Flex PCB Stiffener: A Complete Guide

Introduction

Flat cable with aluminum stiffener
Flex PCB Stiffener

 

Flexible printed circuits (FPCs) are very beneficial due to their flexibility with challenging environmental and other factors. Sometimes, their flexibility may not always provide the rigidity needed for certain applications. The use of mechanical stiffeners or structural backbones is a common approach that is used to increase the stability of flex circuits in some areas that might face mechanical stress. These stiffeners perform the function of stable points to support the connectors and other components.

What Is A PCB Stiffener?

Polyimide stiffener used at the left side of the flex PCB
Polyimide stiffener used at the left side of the flex PCB

 

A PCB stiffener supports a flexible PCB by providing it with mechanical support and stiffness. Its main function is to bolster the flexibility of flex or rigid-flex circuits by improving their longevity and dependability. On the other hand, busbars are slightly different stiffeners, coated with a dielectric material. They have the ability to convey ground and power distribution by supporting currents of up to 128 amps, they eliminate the necessity for thick traces on the PCB provide mechanical support, and ensure electrical conductivity as well.

FPC Stiffener Purposes

In flexible PCB designs, stiffeners perform various roles, including:

  • Making the connector region to end the stress caused by large connectors or frequent connector insertions.
  • Fulfilling the specific thickness requirements for ZIF (Zero Insertion Force) connectors.
  • Providing support against bending in specific areas.
  • Making a flat surface suitable for the placement of SMT (Surface Mount Technology) pads and components.
  • Reducing stress on the components of flex PCB.
  • Helping heat dissipation through the use of aluminum and stainless-steel stiffeners.
  • Ending the need for an SMT carrier by making the panel rigid enough for continuing the SMT assembly.

Types Of Flexible PCB Stiffeners

Flexible PCB with polyimide stiffening
Flexible PCB with polyimide stiffening

 

Flexible PCB Stiffeners are of three primary types: Stainless Steel Stiffener, FR4 Stiffener, and PI Stiffener. They are applied to various positions on the PCB. When developing a PCB for rigid-flex circuits, it is necessary to follow the manufacturing processes and guidelines. Opting for the PI Stiffener is a highly recommended choice in cases where the PCB is not specifically designed for rigid-flex circuits by the manufacturer.

FR4 Stiffeners

The most common and budget-friendly choice for stiffening is FR4 glass epoxy because it uses the same laminate material as the FPC substrate.

Benefits Of FR4

  • Cost-effective: No extra expenditure of money because it utilizes the same base laminate.
  • Simplified assembly: Makes strong bonds with FPC adhesive and lamination procedures.
  • Suitable for soldering: Uses a standard FR4 finish for solder pad plating.
  • Readily accessible: Easily obtainable material with quick delivery times.
  • Sufficient rigidity: Offers great stiffness for the majority of applications.

Cons Of FR4

  • Size limitations: Stressed by the usual panel dimensions and manufacturing tools.
  • Moisture retention: The glass weave can absorb and keep moisture and solvents.
  • Copper vulnerability: The presence of glass fiber increases the risk of copper being torn away from the pad.
  • Fragile: The epoxy resin can crack easily if subjected to excessive loads.

Design Guidelines

  • Maintain a small length for unsupported stiffeners in order to reduce bending,
  • Select thicker cores, such as 1.6mm, to achieve the highest level of stiffness.
  • Be careful to avoid DFM (Design for Manufacturability) violations about minimum trace spacing and hole size.
  • Apply a solder mask or an additional PI overlay for sealing any exposed glass fibers.
  • In general, FR4 stiffeners are the swiftest and most economical means of making flex circuits due to their material combination with the FPC substrate.

Polyimide (PI) Stiffeners

Polyimide films, such as Kapton is the best alternative for flex circuit stiffening materials due to their improved performance.

Pros Of Polyimide

  • Exceptional strength: has the ability to endure repeated stress and flexing.
  • Minimal moisture absorption: It is humidity and liquid-resistant.
  • Remarkable heat tolerance: It can endure temperatures above 260°C.
  • Lightweight: It is a low-density stiffener having an overall low mass.
  • Variety of thin core choices: It has a variety of thin core choices ranging from 25 micrometers to 150 micrometers.
  • Chemical resilience: It has resilience to the majority of solvents and acids.

Cons Of Polyimide

  • Increased expense: Approximately 5-10 times more costly as compared to FR4.
  • Extended lead time: Extended production lead times.
  • Lamination complexity: Difficulty in establishing adhesive bonds with FPC layers.
  • Non-solderable: Requires metallization for component pads to enable soldering.

Design Guidelines

  • It is important for surface treatments in order to enhance adhesion.
  • Be careful to prevent harm during the handling of thin cores.
  • Precise and accurate drilling and routing settings are necessary while using polyimide stiffener.
  • The use of polyimide stiffener reduces the extent of exposed PI edges.
  • Polyimide stiffeners offer optimal strength with minimal thickness for demanding flex circuit applications.

Aluminum Stiffeners

The use of Aluminum stiffeners is the most inflexible choice that involves the use of metal backing plates or frames.

Pros Of Aluminum

  • Optimal stiffness: It provides the highest level of reinforcement with thin materials.
  • More economical compared to PI: It is less costly than polyimide films.
  • Lightweight: Aluminum alloys have a low mass.
  • Versatile machining: Aluminum alloys can be machined or shaped as needed.
  • Efficient heat dissipation: Aluminum alloys exhibit high thermal conductivity.

Cons Of Aluminum

  • CTE disparity: Variations in thermal expansion could lead to distortion in aluminum stiffeners.
  • Complex assembly: Complex assembly in the form of metallization or insulation is required.
  • Susceptible to oxidation: Aluminum stiffener is susceptible to corrosion in the absence of protective plating.
  • Rebound effect: Flexible aluminum (heat-treated) requires care during handling.

Design Guidelines

  • Anodization helps in the insulation and oxidation resistance.
  • Adhesive selection is critical to handle CTE differences.
  • Flexing zones around stiffeners should be cleared for the best results.
  • Minimize rigid portions of assembly for the easiest flex stiffening.
  • Aluminum or metal stiffeners provide the strongest option but they require additional processing steps to settle properly with the flex circuit stackup.

 

Stiffeners Placement In Flex PCBs

 

FR4 stiffener used on the flex PCB
FR4 stiffener used on the flex PCB

Stiffeners On The Top Layer

Stiffeners are placed on the upper layer of a flex PCB. They can offer structural reinforcement for the entire PCB assembly. These stiffeners are commonly used when there is a substantial load applied on one side of the flex PCB. Their purpose is to distribute equal weight across both sides of the flex PCB, ensuring equal support.

Stiffeners On The Bottom Layer

Stiffeners applied to the lower layer of a flex PCB are generally used when increased strength is required in any section of the PCB. For instance, if there’s an active component in one region that demands more support as compared to other parts of the flex PCB layout, then, a stiffener can be applied in that specific area. This helps in providing superior mechanical reinforcement. It also safeguards against wear and tear resulting from repeated usage over time.

 

Stiffener Performance Comparison

Parameter        Fr4 Aluminum Polyimide
Rigidity Medium High Low
Thermal conductivity Medium High Low
CTE Medium High Medium-low
Moisture absorption High Medium Low
Solderable Yes No No
Chemical resistance Medium High High
High temperature Medium High High
Lead time Short Medium Medium
Cost Low Medium High

 

Stiffener Integration And Design

Numerous factors are taken into account for the effective integration of the stiffener structure into the flex PCB assembly:

Layer Position

Stiffeners are typically added as a balanced pair on the external sides of the flex circuit. Stiffeners encase the FPC layers between themselves. This arrangement serves to react against any uneven expansion changes due to moisture or temperature.

Layer Adhesion

Polyimide and aluminum require particular surface treatments or adhesives specifically designed to attach the polymer layers of the FPC. Manufacturers can choose adhesive systems that are compatible with stiffener materials.

Stiffener Contour

By using laser machines, stiffeners can be cut according to the flex circuit designs. It allows selective bending in necessary sections.

Mounting Features

The stiffener should allow mounting features like openings, perforations, and connection points to facilitate component attachment, hardware placement, and linking with the rigid PCB.

Flex Transition

Sufficient space should be allocated along the edges of the stiffener. It allows the flexible section of the circuit to flex easily. It leads to the smooth transition of flex into the rigid regions.

Stiffener Fabrication Process Overview

The essential fabrication steps for integrating stiffeners are:

  • Create the base FPC layers.
  • Create the stiffener layers using FR4, PI, or aluminum.
  • Carefully, cut or route the stiffener layers to achieve the desired shapes.
  • Apply surface treatments to the stiffener for increased adhesion.
  • Align and attach the stiffener layers to the FPC.
  • Use laser cutting to define the outline of the assembled stiffened structure.
  • Proceed with the FPC manufacturing processes, including overlay application, solder mask, plating, and singulation.

 

Flexible PCB Stiffener Requirements

It is necessary to consider that stiffeners must follow all the design regulations of the PCB manufacturing process. Many PCB manufacturers do not offer flex PCB stiffeners to their clients. Therefore, if your PCB design requires a specific stiffener, it is advisable to collaborate with the manufacturing company to ensure that they can supply the required customized stiffener. The rules and restrictions for flex PCB stiffeners can differ from one manufacturer to another due to compliance with OSHA and UL regulations.

Things To Know About Flex PCB Stiffeners

Stiffeners do not require to be made from a single material. The material used for a flex PCB stiffener should have enough rigidity to support the weight of the rigid-flex circuits. Attachment of the stiffener to the rigid PCB is not obligatory. A recommended technique for affixing a flex PCB stiffener to a rigid-flexible circuit without adhesives involves the use of bolts, nuts, or screws.

Coverlay Flex PCB Materials & Construction

Coverlay is a covering material that is applied over an electronic board for protection against corrosion or fluids. A coverlay can be either liquid or solid, the most common materials used for the construction of a coverlay are polyimide, silicones, epoxies, and polyesters in large quantities. The rigidity of a flex coverlay depends on the thickness and flexibility of the material.

FPC Coverlay In Flex Circuits

FPC Coverlay is a flexible circuit near the solder mask side that improves the flexible circuit’s electrical, optical, and mechanical characteristics. Typically, flex circuits are made by printing on various materials that have dielectric properties at high temperatures and pressures. FPC Coverlay allows the designing of a flexible circuit with good mechanical characteristics. It also protects it from environmental effects.

Conclusion

This article provides a comprehensive understanding of Flex PCB design and ways of using different stiffeners to make the circuit flexible. Flex PCB is a new product that manufacturers have been producing in recent years. Stiffeners are used in many applications with limitations on flex circuits, such as size, form factor, and weight. Flex PCB design has unique requirements that are necessary to consider while designing a flexible circuit on a rigid PCB.

FAQs

What are some typical locations for stiffeners on flex circuits?

Stiffeners are applied on reinforcing points like connectors, cable attachments, component pads, and joints with rigid boards.

Can multiple stiffener materials be combined on one flex circuit?

Yes, it is possible to use FR4, polyimide, and aluminum together on a single FPC to enhance the performance.

What adhesive is used to bond aluminum stiffeners to flex layers?

Epoxy is commonly used but other options include etching, anodization, mechanical abrasion, and application of bonding films.

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