Which Flexible PCB Material is Best Suited for Your Project?

With the advancement of electronic engineering, the demand for innovative technologies to meet users’ needs for miniaturization and portability has increased. Flexible circuits provide a solution for this, but PCB engineers need to carefully consider the materials used in their designs. In this article, TechSparks will provide a comprehensive flexible PCB materials selection guide to enhance circuit performance.

Table of Contents

Flexible PCB Copper Clad Laminate Materials

Flexible PCB Copper foil material

The Copper Clad Laminate (CCL) plays a crucial role in PCB as it directly influences circuit reliability, electrical conductivity, and overall performance. It is composed of three layers of copper foil + adhesive+ base material, or “copper foil + base material” of non-adhesive copper clad laminate. In dynamic flexible PCB, it must withstand at least 100,000 cycles of use.

Copper foil material

There are two main options for the copper foil material in flexible PCB: rolled copper and electrolytic copper.

  • Rolled Copper: Utilizing pure copper as the substrate, it undergoes multiple pressure treatments to form a uniform, fine, and flat copper foil.
  • Electrolytic Copper: This process involves immersing a pure copper plate or other substrate in a copper electrolyte. When an electric current is applied, a layer of copper foil is deposited on the surface of the substrate.

For flexible PCB, it is recommended to use rolled copper for the copper foil material. This choice offers better control over thickness during preparation, along with improved ductility, conductivity, and bending properties. Electrolytic copper, on the other hand, is more commonly used for rigid PCB. The typical thickness options for flexible PCB copper foil are 1/3oz, 1/2OZ, 1OZ, and 2OZ.

Advanced Reading: Comprehensive Knowledge of PCB Copper Thickness

Substrate material

The PCB substrate is a critical component of the copper clad laminate, providing support and insulation for electronic components and wires on the circuit board. Polyimide is the most frequently used material for flexible circuit. However, other options include polyester, PTFE, liquid crystal polymer, and polyamic acid ester.

ElementPolyimidePolyesterPTFELCPPAI
Operating Temperature-269°C to 260°C-40°C to 125°C-200°C to 260°C-50°C to 150°C-100°C to 275°C
Dielectric Constant3.3-3.53.0-4.02.1-2.32.8-3.23.5-3.7
Loss Factor0.003-0.0070.008-0.050.0001-0.00030.003-0.020.004-0.012
FlexibilityExcellentGoodPoorExcellentFair
Chemical ResistanceExcellentGoodPoorExcellentFair
Anti-StickingFairFairExcellentFairGood
Application FieldWidelyFairHigh TemperatureFlexible DesignFair
CostHigherLowHigherLowLow

Adhesive

Within the internal structure of a circuit board, adhesive plays a vital role in providing padding, cushioning, and heat conduction. In the construction of multilayer flexible PCB, adhesive becomes the key element in forming a laminated structure, ensuring the stability and integrity of the multi-layer design. Various adhesive materials are used in the flexible PCB manufacturing process, each serving specific purposes:

  • Acrylic: This type of adhesive belongs to light-curing adhesives, enabling quick curing when exposed to ultraviolet or other visible light sources. Its rapid curing capability speeds up the production process.
  • Epoxy: Known for its excellent mechanical properties, offers chemical corrosion resistance, ensuring the circuit board’s performance remains intact even in extreme conditions.
  • Polyurethane: Utilized in applications that require higher abrasion resistance, offers greater flexibility and versatility.
  • Fluoroelastomer: Considered the material of choice for harsh chemical environments.

Flexible PCB Protective Film Materials

Flexible PCB Protective Film Materials

In rigid circuits, the widely recognized protective layer is the solder mask, while in flexible circuits, a specialized protective layer known as a cover film is used. Unlike the rigid solder mask, the cover film is designed to be flexible, allowing it to bend and fold along with the flexible PCB. Common materials used for the cover film include polyimide, polyester, fluoride, PDMS, and polyethersulfone, among others.

When selecting a cover film material for a flexible PCB, several factors come into play. Apart from high temperature resistance and chemical stability, it is crucial to consider the material’s flexibility. A cover film must be able to adapt to the bending, folding, and maneuvering required by the flexible PCB without cracking or becoming brittle.

Reinforcing Material for Flex PCB

Reinforcing Material for Flex PCB​

For local areas on the soft board that require welding parts or additional reinforcement to compensate for thickness, various materials and combinations of reinforcing materials can be selected:

Materials:

  • PI: Exhibits excellent high-temperature performance and mechanical strength, making it suitable for environments and applications that demand elevated temperatures and higher strength.
  • PET: Offers good strength and durability, making it suitable for general reinforcement needs.
  • FR4: Common glass fiber reinforced material known for its exceptional strength and stability, making it suitable for situations requiring high strength and rigidity.
  • Stainless steel (SUS): Boasts excellent corrosion resistance and mechanical strength, making it ideal for special environments and applications that require corrosion resistance.

Combination Methods:

  • PSA (Pressure Sensitive Adhesive): PSA is a sticky material that can be easily applied and fixed to the soft board. It is suitable for temporary fixing and quick applications, and products from the 3M series are commonly used in this category.
  • Thermosetting: Thermosetting reinforcing materials form a strong connection when cured after heating. They offer high bonding strength, solvent resistance, heat resistance, and creep resistance, making them perfect for scenarios that demand long-term durability and stability.

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