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 What is The Flex PCB Stiffener ?

What is The Flex PCB Stiffener ?

Introduction:

Flex PCB stiffeners serve to make parts of a flexible PCB to soldering components easier. It is important to note that flexible PCB stiffeners are not a component of the PCB circuit. Because the usage of Flex PCB stiffeners would affect the total cost of the FPC. Please follow GESFLEX to learn how to choose flex PCB stiffeners in this article.

flexible PCB PI STIFFENER

What is The Flex PCB Stiffener ?

A flex PCB stiffener is a component or material used in flex PCBs. To provide support, reinforcement, and rigidity to specific areas of the board. Flex PCBs designed to be flexible and bendable. allowing them to conform to irregular shapes and fit into tight spaces. However, in some applications, it may be necessary to add stiffness. to certain regions of the flex PCB to protect delicate components, improve durability. or ensure that the board maintains its desired shape during use.

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Benefits of Flex PCB Stiffener

Flex PCB stiffeners offer several benefits in electronic and electrical applications:

why we need FLEX PCB STIFFENER

※ Mechanical Support:

The primary benefit of flex PCB stiffeners is to provide mechanical support. and rigidity to specific areas of the flexible board. This is particularly useful when there are components and connectors. or other sensitive elements that need protection from bending, flexing, or mechanical stress.

※ Component Protection:

Stiffeners help protect delicate or sensitive components (such as integrated circuits or connectors). from physical damage or stress caused by bending or flexing. This can improve the overall reliability and longevity of the PCB and the components it carries.

※ Improved Durability:

By reducing the risk of mechanical damage. stiffeners contribute to the increased durability and robustness of flex PCBs. making them suitable for applications with demanding environmental conditions or repeated flexing.

※ Maintaining Shape:

In some applications, it’s essential to maintain a particular shape or form for the flex PCB. Stiffeners help preserve the desired shape and prevent deformation during use.

※ Enhanced Electrical Performance:

Stiffeners can help maintain consistent electrical performance. by reducing the risk of signal interference. or impedance changes caused by flexing or bending of the PCB.

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Form of Flex PCB Stiffener

Rigid Substrates:

These are solid, non-flexible materials that added to the flexible PCB to provide rigidity in specific areas. Common rigid substrate materials include FR-4 (a type of fiberglass epoxy laminate). polyimide, or other rigid laminates. These are typically attached to the FPC using adhesives or by lamination during the PCB manufacturing process.

Stiffener Bars or Beams:

These are narrow strips of rigid material. often made of the same materials as rigid substrates. which attached to the flexible PCB along specific edges or regions. They provide structural support to prevent bending or flexing in those areas.

Rigid Components:

In some cases, rigid components like connectors or connectors with built-in stiffeners used. These components mounted on the FPC and provide rigidity where they attached.

Kapton Tape:

Kapton tape is a high-temperature-resistant polyimide film with adhesive backing. It used to reinforce and stiffen specific areas of a flex PCB by applying it to the flexible substrate.

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Types of Materials of Flex PCB Stiffener

Flex PCB stiffeners made from various materials. each with its own set of characteristics and advantages. The choice of material depends on the specific requirements of the application. and the design of the flexible printed circuit board (flex PCB). Some common materials used for flex PCB stiffeners include:

Polyimide (PI):

Polyimide is a popular material for flex PCB stiffeners and is often used in flexible circuitry itself. It known for its excellent thermal stability, high-temperature resistance, and flexibility. Polyimide stiffeners can withstand harsh environments and extreme temperatures.

PI stiffener is commonly used to improve the thickness of a ZIF connector. where the gold fingers make contact and provide enough support. As a result, tolerances in bending thickness. and component profile at the contact are possible. In the finger area, bend thickness tolerances are typically 0.002″ with a width profile of 0.003″. For solder resistance and good bond strength, PI stiffener is an excellent choice. The backside of the implanted gold finger is the most preferable for stiffener.

flex PCB stiffener- PI stiffener

FR-4 (Glass Epoxy):

FR-4 is a common material used in rigid PCBs, but it can also used as a stiffener in flex PCBs. It offers good rigidity, mechanical strength, and excellent electrical properties. FR-4 stiffeners are especially useful when a portion of the flex PCB needs to remain rigid.

The FR4 reinforcement holes are the same size as the FPC holes. These holes would be extremely difficult to install and might result in misalignment. To ensure correct alignment, GESFLEX, a leading manufacturer of flexible circuits. constantly ensures that the holes in the FR4 stiffeners are 0.3 mm larger than the appropriate holes in the FPC.

flex PCB stiffener- FR4 stiffener

Stainless steel stiffener

At the same time, stainless steel stiffener is commonly used in flexible circuit designs. that require corrosion resistance, performance, robustness, and formability. Stainless steel stiffener in flexible circuits is always added to the connecting fingers. to reinforce the flexible printed board for easy assembly.

flex PCB stiffener-steel stiffener

PET (Polyethylene Terephthalate):

PET is a flexible and lightweight material that used as a stiffener. It is often used in consumer electronics. and applications where weight and flexibility are important considerations.

Polycarbonate:

Polycarbonate known for its impact resistance and optical clarity. It used as a stiffener when transparency that required. or when additional impact resistance needed.

PVC (Polyvinyl Chloride):

PVC is a cost-effective material used in various applications. including as a stiffener for flex PCBs. It offers good flexibility and rigidity. but it may not be suitable for high-temperature environments.

Adhesive Films:

Stiffeners made from adhesive films that combine a layer of adhesive. with a reinforcing material, such as polyester or fiberglass. These adhesive films applied to the flexible PCB to add rigidity.

Kapton Tape:

Kapton tape, made from polyimide. can also serve as a flexible stiffener when wrapped around the flexible PCB in specific areas. It is often used in applications where flexibility is essential. such as aerospace and electronics.

Thermoplastics:

Various thermoplastic materials, including ABS (Acrylonitrile Butadiene Styrene) and PVC. That heated and molded into the desired shape. making them suitable for customized stiffeners.

Flex PCB Stiffener Thickness

For designers, the thickness of PCB stiffeners is an important factor to consider because it affects the PCB specs, especially mil-specs.

  • PI stiffener thickness: 0.001″ (25μm), 0.002″ (50μm), 0.003″ (75μm) and 0.005″ (125μm) available;
  • FR-4 stiffener thickness: 0.003″ (0.08mm) – 0.125″ (3.18mm);
  • Stainless steel/aluminum stiffener thickness: 0.1 mm (4 mils) – 0.45 mm (18 mils).
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Design Considerations of Flex PCB Stiffener

When designing a flexible printed circuit board (flex PCB) with stiffeners. There are several important considerations to keep in mind. to ensure that the stiffeners perform their intended functions effectively. Here are some key design considerations for flex PCB stiffeners:

flex PCB Stiffener Placement and Size

Stiffener Placement and Size:

Determine where stiffeners needed and the appropriate size and shape for each stiffener. Stiffeners placed along specific edges, near components. or in areas where additional rigidity required.

FLEX pcb Material Selection

Material Selection:

Choose the right material for the stiffeners based on the application’s requirements. Consider factors such as thermal properties, electrical insulation, and environmental conditions. Polyimide, FR-4, PET, and other materials used.

flex PCB Thickness

Thickness:

Determine the appropriate thickness of the stiffeners based on the degree of rigidity. That needed and the flexibility requirements of the flex PCB. Consider the size and weight of components that require support.

FLEX PCB Adhesion

Adhesion:

Select the appropriate adhesive method or material to attach the stiffeners to the flexible PCB. Common adhesive methods include pressure-sensitive adhesive (PSA), heat-activated adhesive, and epoxy adhesives. Ensure that the adhesive chosen is compatible with the flex PCB material.

Tolerance and Alignment

Tolerance and Alignment:

Pay attention to tolerances and alignment when attaching stiffeners. Precise placement is crucial to ensure the stiffeners provide the intended support. and do not interfere with other components or connections on the PCB.

Corners and Cutouts

Corners and Cutouts:

When using stiffeners with cutouts or notches. ensure that the corners are adequately rounded. or chamfered to prevent stress concentration, which can lead to cracking or delamination.

Cutout Shapes

Cutout Shapes:

Design cutouts in the stiffeners to accommodate components, connectors. or areas where the PCB needs to flex. The shape and size of cutouts should match the components they are protecting.

Bend Radius

Bend Radius:

Consider the minimum bend radius of the flex PCB when designing the stiffeners. Ensure that the stiffeners do not impede the natural bending or flexing of the PCB. The stiffener should not cause stress concentration points that could lead to cracking.

FLEX PCB Thermal Considerations

Thermal Considerations:

If the PCB will experience temperature fluctuations. choose a stiffener material that can withstand those temperatures without deformation or degradation. Polyimide stiffeners are often suitable for high-temperature environments.

FLEX PCB Cost and Manufacturing Constraints

Cost and Manufacturing Constraints:

Keep in mind the cost implications and manufacturability of the design. Some materials and manufacturing methods may be more expensive or complex than others.

flex PCB test

Testing and Prototyping:

Test the design and prototype with the chosen stiffeners. to ensure that they meet the mechanical and electrical requirements of the application. Evaluate their impact on the overall performance of the flex PCB.

Compliance and Standards

Compliance and Standards:

Ensure that the design complies with industry standards. and regulations relevant to the specific application. especially if the PCB used in safety-critical or regulated environments.

It’s important to collaborate closely with PCB designers. and manufacturers with experience in flex PCBs. to ensure that the stiffeners integrated effectively into the design. Prototyping and testing can help identify and address any issues before mass production.

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Application of Flex PCB Stiffener

Display Driver FPCs

Reinforcing bars reinforce the area under the driver ICs and connectors to prevent damage to devices. such as cell phones, tablets, televisions, etc. from handling and vibration.

Battery FPC Interconnects -

Reinforcement is essential for battery connector FPCs. as it can withstand vibration and shock over the long life of the device.

Medical Products -

For patient monitoring patches, implants and wearable devices. reinforcement enables robust interface connectors while maintaining sensor flexibility.

Automotive FPCs -

The use of reinforcement in wiring and sensors prevents damage. from constant vibration and thermal cycling over the life of the vehicle.

Robotics -

Frequent movements of robot arms and grippers require localized reinforcement of flexible cables. connecting movable parts to extend mechanical durability.

Consumer Electronics -

In smartphones, laptops, and portable electronics, rigid reinforcement. reinforces FPC connections to printed circuit boards to withstand drops and shocks.

Industrial -

FPCs used in motion control systems, pneumatic valves, and instrumentation. and more use reinforcements to create stable component mounting interfaces and robust wiring.

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FPC Stiffener Attachment Methods

Attaching stiffener to a flex PCB is a critical step in the design and manufacturing process. The attachment method chosen should provide secure bonding. while maintaining the mechanical and electrical integrity of the FPC. There are several methods for attaching FPC stiffeners, including:

Adhesive Bonding:

Pressure-Sensitive Adhesive (PSA): PSA is a common and straightforward method. It involves applying a double-sided adhesive tape with one side. adhering to the stiffener and the other to the flexible PCB. The stiffener is then carefully positioned and pressed onto the FPC to create a bond.

  • Epoxy Adhesive: Epoxy adhesives offer a strong and permanent placed onto the FPC. After curing, the epoxy hardens and securely attaches the stiffener to the PCB.
  • Heat-Activated Adhesive: Some adhesives require heat to activate the bonding process. This can involve a thermal press or a heated lamination process. where the adhesive is pre-applied to either the stiffener or the FPC. and then heat and pressure applied to bond them together.

Soldering:

For applications where electrical connections maintained across the stiffener, soldering is an option. Tabs or contacts on the stiffener soldered to corresponding pads on the FPC. To ensure that the soldering process does not damage the flexible components.

Mechanical Fasteners:

In cases where the stiffener needs to be removable or repositionable. mechanical fasteners like screws, bolts, or rivets used. These fasteners are typically attached through holes in the stiffener and the FPC.

Rivet or Eyelet:

A small metal eyelet or rivet inserted through holes in both the stiffener and the FPC. creating a secure mechanical connection. This method is commonly used in high-stress applications.

VHB Tape (Very High Bond): VHB tape is a type of acrylic foam tape with excellent adhesive properties. It provides a strong and durable bond between the stiffener and the FPC.

Velcro or Hook-and-Loop Fasteners:

In some applications, Velcro or similar hook-and-loop fasteners used to attach the stiffener. This method allows for repositioning and replacement of the stiffener.

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Conclusion

FPC stiffeners enable flexible circuits to function more efficiently in the assembly process. Depending on the final design, there are the various combinations of FPC stiffeners. If you are unsure how PCB reinforcement will work with your flexible PCB design. Please contact GESFLEX and our experts will advise how to effectively use stiffener. Contact GESFLEX today via e-mail at marketing@gespcb.com for more information.

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