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 How to Choose the Right Material for Flex Circuits ?

How to Choose the Right Material for Flex Circuits ?

Introduction:

The choice of flex circuit material is critically important to a flexible PCB manufacturer. The choice of FPC materials directly impacts the performance of the final product. The materials must meet the electrical, and thermal. and mechanical requirements of the specific application. Different applications may require FPCs with specific characteristics. such as high-temperature resistance, flexibility, or resistance to environmental factors. Selecting the right flex circuit material ensures that the FPC is well-suited for its intended use. Choosing the appropriate materials that meet the performance and reliability requirements. while staying within budget is essential for profitability. So follow us to learn more about flex circuit material.

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What Are The Flex Circuit Material?

Flexible circuit boards (FPCBs) constructed using several key raw materials. To achieve their flexible and conductive properties. The primary raw materials for FPCBs include:

Substrate Material:

Flexible circuit substrates made from various materials. each offering unique properties to meet the specific requirements of different applications. The choice of flex circuit substrate material depends on factors. such as flexibility, temperature resistance, cost, electrical performance, and environmental conditions. Here are some common types of flex circuit substrate materials:

※ Polyimide (PI):

Polyimide is one of the most popular substrate materials for flexible circuits. Due to its excellent combination of flexibility, high-temperature resistance, and electrical insulation properties. It can withstand a wide temperature range and is well-suited for demanding applications.

※ Polyester (PET):

Polyester-based substrates, such as PET, are cost-effective options with good flexibility. While not as thermally stable as polyimide. they are suitable for less demanding applications and can be a budget-friendly choice.

※ Liquid Crystal Polymer (LCP):

LCP known for its exceptional electrical properties. including low dielectric constant and low dissipation factor. It is suitable for high-frequency applications and can provide good thermal stability.

※ Polyethylene Naphthalate (PEN):

PEN substrates offer good thermal stability and electrical insulation properties. They can withstand higher temperatures than PET. and are suitable for applications requiring increased heat resistance.

※ Fluorinated Ethylene Propylene (FEP):

FEP is a thermoplastic fluoropolymer that offers excellent chemical resistance. and used as a flexible substrate material in applications where resistance to harsh chemicals is essential.

※ Polyetherimide (PEI):

PEI substrates provide good mechanical strength, high-temperature resistance, and electrical insulation. They are suitable for applications that require a balance of flexibility and robustness.

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Features of Flex Circuit Substrate Material:

Property (Typical)UnitsPolymidePolymide (Adhesiveless)Polyester
Representative Trade Name KAPTONKAPTONMYLAR
Physical
Thickness Rangemil0.5 to 51–62—5
Tensile Strength (@25°C)psi25,00050,00020,000 to 35,000
Break Elongation%705060 to 165
Tensile Modulus (@25°C)100,000 psi4.30.75
Tear Initiation Strengthlb/in1000700-12001000 to 1500
Tear Propagation Strengthg/mil82012 to 25
Chemical
Resistance to:    
Strong Acids GoodGoodGood
Strong Alkalis PoorGoodPoor
Grease and Oil GoodGoodGood
Organic Solvents GoodGoodGood
Water GoodGoodGood
Sunlight GoodGoodFair
Fungus Non-nutrientNon-nutrientNon-nutrient
Water Absorption% (24 hours)2.90.8<0.8
Thermal
Service Temperature (min/max)degree C-0.625-0.625-0.571428571
Coefficient of Thermal Expansion (@22°C)PPM/degree C202027
Change in Linear Dimensions (100°C, 30min)%<0.30.04-0.02<0.5
Electrical
Dielectric Constant (ASTM D150) 1MHz 3.43.43
Dissipation Factor (ASTM D150) 1MHz 0.010.0030.018
Dielectric Strength (ASTM D149)V/mil600060003400
@ 1 mil thickness
Volume Resistivity (ASTM D257)
ohm-cm1.00E+161.00E+16

1.00E+16

Data From https://www.epectec.com/

Conductive Material (Copper Foil):

Conductor materials in Flexible Printed Circuits chosen for their electrical conductivity. and flexibility, which are essential properties for these circuits. The most common conductor material used in FPCs is copper, specifically copper foil. Copper offers excellent electrical conductivity and made very thin to suit the flexible nature of FPCs.

Copper foil is the primary conductor material used in FPCs. It is typically very thin, often measured in ounces per square foot (oz/ft²). Common thicknesses include 1oz, 2oz, and 3oz copper foils. Thinner foils are more flexible but may have higher resistance.

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Adhesive :

Adhesive materials play a crucial role in Flex circuit (FPC) construction. by bonding various layers of the FPC together and providing mechanical stability. The selection of adhesive materials depends on the specific requirements. Here are some common adhesive materials used in FPCs:

※ Pressure-Sensitive Adhesives (PSAs):

PSAs are adhesive materials that bond when pressure applied. They are commonly used to attach components. such as connectors or integrated circuits, to the FPC. PSAs are available in various formulations, including acrylic, rubber. or silicone-based adhesives, and can be single-sided or double-sided.

※ Epoxy Resins:

Epoxy resins are thermosetting adhesives used in FPCs for their strong bonding capabilities. and resistance to heat and chemicals. They are often used in applications where components need to be securely attached to the FPC. and the adhesive must withstand high temperatures during soldering processes.

※ Polyurethane Adhesives:

Polyurethane adhesives are flexible and have good adhesion properties. They used in FPCs when flexibility and mechanical durability are important. Polyurethane adhesives can tolerate some degree of flexing. and movement without losing their bond.

※ Cyanoacrylate Adhesives (Super Glue):

Cyanoacrylate adhesives are fast-curing adhesives that provide strong bonds. They are sometimes used in FPC assembly for their rapid bonding properties. but they may not be suitable for all FPC materials due to their rigidity.

※ Anisotropic Conductive Adhesives (ACAs):

ACAs specialized adhesives that contain conductive particles, such as silver-coated glass spheres. They used to bond components with fine-pitch electrical connections to FPCs. enabling both mechanical attachment and electrical conduction.

※ Hot-Melt Adhesives:

Hot-melt adhesives are solid at room temperature but become liquid when heated. They are often used in applications where a strong bond needed. and components heated during the assembly process to activate the adhesive.

※ Thermoplastic Adhesives:

Thermoplastic adhesives, such as polyethylene, polypropylene. or PVC, and used when a bond required but not at high temperatures. They melted and reactivated multiple times, making them suitable for rework or repair.

※ Acrylic Adhesives:

Acrylic adhesives are versatile and offer good adhesion. To various surfaces, including FPC materials. They are often used for bonding layers of FPC substrates together.

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Stiffeners:

※ FR-4:

FR-4 is a commonly used material in traditional rigid printed circuit boards (PCBs). It is a fiberglass-reinforced epoxy laminate that offers high rigidity and dimensional stability. FR-4 stiffeners are relatively rigid and used in FPCs. To provide extra support in areas where components attached. or where mechanical reinforcement required. FR-4 stiffeners are often used in conjunction with flexible substrates like polyimide (PI).

※ Polyimide (PI):

PI stiffeners made from the same material as the flexible substrate of the FPC itself. They provide a balance between flexibility and rigidity and are suitable for applications. where mechanical reinforcement needed without sacrificing too much flexibility. PI stiffeners are often preferred when maintaining overall circuit flexibility is essential.

※ Polyester (PET):

PET stiffeners are lightweight and offer some degree of flexibility. They used in applications where rigidity required, but not to the extent provided by FR-4. PET stiffeners are a good compromise between flexibility and mechanical reinforcement.

※ Metal Stiffeners:

In some applications, especially those requiring high rigidity. Or heat dissipation, metal stiffeners used. Materials like aluminum or stainless steel employed. To add substantial strength and stability to the FPC. Metal stiffeners are also useful for heat-spreading purposes.

※ Thermoplastic Materials:

Thermoplastic materials, such as ABS (Acrylonitrile Butadiene Styrene). or PVC (Polyvinyl Chloride), used as stiffeners. These materials are lightweight and offer moderate rigidity. They can be a good choice when cost-effective reinforcement needed.

※ Composite Materials:

Composite materials used to create custom stiffeners. with specific properties tailored to the application’s requirements. These materials often combine layers of different substrates. or materials to achieve desired characteristics.

※Adhesive-Backed Stiffeners:

Stiffeners may come with adhesive backing (e.g., pressure-sensitive adhesive) for easy attachment to the FPC. This adhesive layer ensures a secure bond between the stiffener and the flexible circuit.

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Typical single Layer Flex Circuit Stack-up

1 Layer Flex PCB with Optional FR-4 Stiffeners

A typical single-layer flexible printed circuit (FPC) stack-up consists of several layers. each serving a specific purpose in the construction and functionality of the FPC. Here is a simplified description of a typical single-layer FPC stack-up:

Top Coverlay (Solder Mask):

  • This is the topmost layer of the FPC and serves to protect the conductive traces. and components from environmental factors like moisture, dust, and physical damage.
  • The coverlay is typically made from materials like polyimide (PI) or polyester (PET).
  • It is also where any silkscreen printing or component markings added if required.

Adhesive Layer (Optional):

  • An adhesive layer may be present if the FPC needs attached to another surface. or if additional components need to bonded.
  • Pressure-sensitive adhesive (PSA) or thermally conductive adhesive (TCA) used, depending on the application.

Flexible Substrate Material:

  • The flexible substrate is the core layer of the FPC. and it provides mechanical support and flexibility.
  • Common substrate materials include polyimide (PI). polyester (PET), liquid crystal polymer (LCP), or polyethylene naphthalate (PEN).
  • This layer is where the conductive traces patterned or etched.

Conductive Traces:

  • The conductive traces are thin, copper-based pathways that carry electrical signals across the FPC.
  • These traces patterned on the flexible substrate using processes like etching or printing.
  • The thickness and width of the traces depend on the specific design requirements.

Adhesive Layer (Optional):

Another adhesive layer may be present on the backside of the flexible substrate if additional bonding required. or if the FPC needs to attached to another surface.

Bottom Coverlay (Solder Mask):

  • The bottom coverlay provides protection to the conductive traces on the opposite side of the FPC.
  • It may use the same or a different material as the top coverlay.

Typical Multi-layer Flex Circuit Stack-up

A typical multi-layer flexible printed circuit (FPC) stack-up involves the use of multiple layers. to create a complex and versatile flexible circuit. Multi-layer FPCs designed to provide enhanced functionality, accommodate a higher density of components. and handle more complex interconnections. Here’s a simplified description of a typical multi-layer FPC stack-up:

4 Layer Rigid-Flex PCB (2 Flex Layers)

Top Coverlay (Solder Mask):

Similar to the single-layer FPC stack-up, a top coverlay provides protection to the topmost conductive traces. and components from environmental factors and physical damage.

Adhesive Layer (Optional):

An adhesive layer may be present on the top coverlay. allowing for the attachment of components or additional bonding if needed.

Flexible Substrate Layers (Multiple):

  • Multi-layer FPCs consist of several flexible substrate layers stacked on top of each other.
  • Each substrate layer includes a pattern of conductive traces and provides mechanical support.
  • The number of substrate layers can vary depending on the complexity of the circuit.

Interconnection Layers (Core Layers):

  • Core layers, also known as interconnection layers. used to route signals between different substrate layers.
  • They consist of conductive traces that connect various sections of the circuit. allowing for complex interconnections.
  • Core layers positioned between the flexible substrate layers.

Adhesive Layers (Optional):

Adhesive layers interspersed between substrate layers for bonding and mechanical stability.

Bottom Coverlay (Solder Mask):

Similar to the top coverlay, the bottom coverlay provides protection. to the conductive traces on the bottommost substrate layer.

Stiffeners (Optional):

Stiffeners made from materials like FR-4, polyimide. or other rigid materials added to reinforce specific areas. such as where connectors or components mounted.

Adhesive Layers (Optional):

Adhesive layers included between stiffeners. and the adjacent substrate layers for bonding and reinforcement.

Additional Layers (Optional):

Depending on the complexity of the design and the specific requirements. additional layers such as shielding layers, shielding films, or additional core layers added.

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Tips For Select Flex Circuit Material

Selecting the right flex circuit material is a crucial decision in the design and manufacturing process. The choice of material significantly impacts the performance, reliability. and cost of the flex circuit. When selecting flex circuit material, consider the following key factors:

Electrical Properties:

  • Dielectric Constant (Dk): The dielectric constant of the material affects signal propagation. Choose a material with a consistent. and appropriate Dk for your application’s electrical requirements.
  • Dissipation Factor (Df): A low Df is essential to minimize signal loss. and ensure signal integrity in high-frequency applications.

Mechanical Flexibility:

  • Flexibility: The material should allow the flex circuit to bend, twist. and flex without damage or excessive stress on the conductive traces. Polyimide (PI) and some other flexible substrates known for their flexibility.
  • Bending Radius: Consider the minimum bending radius required for your application. and choose a material that can meet this requirement without compromising mechanical integrity.

Thermal Properties:

  • Thermal Stability: Ensure that the material can withstand the temperatures. encountered during manufacturing processes (e.g., soldering). and operating conditions without deformation or degradation.
  • Thermal Conductivity: In some applications, the material’s thermal conductivity important for heat dissipation.

Environmental Resistance:

  • Chemical Resistance: Assess the material’s resistance to chemicals, solvents. and other substances it may encounter in the environment.
  • Moisture Resistance: Consider the moisture absorption properties of the material. and whether a protective coating or coverlay needed.

Adhesion and Bonding:

  • Adhesive Compatibility: If adhesive layers used for bonding or attaching components. ensure that the material is compatible with the chosen adhesive.
  • Solderability: If soldering involved, the material conducive to solder bonding without adverse effects.

Cost Considerations:

  • Material Cost: Evaluate the cost of the material in relation to your project budget. Some materials are more cost-effective than others.
  • Manufacturing Processes: Consider any additional manufacturing steps. or costs associated with the chosen material.

Copper Foil Thickness:

The thickness of the copper foil used for conductive traces. That should match the current-carrying requirements of the circuit. Thicker copper foils have lower resistance but may reduce flexibility.

Application-Specific Requirements:

Unique Application Needs: Assess any specific requirements of your application. such as high-frequency performance, aerospace certifications, medical-grade materials, or extreme temperature conditions.

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Conclusion:

Remember that material selection is a critical aspect of flex circuit design. and making the right choice can significantly impact the success of your project. Careful consideration of the factors mentioned above. along with collaboration with experienced professionals. That can help you choose the optimal material for your specific flex circuit application.

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