PCB Stiffeners: An Essential Requirement for Flexible Circuits

A printed circuit board stiffener plays an important role when it comes to offering mechanical support to a PCB. They are particularly helpful for flexible circuit boards, that given their name, are flexible and in need of stiffness in certain places. Stiffeners are particularly required when components are placed in a flex zone & the weight of these components stresses the flex material. These components provide a rigid connection from the flexible portion to the solid board. This solidity makes applying solder for interconnects or components easier and completes the circuit run through the flexible part.

They are also of use where a rigid printed circuit board surface needs to be created to place SMT pad components. Also, connectors that need multiple insertions need stiffeners so that the stress of the pads is reduced. Let us look at the uses of stiffeners in some more detail. A flexible PCB requires a stiffener when:

• The Flex needs to connect to another board or power source.
• You need to attach a component to the flexible material of the PCB.
• The attached components will place too much weight on the flexible PCB.
• Connectors requiring multiple insertions might stress the connection pads.

Role of Stiffener in Flex PCB Design

A stiffener in a flexible printed circuit (FPC) is a rigid layer, typically made of materials like polyimide or FR4, added to specific areas of the flexible PCB. It provides mechanical support and reinforcement to prevent bending or flexing in delicate regions, such as connector areas or mounting points. Stiffeners improve durability, enhance reliability in high-stress areas, and help maintain the overall form factor of the FPC in critical sections.

What are the Various Uses of PCB Stiffeners Materials?

The type of stiffener your PCB needs depends on many factors. There are three types of stiffeners, each with its pros and cons. Each type of stiffener has a typical range of thicknesses available. The thicker the required stiffener, the more it will affect the overall design of the PCB and circuit. Let's take a look at each one.

1. FR4 Stiffeners

FR4 is the same substrate that you typically use to build PCBs. It is a solid material made of epoxy resin, reinforced fiberglass, and copper. For this reason, it is also the most widely-used material for stiffeners. FR4 thickness ranges in thickness between 0.003" and 0.125". The main benefits of FR4 stiffeners are:

• Provide solid, flat support during pick-and-place and reflow assembly processes.
• Facilitate easy application of Plated Through Hole (PTH) connectors due to their solid, conductive material.
• Offer a cost-effective solution, allowing flexible PCBs to seamlessly integrate with solid PCBs of similar material.

2. Polyamide (PI)

Polyamide stiffeners are made by creating thin, firm layers with high-temperature polymers, such as Kapton. These polymer layers can be stacked up to create a much thicker, firmer stiffener, depending on the requirements of the final product. Polyamide stiffeners are usually available in thicknesses of 0.001", 0.002", 0.003", and 0.005". The main reasons for using a PI stiffener are:

• Excellent for creating solder resistance and high bond strength.  
• Higher layer count limits PCB bending, making assembly easier.  
• Adds wear resistance to hole locations, reducing long-term damage.  
• Easily adaptable for further insertions like ZIF connectors or sockets.

3. Aluminum/ Stainless Steel

Though much more expensive than either PI or FR4, stainless Steel and aluminum provide many benefits that make them ideal for challenging circuits. Aluminum/Stainless Steel thickness varies between 0.1mm and 0.45mm. Some of these benefits include:

• Much tougher and more robust security in the connector
• Resistance to corrosion
• Greater formability for components
• Stiffener Thickness

Design Considerations for PCB Stiffeners

When it comes to attachment of a stiffener to a circuit, heat and pressure is used. A pressure-sensitive adhesive can also be used to attach PCB stiffeners.  While incorporating stiffeners into PCB design, several factors must be considered:

Board Size and Thickness: Larger and thinner boards generally require more substantial stiffening.

Component Layout: The distribution and weight of components on the PCB affect the required stiffener placement and design.

Environmental Conditions: Temperature fluctuations, humidity, and vibration levels in the intended operating environment must be accounted for.

Electrical Performance: Stiffeners should not interfere with the electrical characteristics of the PCB, such as signal integrity or EMI shielding.

Manufacturing Processes: The stiffener design must be compatible with the PCB manufacturing and assembly processes.

Cost Considerations: The choice of stiffener material and design complexity impacts the overall cost of the PCB.

Integration with PCB Design

Integrating stiffeners into the PCB design process involves several steps:

1- Early Planning: Consider stiffener requirements during the initial PCB layout phase.

2- Material Selection: Choose stiffener materials compatible with the PCB substrate and intended application.

3- Thermal Analysis: Evaluate the impact of stiffeners on the thermal characteristics of the PCB.

4- Mechanical Simulation: Use finite element analysis (FEA) to optimize stiffener design and placement.

5- Manufacturing Considerations: Ensure the stiffener design is compatible with PCB fabrication and assembly processes.

6- Testing and Validation: Conduct prototyping and testing to verify the effectiveness of the stiffener design.

How to Attach a Stiffener

Stiffeners are attached to a flexible PCB on the same side as the components when using plated through hole assembly components. This allows access to the solder pads underneath the connector without requiring disassembly, making it easier to attach a second stiffener on the other side, if necessary.

The stiffeners are then sealed in place using either thermal bonding or pressure-sensitive adhesives.

Thermal bonding: The stiffener effectively melts in place using heat and pressure without damaging any components. This provides a much stronger bond making them more secure when used. However, this method is a more expensive and time-consuming option. You cannot remove the stiffener without significant damage to the PCB and its components.

Pressure Sensitive-Adhesives: This method only uses pressure to affix the stiffener, which is cheaper and quicker, though less permanent. As a result, this option is ideal for consumer electronics, and you can remove them fairly easily if issues arise.

Stiffener Placement and Configuration

The effective placement of stiffeners is crucial for optimal performance. Common configurations include:

Full Board Stiffeners: Cover the entire PCB area, providing maximum support but potentially increasing weight and cost.

Partial Stiffeners: Applied to specific areas of the PCB that require additional support, such as areas with heavy components or prone to flexing.

Edge Stiffeners: Reinforcing the edges of the PCB to prevent bending and improve handling during assembly.

Localized Stiffeners: Small, targeted stiffeners used to support specific components or areas of concern.

Designing Tips for Rigid-Flexible Printed Circuit Boards

1. Keep the max bending radius in mind while working with FPC of different thickness.

2. In areas where the substrate or overlay bends, avoid discontinuities.

3. Tear drops can be added to make the through holes more stable and rigid. To know more about how to add teardrops to your design have a look on this EasyEDA tutorial page.

4. Use a stiffener under the main electronics circuitry of the PCB.

5. Make the corners round and a little bigger to avoid any corner tear of the PCB.

6. Around stiffeners, it allowed etched markings, while on the other hand, silkscreen markings are not.

For more design related information, see a full article covered on an Arduino board fully flexed PCB.

The Difference Between Rigid-Flex and Rigidized Flex PCBs

How you use stiffeners will affect the type of PCB you get. These are known as either Rigid-Flex or Rigidized Flex PCBs. Improving PCB rigidity requires careful planning based on the assembly's function and support needs, ensuring the right stiffener is used for the task. The differences between these are:

• Rigid-Flex: Rigid sections with traces connected via holes; stiffeners provide electrical connections and become functional parts of the PCB.

• Rigidized Flex: Stiffeners only provide mechanical support without impacting PCB functionality, aiding assembly and adding strength.

Conclusion:

The advantage of using a PCB stiffener when we need a rigid area in the circuit, in order to protect components or connectors, cannot be overstated. By doing so you can ensure that the circuit doesn't bend and that the integrity of solder joints is protected. It is common to use FR4 as stiffener material for rigidizing specific areas of the circuit.

With over 3 decades of experience in rigid-flex PCBs with differing levels of complexities, our team of experts and state-of-the-art equipment ensure that all our PCBs meet the industry standards of quality and testing. Get the best PCB Quote for your designs.