices, from damage caused by bending or twisting of the flexible PCB. This ensures the longevity and reliability of the electronic assembly.

  • Connector Stability: Stiffeners help maintain the alignment and stability of connectors, ensuring reliable electrical connections and reducing the risk of intermittent or faulty connections.

  • Improved Rigidity: By adding rigidity to specific areas of the flexible PCB, stiffeners enhance the overall structural integrity, making the assembly easier to handle during manufacturing and installation processes.

  • Enhanced Heat Dissipation: In some cases, stiffeners can act as heat sinks, aiding in the dissipation of heat from heat-generating components, thereby contributing to better thermal management.

  • Higher Board Durability: The use of stiffeners prevents the flexible PCB from experiencing excessive wear and tear, resulting in a longer lifespan and increased durability of the electronic system.

  • Design Flexibility: PCB stiffeners allow designers to incorporate both flexible and rigid areas within the same PCB, offering a versatile design approach that optimizes both mechanical and electrical requirements.

  • Cost-Effective Solution: By adding localized reinforcement to a flexible PCB instead of using a fully rigid board, the overall manufacturing cost can be lower while still achieving the desired mechanical stability.

  • Wider Application Range: With the use of stiffeners, flexible PCBs can be applied in a broader range of industries and applications, including those that demand a combination of flexibility and rigidity.

PCB Stiffener Materials

PCB stiffeners usually use polyimide, FR-4, stainless steel, or aluminum as the construction material. Each technology has different characteristics and advantages and is suitable for different applications.

FR-4 Stiffeners

FR4 is a widely used rigid material in PCB manufacturing. FR4 stiffeners are typically made from the same glass-reinforced epoxy laminate material used to make rigid PCBs.

The primary purpose of using an FR4 stiffener is to:

  • Provide flat support when the flex circuit board is in the reflow and pick-and-place processes

  • Support areas with connectors attached

  • Hold the parts where plated-through hole components get inserted to direct access to the solder pad.

Fr-4 Stiffener Thickness: 0.2mm, 0.3mm, 0.4mm, 0.5mm, 0.6mm, 0.7mm, 0.8mm, 1.0mm, 1.2mm, 1.5mm.

Polyimide Stiffeners

Polyimide stiffeners are made from the same flexible material used in the PCB itself. They are well-suited for flexible PCB applications, ensuring that the stiffened areas remain flexible enough while providing the necessary reinforcement.

A Polyimide (PI) stiffener is commonly used to increase the thickness of ZIF connectors, offering full support where the fingers contact. It allows tolerances in bending thickness and contours the component at the contact.

PI stiffeners limit bending in specific PCB areas for easy installation in the final assembly, often enhancing hole locations for added wear resistance. They are preferred for their solder resistance and high bond strength, making them ideal for the backside of insertion gold fingers in flexible printed circuit connectors. PI stiffeners ensure secure connections and reliable performance.

Polyimide Stiffener Thickness: 0.05mm, 0.075mm, 0.1mm, 0.125mm, 0.15mm, 0.2mm, 0.25mm

Aluminum Stiffeners

Aluminum stiffeners offer lightweight yet robust support for flexible PCBs. They are particularly useful for applications that require heat dissipation as aluminum has good thermal conductivity.

Aluminum Stiffeners Thickness: 0.2mm, 0.25mm, 0.3mm, 0.35mm, 0.4mm, 0.5mm, 1.5mm

Steel Stiffeners

Steel stiffeners are sturdy and provide high mechanical strength. They are commonly used in applications where the PCB needs to withstand significant stress or weight.

Steel Stiffeners Thickness: 0.1mm, 0.15mm, 0.2mm, 0.25mm, 0.3mm, 0.35mm, 0.4mm, 0.5mm, 1.5mm

Choosing the Right PCB Stiffener for Your Design

Selecting the appropriate PCB stiffener is a critical step in ensuring the mechanical strength, reliability, and overall performance of a flexible PCB design. Making the right choice involves careful analysis of the mechanical requirements, material options, attachment methods, heat dissipation needs, and electrical considerations. Additionally, cost analysis, environmental factors, and adherence to industry standards play a vital role in the decision-making process.

Common Mistakes to Avoid When Using PCB Stiffeners

When using PCB stiffeners, it's essential to be mindful of potential mistakes that could compromise the performance and reliability of the flexible PCB assembly.

The most important point of common mistakes to avoid when using PCB stiffeners is ensuring proper placement and material selection to provide adequate mechanical support and thermal considerations for effective heat dissipation.

Additionally, attention to electrical connectivity, adherence to industry standards, and thorough prototyping and testing are crucial to avoid potential issues and ensure the reliability of the flexible PCB assembly.

By avoiding these common mistakes and following best practices in PCB design and assembly, engineers can ensure that PCB stiffeners effectively enhance the mechanical strength, stability, and reliability of flexible PCBs without introducing unintended issues or complications.

PCB Stiffeners vs. Other Reinforcement Techniques

PCB stiffeners are just one of several techniques used to reinforce flexible printed circuit boards (PCBs) and improve their mechanical strength and stability. Let's compare PCB stiffeners with other common reinforcement techniques:

Reinforcement TechniqueProsCons
PCB Stiffeners- Relatively simple and cost-effective.- May not offer uniform reinforcement throughout the entire PCB.
- Can be used in specific targeted areas.- Requires careful design to avoid interference with other components.
- Does not add excessive weight or thickness.
Polymeric Coatings or Potting- Provides comprehensive reinforcement and environmental protection.- Adds weight and thickness to the PCB.
- Suitable for rugged environments.- May require additional steps in the manufacturing process.
- Protects against moisture, dust, and chemical exposure.- May limit flexibility.
Reinforcement Tapes or Adhesive Films- Can be customized for targeted reinforcement.- May not offer as much support as stiffeners.
- Relatively easy to apply.- Requires careful attention to ensure proper application and adhesion.
Rigid-Flex PCBs- Offers comprehensive mechanical reinforcement and electrical connections.- More complex and costly to design and manufacture.
- Reduces assembly complexity and saves space.- May not be suitable for all applications.
Reinforced Conductor Traces- Integrates reinforcement directly into the circuit layout.- Limited to strengthening individual traces.
- Cost-effective for specific traces requiring more support.- May not address overall mechanical stability needs.

Attaching Your Stiffeners to Flex PCB

When it comes to reinforcing Flexible Printed Circuit Boards PCB stiffeners play a vital role in providing mechanical support and stability. Attaching these stiffeners to the flexible PCBs requires careful consideration of design constraints and application requirements. The two primary methods used for attachment are thermally bonding adhesives and double-sided PSA (Pressure-Sensitive Adhesive) tapes.

While thermal bonding adhesives offer a permanent and cost-effective solution, certain design limitations might necessitate the use of PSA tapes. For designs with heat dissipation needs, thermally conductive PSAs become a necessity. It is important to note that these PSAs should be applied after component assembly as they are not suitable for withstanding reflow temperatures.

Attachment MethodPressure Sensitive Adhesives (PSA)Thermal Bonding
DescriptionPSA (Double-sided tape) is applied to the stiffener, allowing for easy manual placement and bonding to the flexible PCB.The stiffener is laminated to the flexible PCB using a thermally activated adhesive, creating a permanent and durable bond.
Assembly ProcessQuick and simple manual application, no curing required.Involves a heating process to activate the adhesive and create a strong bond. It may require additional curing for maximum strength.
ReversibilityGenerally, PSA attachment is reversible, allowing for repositioning or replacement.Thermal bonding results in a permanent bond, making it difficult to remove or reposition the stiffener.
Mechanical StrengthAdequate for many applications, but may not offer the same level of mechanical stability as thermal bonding.Provides excellent mechanical support, enhancing the rigidity and durability of the flexible PCB.
Heat DissipationPSA tapes with thermally conductive properties can be used for heat dissipation needs.Thermally activated adhesives can facilitate heat transfer, enhancing heat dissipation capabilities.
CostRelatively lower cost due to simplified application and material expenses.Slightly higher cost due to the additional heating and curing process.
Application ConsiderationsIdeal for designs with design iterations or prototypes that may require repositioning or removal of the stiffener.Suited for applications where a permanent and robust bond is necessary, particularly in rugged or high-stress environments.
Electrical ConductivityCan be selected with conductive properties if electrical continuity is required.The adhesive used may or may not offer electrical conductivity; separate measures might be needed for electrical connections.
Environmental ConsiderationsMay require careful attention to environmental factors such as temperature and humidity affecting adhesive properties.The thermal bonding process may have specific environmental requirements for proper curing and bonding.

Difference between rigid-flex and rigidized PCBs

Rigidized flex PCBs and rigid-flex PCBs are two types of hybrid circuits that incorporate both rigid and flexible elements.

  • Rigidized flex PCBs use a flex PCB bonded with an FR4 stiffener to provide mechanical support to specific locations, strengthening the rigidity during assembly. In contrast, rigid-flex PCBs combine rigid and flexible substrates with traces on both parts, connected through holes to establish electrical connections rather than providing mechanical support.

  • Rigidized flex PCBs have no traces on the rigid part, while rigid-flex PCBs have traces on both rigid and flexible segments for electrical connectivity. These distinctions make each type suitable for different applications based on their unique mechanical and electrical requirements.

Conclusion

PCB stiffeners are crucial components in enhancing the mechanical strength and stability of flexible printed circuit boards. They provide targeted reinforcement to specific areas, preventing excessive bending or flexing during assembly and use. The correct selection and application of stiffeners can significantly improve the reliability and durability of electronic assemblies, ensuring optimal performance even in challenging environments.

At VictoryPCB, we take pride in being a professional PCB manufacturer with a strong focus on quality and customer satisfaction. If you're in need of high-quality PCB stiffeners or any other PCB solutions, we are here to help. Our experienced team is ready to assist you in finding the right stiffener materials, dimensions, and attachment methods to suit your specific design requirements. Contact us today to explore how our expertise and dedication to excellence can bring success to your PCB projects. Let's work together to achieve victory in your electronic endeavors!

About The Author

I am the Engineering and Sales supervisor working in Victorypcb from 2015. During the past years, I have been reponsible for all oversea exhibitions like USA(IPC Apex Expo), Europe(Munich Electronica) and Japan(Nepcon) etc. Our factory founded in 2005, now have 1521 clients all over the world and occupied very good reputation among them.

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