Stiffeners are critical mechanical features used in flexible circuits to add rigidity to specific areas without sacrificing the overall flexibility of the design. They help meet assembly thickness requirements, support components and plated features, improve handling, and create stable, flat regions that enhance reliability.
What Are Stiffeners in Flexible Circuits?
A stiffener is a piece of material bonded to a flexible circuit to rigidize a specific area of the design. Although stiffeners provide no electrical function, they play an important mechanical role by reinforcing areas that require additional support.
Example of a flexible circuit board with stiffeners.
Stiffeners are commonly used to:
- Meet assembly thickness requirements
- Support ZIF connector interfaces
- Create stable and flat areas
- Improve handling during manufacturing and assembly
- Protect plated holes and component locations from bending-related damage
Without stiffeners, many flexible circuit applications would be significantly more difficult to manufacture, assemble, and operate reliably.
Meeting Thickness Requirements
Supporting Assembly Requirements
Many assemblies require a specific board thickness to ensure proper mechanical fit and repeatable installation. Common examples include mounting locations where hardware dimensions, such as screw lengths or enclosure features, depend on a consistent thickness.
Flexible circuits are often much thinner than rigid circuit boards because flexibility is a primary design objective. While beneficial for routing and packaging, this reduced thickness can create challenges during assembly.
By adding a stiffener to a localized area, the required thickness can be achieved without affecting the flexibility of the rest of the circuit.
Meeting ZIF Connector Thickness Requirements
Zero insertion force (ZIF) connectors are a common reason for using stiffeners in flexible circuit designs.
These connectors require a specific thickness to properly clamp and retain the circuit. When the flexible circuit alone cannot provide the required thickness, a polyimide stiffener can be added to achieve the necessary dimension and ensure proper connector engagement.
Creating Stable and Flat Areas
Flexibility is one of the primary advantages of a flexible circuit, but it can create challenges in applications that require dimensional stability.
In suspended installations, unsupported sections of a flexible circuit may sag under the weight of components. Excessive movement can reduce stability and increase the potential for mechanical issues.
Adding a stiffener creates a rigidized area that remains flat during use. This can help maintain separation between surface mount components and other features if the circuit bends, reducing the likelihood of contact-related failures and improving overall mechanical stability.
Improving Handling During Manufacturing and Assembly
Flexible circuits are inherently more susceptible to damage than rigid structures because of their thin construction.
Throughout manufacturing, assembly, installation, and routine handling, a flexible circuit may be exposed to conditions that increase the risk of tearing, ripping, or accidental damage.
Flex PCB with stiffeners applied for support and ease of handling during assembly.
Stiffeners improve handling by reinforcing selected areas of the circuit, making them easier to manipulate during production and installation. This benefit becomes increasingly important as circuit thickness decreases, since thinner constructions are generally more vulnerable to handling damage.
Supporting Plated Areas
Why Plated Features Require Protection
Many flexible circuits use pad plating, where plating is limited to through-holes, via holes, and associated pads.
While this method is effective for flexible circuit construction, plated holes located in areas that repeatedly bend can present reliability concerns. Plated copper is not well suited for continuous flexing and may be more susceptible to fracture than rolled annealed copper used in flexible regions.
Preventing Intermittent and Open Circuits
Stiffeners are commonly added beneath plated-hole regions to prevent bending in those locations.
By restricting movement, the stiffener helps protect hole plating from cracking during flexing. This reduces the risk of intermittent opens and complete open circuits that may occur if plated structures fracture over time.
Supporting Component Areas
IPC-2223 Recommendations
IPC-2223 recommends installing stiffeners in locations where components are mounted on flexible circuits.
This recommendation helps improve reliability in areas where mechanical and soldered connections must withstand assembly and operational stresses.
Protecting Solder Joints and Copper Features
Component locations often contain plated holes and solder joints, both of which can be affected by bending.
Although solder joints are designed to secure components, excessive flexing can:
- Damage solder joints
- Break copper conductors
- Lift copper from the circuit
- Cause component attachment failures
Adding a stiffener beneath component areas helps prevent these bending-related issues by providing a more stable mounting surface.
Why Stiffeners Are Essential in Flexible Circuit Design
Stiffeners enable many of the functions that make flexible circuits practical in demanding applications. They allow designers to add mechanical strength only where needed while preserving flexibility throughout the rest of the circuit.
Key benefits include:
- Achieving required assembly thicknesses
- Supporting ZIF connector interfaces
- Creating flat, stable regions
- Improving handling during manufacturing and installation
- Protecting plated holes from bending damage
- Supporting components and solder joints
- Improving overall circuit reliability
Without stiffeners, many flexible circuit assemblies would experience increased handling challenges, reduced durability, and significant assembly limitations.
Video: How Automated Stiffener Placement Enhances Flex Circuit Reliability
Summary
One of the most crucial parts of a flexible circuit board is the stiffener. Without a stiffener, many functions and applications of the circuit would be heavily limited or even impossible. Without stiffeners, components would be harder to install, the boards flexible circuit areas would be harder to handle, plated holes would run the risk of breaking, ZIF connectors and components would be harder to install, or would not exist, and in some applications the board would be unstable. But because of stiffeners, all of these are possible and more.
Key Takeaways
- Stiffeners provide necessary thickness: Flexible circuits are often too thin to meet assembly or ZIF connector requirements, and stiffeners help achieve the strict thickness tolerances without sacrificing flexibility.
- They create stable and flat areas: Adding stiffeners prevents sagging and ensures surface mount components remain supported and protected from contact or bending stresses.
- Stiffeners improve handling: Flexible circuits are fragile and prone to tearing, but stiffeners make them easier to handle during manufacturing, assembly, and installation.
- They protect plated areas: Plated through-holes are vulnerable to cracking under bending; stiffeners prevent flexing in those regions, reducing the risk of intermittent or full opens.
- They support component regions: Per IPC-2223, stiffeners are recommended under component areas to protect solder joints and copper pads from damage due to bending stresses.
