Flexible PCBs have become increasingly popular in various industries due to their numerous advantages. In this article, we will provide an overview of flexible printed circuit boards, including their different types, manufacturing processes, advantages, and disadvantages. We will also highlight the business segments that typically demand these circuit boards, as well as the capabilities and services offered by three different FPC manufacturers.
Introduction Flexible printed circuit boards (FPCs) are a type of printed circuit board (PCB) that utilize substrates made from flexible materials such as polyimide films. As opposed to rigid PCBs, which are used in applications that require a fixed shape or structure, FPCs can bend and flex to conform to the needs of the application. This flexibility allows for more compact designs, increased reliability, and improved thermal management. FPCs are commonly used in industries such as medical, defense, industrial, automotive, and consumer electronics.
Types of Flexible PCBs There are three main classifications of FPCs: flex, flex with stiffener(s), and rigid-flex.
A flex circuit is 100% flexible without any rigid sections. A flex with stiffener(s) is a flexible circuit with added FR4 or polyimide stiffeners for non-flexing regions. Lastly, a rigid-flex board is a combination of a rigid board and flex circuit, where the flexible layers are sandwiched between two rigid boards using no flow prepreg. The appropriate type of FPC depends on the specific needs of the application.
Manufacturing Processes The manufacturing process for FPCs involves etching copper traces onto a flexible substrate material. The most common substrate material used is polyimide film due to its high resistance to traction and temperature. The substrate is then laminated with adhesive material to join the conductive tracks with the dielectric substrate. The adhesive film can also be used for protective purposes to prevent moisture, dust, or other external agents from damaging the flexible circuit. If the circuit has a high complexity or requires the presence of copper ground shields, it is necessary to switch to a double layer or multi-layer FPC. In multi-layer FPCs, PTH (Plated Through Hole) is used to create electrical connections between different conductive layers.
Advantages of Flexible PCBs One of the primary advantages of FPCs is their flexibility, which allows for more compact designs and greater freedom in the design and operation of the application. FPCs can also adapt to small or irregularly shaped spaces, which is not possible with rigid boards. They take up less space, reducing the weight on the application’s motherboard, and provide better thermal management. Compared to rigid PCBs, FPCs are more reliable and long-lasting, especially in applications where the circuits are subjected to continuous vibrations and mechanical stress. They are also resistant to high temperatures, oils, gases, and acids.
Disadvantages of Flexible PCBs The initial cost of FPCs is typically higher than that of rigid PCBs due to their specific design requirements. Additionally, repairing or modifying FPCs can be challenging, and great care must be taken during the assembly phase as the circuit can be easily damaged if handled incorrectly.
Business Segments and Market Demand Flexible PCBs are commonly used in industries such as medical, defense, industrial, automotive, and consumer electronics. The demand for FPCs is increasing in all business segments with strong demand from medical, defense, and industrial markets. Since the volumes are usually quite small for these segments, manufacturers work with a number of HMLV (high-mix, low-volume) factories that fit the technology and volume requirements.
FPC Fabrication Technical Requirements List
As an FPC manufacturer, we require certain information and specifications from our customers to manufacture high-quality and reliable FPCs. The following are the details that we would need:
Design files: We require design files in Gerber format or other standard layout formats that outline the board’s actual size, shape, and layer stackup.
Bill of Materials (BOM): A complete BOM including part numbers, quantities, values, and descriptions of all components required for the assembly.
Schematics: A set of schematics showing the electrical connections between components on the FPC.
Material specifications: Specific material requirements, such as base film, adhesive, coverlay, copper thickness, etc., should be provided as per the customer’s requirement.
Layer count: The number of layers required for the FPC must be specified, along with details of any desired stiffeners, vias, or pads.
Tolerance requirements: Customers should provide the required tolerances for the finished product to ensure that the FPC is manufactured according to their specification.
Test requirements: Any specific testing requirements or standards that the FPC must conform to should be specified by the customer.
Quantity requirements: The expected quantity of boards to be produced should be communicated upfront to help us plan for materials procurement and manufacturing processes.
Lead times: The expected delivery date should be communicated to us, so we can schedule the production process accordingly.
Any other special requirements: If there are any unique or special requirements, such as non-disclosure agreements, certifications, or approvals, they should be communicated to us upfront.
It is essential to communicate these details clearly and accurately to ensure that the final product meets the customer’s expectations. Our engineers will review the customer’s specifications and prepare a quote based on the complexity, materials, and quantities of the FPCs required. We may also suggest design modifications or material upgrades to ensure the highest reliability and performance of the final product. By working closely with our customers and providing them with the necessary support throughout the entire process, we can deliver high-quality FPCs that meet their needs and expectations.
Ten Design Considerations for FPC Circuit Design
Trace Routing: Trace routing is critical in FPC design as it determines the electrical performance of the circuit. It is essential to maintain consistent trace widths and spacing, especially when dealing with high-speed signals.
Component Placement: Proper component placement is crucial in FPC design as it affects the reliability of the circuit. Components should be placed in areas where there is no bending or flexing.
Bend Radius: The bend radius is the minimum radius an FPC can be bent without causing damage. It is important to ensure that the FPC’s bend radius is not exceeded during installation or use.
Stiffeners: Stiffeners are often used in FPC design to provide additional support to areas that are prone to bending or flexing. When using stiffeners, it is important to ensure that they are properly aligned with the FPC.
Ground Planes: Ground planes are essential in FPC design as they help reduce electromagnetic interference (EMI) and improve signal integrity. A continuous ground plane should be used wherever possible.
Thermal Management: FPCs generate heat during operation which can affect the circuit’s performance and reliability. It is important to consider thermal management solutions such as adding vias or using thicker copper layers to dissipate heat.
Signal Integrity: Signal integrity is critical in FPC design, particularly for high-speed signals. It is essential to consider impedance control, controlled impedance, and signal termination to minimize signal reflections and maintain signal quality.
Environmental Factors: FPCs are often exposed to harsh environments such as extreme temperatures, humidity, and vibration. It is important to consider these factors when designing an FPC and select materials that can withstand these conditions.
EMI Shielding: EMI can cause interference with a circuit’s operation, so it is important to include EMI shielding measures such as using conductive coatings, metal foils, or magnetic shielding.
Design for Manufacturability: FPCs are typically manufactured using a complex process that requires attention to detail. It is important to consider the manufacturing process during the design phase to ensure that the FPC can be manufactured efficiently and cost-effectively.
FPC Material Selection Standards and Specifications
Substrate Thickness: The substrate thickness is an essential factor in determining the FPC’s flexibility, durability, and thermal stability. Typically, substrates with a thickness of 0.1-0.5mm are used for FPC design.
Material Tensile Strength: The material’s tensile strength determines its ability to withstand physical stress without breaking or tearing. It is essential to select a material with high tensile strength for FPCs that will be exposed to bending or flexing.
Dielectric Constant: The dielectric constant refers to the material’s ability to store electrical energy. A low dielectric constant is ideal for FPCs as it helps reduce signal loss and crosstalk.
Glass Transition Temperature (Tg): Tg is the temperature at which a material changes from a rigid to a flexible state. It is crucial to select a material with a Tg that is higher than the operating temperature to maintain the FPC’s dimensional stability.
Coefficient of Thermal Expansion (CTE): CTE refers to the material’s ability to expand or contract in response to temperature changes. A material with a low CTE is preferred for FPCs that will be exposed to extreme temperatures.
Chemical Resistance: FPCs may be exposed to various chemicals such as solvents, oils, and acids. It is important to select a material that can resist these substances without degrading or delaminating.
Surface Finish: The surface finish on an FPC affects its ability to adhere to other materials and components. It is important to select a surface finish that is compatible with the assembly process and ensures good solderability.
Environmental Compatibility: FPCs may be exposed to various environmental conditions such as humidity, moisture, and UV light. It is important to select a material that can withstand these conditions without degrading or delaminating.
Cost: The cost of an FPC is influenced by the choice of materials. It is essential to balance the performance requirements with the available budget to ensure a cost-effective design.
Availability: The availability of the chosen material can affect the production timeline and cost. It is important to select a readily available material to avoid delays in production or increased costs due to material scarcity.