PCB stiffeners are not an original part of the PCB but are additional components added to the circuit board to provide support. They serve multiple functions, including reinforcing specific sections of the PCB and facilitating easier handling of the circuit board. Stiffeners are used to enhance the durability of the PCB to address various challenges encountered during manufacturing and use. When using flexible PCBs, there are times when certain portions of the flexible PCB need to be rigid. This is achieved by adding mechanical support to specific areas of the PCB, which is referred to as PCB stiffeners.
In summary, PCB stiffeners are an essential tool for enhancing the performance and reliability of flexible circuit boards, especially in applications where specific sections require higher mechanical strength and stability.
Depending on the requirements of your flexible PCB project, you may need to consider various types of stiffeners. Understanding the different types of stiffeners can help you choose the appropriate type for your flexible PCB project. There are several types of PCB stiffeners:
Polyimide (PI) Stiffeners
Polyimide is a polymer that forms a thin film layer at high temperatures. Multiple layers of polyimide can be stacked to increase thickness. PI stiffeners are among the most commonly used stiffeners, known for their resistance to solder and high bond strength. They allow manufacturers to establish bend thickness and component contour tolerances at contact points during assembly.
They restrict the flexibility of the PCB areas needed for installation in the final assembly process. They are typically used in areas targeted for inserting gold finger holes or on the backside, providing additional resistance to wear.
Kapton is a common polyimide material widely used for PCB stiffeners. It is especially preferred when there is a need to increase the flexibility of the PCB's ends and insert Zero Insertion Force (ZIF) connectors. In such cases, Kapton provides the required support and stability, ensuring the performance and reliability of the PCB.
FR-4 Stiffeners
FR-4 is a woven glass fiber laminate filled with epoxy resin. It is the most commonly used material for flexible circuit stiffeners. FR-4 stiffeners are used because they provide flat support for flexible PCBs during pick and place and reflow processes.
To save costs during project assembly and allow plated through-hole (PTH) components direct access to solder pads, an FR-4 frame is added to the edges of the PCB, replacing SMT carriers.
Stainless Steel or Aluminum Stiffeners
Stainless steel or aluminum stiffeners are high-quality stiffeners used in flexible circuit designs that require formability, corrosion resistance, and durability. Adding stainless steel or aluminum stiffeners to flexible circuits enhances the assembly process by reinforcing the flexible PCB.
The PCBWay flexible PCB pricing page offers the following options for stiffener placement:
Stiffeners on Top Layer
Stiffeners on the top layer of a flexible PCB are used to provide structural support to the entire flexible PCB circuit board. These are typically employed when one side of the flexible PCB carries significant weight, such as when it is connected to devices like LED displays. Top layer stiffeners help evenly distribute the weight across both sides of the flexible PCB circuit board.
Stiffeners on Bottom Layer
Stiffeners on the bottom layer are used when additional strength is needed in a specific area of the flexible PCB board. For example, if a specific area with active components requires more support than other parts of the flexible PCB design, you can add stiffeners here to enhance mechanical support and prevent fatigue damage over time.
Stiffeners on both Top and Bottom Layer
Using both top and bottom layer stiffeners provides higher rigidity and strength to the flexible PCB. They are typically placed at critical locations on the circuit board and can be composed of various materials, including metal and plastic. A key step in designing a layout with multiple stiffeners is determining which areas of the flex PCB circuit require greater rigidity than others. This can be done through mechanical analysis of each area of interest or by examining positions where flexing is more likely to occur in the flexible PCB product.
In some cases, it is necessary to add stiffeners to both the top and bottom layers of the flexible PCB to enhance the overall structural integrity. This is particularly useful for applications that need to resist bending or compressive forces in different directions. For instance, if a flexible PCB needs to withstand mechanical stresses in multiple directions, adding stiffeners to both the top and bottom layers can evenly distribute these stresses, improving overall rigidity. A multi-layer stiffener layout can enhance the overall stability of the flexible PCB, which is crucial for applications that operate across various temperature ranges or experience vibration or impact. Certain applications may require higher rigidity in multiple directions, allowing for greater design flexibility to meet complex requirements when adding stiffeners to both the top and bottom layers simultaneously.
There are two primary methods for attaching PCB stiffeners to a flexible PCB – Thermal Bonding and Pressure-sensitive Adhesive (PSA). The table below highlights the characteristics and differences of each method:
Both of these methods have their own advantages and disadvantages, and the choice should be based on specific design requirements, application environments, and material characteristics. Thermal bonding provides a stronger mechanical connection, suitable for applications that require high strength and stability, but it may require specialized equipment. PSA is simpler and more cost-effective, suitable for some light-stress or removable applications, but it may be unstable in specific environments.
