Do you know what PCB stiffeners are? They are widely used in flex and rigid-flex circuit boards. Though flexible PCBs have remarkable adaptability and can be bent, folded, and twisted to meet complex design requirements, their flexibility creates challenges when assembling components on the board, making interconnect integration more difficult. This is where PCB stiffeners come into play, providing targeted structural support and enhancing the board’s functionality.
What Is Stiffener in PCB?
First, we need to understand that PCB stiffener is not an inherent part of the circuit board, instead, it is a layer of material added to reinforce or strengthen the PCB. Generally, it is used to the areas that require rigidity or structural support. Importantly, it helps resolve manufacturing and operational issues faced by flexible circuits, and finally leads to the improvement of their entire performance and reliability.
3 Types of PCB Stiffeners
According to the different materials of stiffeners, they can be divided into 3 types:
-
Polyimide (PI) Stiffeners
Polyimide stiffeners are a specialized solution for flexible circuit design, widely used in applications demanding precise thickness and lightweight properties, such as ZIF (Zero Insertion Force) connector applications. Their major advantage is that they provide targeted reinforcement while maintaining the flexibility of the circuit. Moreover, PI flex PCB stiffeners ensure reliable connector engagement and optimal electrical performance, which are critical for precision and adaptable applications.
-
FR-4 Stiffeners
For flexible circuits with complex component configurations, FR-4 stiffeners are the standard reinforcement method. Their primary purpose is to avoid circuit bending near sensitive areas, to protect delicate solder joints from mechanical stress. FR-4 stiffeners possess a broader thickness range and a more versatile application, which mainly used for consumer electronics and industrial equipment.
-
Specialized Metal Stiffeners: Aluminum and Stainless Steel
Aluminum stiffeners are excellent heat dissipation materials, suitable for applications where efficient thermal management is important. And stainless steel stiffeners are particularly useful in space constrained designs, providing superior component area support beyond traditional FR-4 options. These metal stiffeners are more expensive, and take more time to make, but have terrific benefits for electronic specialized applications that need extraordinary mechanical strength and thermal performance.
Common Uses of PCB Stiffeners
PCB board stiffeners serve multiple essential purposes in flexible circuit design:
- Mechanical Reinforcement: They provide critical support forarea that need to assemble PTH and SMT components.
- Thickness Maintenance: Flex circuit stiffeners help maintain consistent circuit thickness.
- Component Support: Offer support for a wide variety of PCB componentsand connectors.
- Flexibility Constraint: Stiffeners can restrict flexible sections to predetermined rigid areas as design requirements dictate.
- Handling Improvement: They facilitate easier manipulation of thin and delicate circuit boards.
- Stability Enhancement: Certain areas of flexible circuits can be kept flat and stable using strategic stiffener placement.
- Connector Compatibility: They help meet Zero Insertion Force (ZIF) connector specifications.
- Stress Mitigation: Stiffeners can increase bend radius at rigid-flex intersections, reducing stress during repeated bending operations.
How to Attach Stiffeners to a Flexible PCB?
There are 2 primary methods to attach stiffeners to flexible printed circuit boards: Thermal Bonding and Pressure Sensitive Adhesives (PSA). Each of them has distinct features and applications as the below chart shows:
| Characteristic | Thermal Bonding | Pressure Sensitive Adhesives (PSA) |
| Bonding Method | Utilizes a combination of heat and pressure to create a permanent bond | Relies exclusively on pressure application |
| Bond Strength | Produces the most secure and durable connection between stiffener and flexible circuit | Provides a functional but less robust connection compared to thermal bonding |
| Cost | Higher Cost | Lower Cost |
| Processing Time | Longer | Shorter |
| Applications | · Military equipment
· Aerospace systems · Critical medical devices · Telecommunications infrastructure |
· Television manufacturing
· Computer and laptop production · Consumer electronic devices · General commercial electronics |
| Advantages | · Maximum structural integrity
·Exceptional resistance to environmental stressors · Ideal for applications requiring extreme durability |
· Lower implementation costs
· Faster attachment process · Easier stiffener removal with minimal circuit damage · More adaptable to various design requirements |
| Limitations | · Higher implementation costs
· More time-consuming process · Potential for significant circuit damage during stiffener removal · Requires specialized equipment and precise temperature control |
· Less permanent bond
·Reduced long-term reliability · May not withstand extreme environmental conditions |
The Difference Between Rigid-Flex and Rigidized Flex PCBs
Are you confused about the difference between rigid-flex and rigidized flex PCBs? Many people wonder if they are the same, but the answer is NO. They have different designs and functionalities:
Rigidized flex circuits are basically flex PCBs strengthened by FR4 stiffener to support mechanics assembly but with no electrical trace on the rigid parts. In contrast, rigid-flex PCBs are sophisticated hybrid circuits, which a combination of a rigid and a flexible substrates to form a single laminated structure. And there are electrical traces and interconnections in both a rigid and flexible piece.
Rigidized flex PCBs are primarily intended for mechanical reinforcement, and rigid-flex PCBs deliver total electrical connectivity and structural integration, thus suitable for more complicated electronic applications involving both electrical performance and structural adaptability.
Recommend reading: Rigid Flex PCB vs. Flexible PCB
Typical Stiffener Thicknesses
Selecting the right stiffener thickness is crucial as it affects board flexibility, performance and reliability. Stiffener thicknesses in PCBs vary depending on the application and the type of material used:
- Polyimide Stiffeners: Normally thinner to maintain flexibility, with layers that span from as thin as 25μm to 125μm (0.025mm to 0.125mm). They also offer a custom thickness range from 0.025mm to 0.25mm for flexible PCB designs.
- FR4 Stiffeners: These stiffeners are designed with rigidity in mind and usually with a broader range of thicknesses (0.08mm to 3.18mm).
- Metal Stiffeners: For heavy duty support, aluminum or stainless steel stiffeners havethicknessesgenerally ranging from 0.1mm to 0.45mm to retain robust mechanical strength in certain areas.
Bottom Line
PCB stiffeners represent a sophisticated solution in modern flexible circuit design, helping to enhance overall functionality, streamline the assembly process, and create a more reliable product. If you’re walking the gauntlet of flexible circuit design, consulting with experienced PCB manufacturers like MOKO Technology can assist in selecting and implementing the most suitable stiffener option. If you are interested to hear more or would like a quote for flex PCB with stiffeners, contact us now!
