When you use rigid-flex PCB design guidelines to integrate multiple components and complex circuits into a single, space-saving unit, you win.
Suppose you’re looking to create highly efficient and compact electronic devices. In that case, rigid-flex PCBs might be the solution you’ve been searching for. With their unique combination of rigid and flexible materials, these innovative circuit boards offer unparalleled design possibilities and numerous advantages.
Rigid-Flex PCBs and Why They’re Worthy
Rigid-flex PCBs allow for seamless connections between rigid sections and flexible circuit board areas. The need for bulky connectors and cables is eliminated. As a result of fewer potential failure sites, your gadget is smaller and lighter overall, improving reliability.
In our upcoming sections, we’ll guide you through the world of rigid-flex PCB design guidelines, providing you with essential tips and insights to ensure success in your projects. From design guidelines to IPC recommendations, we’ll cover it all.
Are you ready to take your PCB designs to the next level? Then, this blog is packed with valuable information to help you unlock the full potential of rigid-flex PCBs.
Layer Stackup
Layer Stackup Rigid-Flex PCB Design Guidelines
The layer stackup is a critical aspect of designing rigid-flex PCBs as it determines the arrangement and order of the different layers in the board. A well-planned layer stackup can significantly impact your design’s overall performance, reliability, and manufacturability.
To help you achieve the best results, we’ve compiled a set of guidelines for layer stackup in rigid-flex PCBs. Alongside, we will use an example case, that of designing a rigid-flex PCB for a smart wearable device.
Consider the functional requirements
Start by identifying the specific functions and components that will be placed on each layer. This includes determining the signal, power, and ground plane requirements to ensure proper signal integrity and power distribution throughout the board.
For the watch, this involves placing the microcontroller and sensors on the rigid layers for stability and mounting the flexible display and interconnects on the flexible layers for flexibility. By considering these functional requirements, you can ensure optimal performance and functionality.
Balance flexibility and rigidity
It’s essential to strike the right balance between flexibility and rigidity in your layer stackup. This means designing the board to maintain flexibility in the flexible areas while providing sufficient rigidity in the rigid sections. This helps ensure mechanical stability and prevents issues like twisting or warping.
You need to strike the proper balance between rigidity and flexibility. So the layer stackup would be designed to have more flexible layers in the watch’s wearable area while maintaining rigid layers in the watch’s housing sections. A balanced approach would avoid problems like over-bending when using a gadget.
Signal layer placement
Place signal layers close to each other to minimize signal crosstalk and impedance mismatch. Follow industry rigid-flex PCB design guidelines, such as those provided by the IPC, for trace spacing, differential pair routing, and controlled impedance to achieve optimal signal integrity.
Our smartwatch’s signal layers would be placed close to minimize signal crosstalk and impedance mismatch.
Power and ground planes
Properly allocating power and ground planes is crucial for minimizing noise, providing stable power distribution, and enhancing electromagnetic compatibility (EMC). Incorporate solid ground planes and use multiple vias for low-impedance connections. For our watch, we will put adequate power and ground planes so that the watch will not run out of power when in use.
Thermal considerations
To manage heat generated by power components or high-speed circuits, incorporate thermal vias and heat dissipation techniques in the layer stackup. For our watch, we will use smooth and gradual transitions between rigid and flexible sections to avoid stress concentration and potential failure points.
Layer transition zones
Design smooth and gradual transitions between rigid and flexible sections to avoid stress concentration and potentially weak sections. For smooth integration, utilize tapered traces and choose appropriate materials for the transition zones. The potential failure points can occur at the locations with abrupt transitions between the watch’s rigid and flexible sections. These areas are susceptible to stress concentration.
Bend Radius
Bend Radius Rigid-flex PCB Design Guidelines
The bend radius refers to the minimum radius that the flexible portion of the PCB can safely withstand without causing damage or compromising performance.
Folding Considerations
In some applications, the rigid-flex PCB may require folding or multiple bending points to fit into compact spaces or conform to specific shapes. Factors that need to be paid attention to are:
Number of Folds
The number of folds refers to the total count of bending points or creases in the flexible portion of the PCB. Although there is no set standard for the number of folds, it is generally advised to decrease the number of folds to lower the danger of mechanical fatigue. Ideally, fewer folds help maintain the structural integrity of the PCB.
Fold Angle
The fold angle represents the degree of bending or the angle at which the PCB is folded. The fold angle should be within the acceptable range defined by the materials used and the manufacturer’s specifications.
Here are three materials used in rigid-flex PCBs and their respective fold angle rigid-flex PCB design guidelines:
Polyimide (PI)
The fold angle guideline for polyimide typically ranges from 0.5 to 2 degrees per layer. This means that each individual layer should be bent within this specified range to avoid excessive strain and potential damage to the circuitry.
Liquid Crystal Polymer (LCP)
For LCP-based rigid-flex PCBs, the fold angle guideline can vary but is typically recommended to be within the range of 1 to 3 degrees per layer.
Polyester (PET)
The fold angle guideline for PET-based PCBs usually falls within the range of 2 to 4 degrees per layer.
PEEK (Polyether Ether Ketone)
The fold angle guideline for PEEK-based rigid-flex PCBs typically ranges from 1 to 3 degrees per layer.
PEN (Polyethylene Naphthalate)
For PEN-based PCBs, the recommended fold angle ranges from 0.5 to 2 degrees per layer.
Polyurethane (PU)
For rigid-flex PCBs made of polyurethane, the recommended fold angle can change depending on the material’s hardness and individual formulation. Generally, a fold angle range of 1 to 3 degrees per layer is recommended.
Folding Mechanism
The folding mechanism refers to the method or mechanism used to achieve the desired fold or bending of the PCB. The choice of the folding mechanism depends on factors such as design constraints, space limitations, and the overall functionality of the rigid-flex PCB. Common folding tools include:
Simple Bending:
Flexible Polyimide (PI): Typically allows for a bend radius of around 10 to 20 times the thickness of the material. For example, if the PI thickness is 0.1mm, the bend radius would be about 1mm to 2mm.
Polyester (PET): Can have a similar bend radius range as PI, typically around 10 to 20 times the thickness.
Accordion-Style Folding:
Polyimide (PI): Similar to simple bending, a bend radius of approximately 10 to 20 times the material thickness is often recommended.
Polyester (PET): The acceptable bend radius range is usually similar to that of PI, around 10 to 20 times the thickness.
Z-Folding:
Polyimide (PI): The recommended Z-fold angle is typically within the range of 120 to 180 degrees.
Polyester (PET): Similar to PI, the fold angle for Z-folding is often within the range of 120 to 180 degrees.
Mechanical Stress Analysis
By simulating the mechanical behavior of the PCB under different bending scenarios, you can identify potential areas of stress concentration or excessive strain. This analysis aids in optimizing the bend radius and identifying critical folding points.
Material Selection
Flexible substrates with high ductility and low modulus of elasticity, such as polyimide (PI) or liquid crystal polymer (LCP), are commonly used to withstand repeated bending without compromising electrical performance. Additionally, selecting appropriate coverlay materials that can withstand folding and protect the flexible areas is essential. If you want to know more about Rigid-Flex PCB materials, please refer to our blog here.
Component Placement
Component Placement in Rigid-Flex PCB Design Guidelines
The rigid-flex PCB design guidelines are as follows:
Connectors
Board-to-Board Connectors: Ensure a minimum spacing of 2mm (80 mils) between board-to-board connectors to allow for proper mating and prevent mechanical interference.
Flex-to-Board Connectors: Align flex-to-board connectors with the flex regions, leaving a minimum distance of 2mm (80 mils) between the connector and the edge of the flex area.
Electrolytic Capacitors
Keep a minimum distance of 3mm (120 mils) between electrolytic capacitors and the edges of flexible areas to prevent stress concentration.
Maintain a minimum spacing of 1mm (40 mils) between electrolytic capacitors to allow for proper airflow and heat dissipation.
Ball Grid Array (BGA) Components
Minimum BGA Pitch: Follow the component datasheet guidelines but typically maintain a minimum pitch of 0.8mm (32 mils) for BGA components.
Clearance Requirements: Maintain a minimum clearance of 0.3mm (12 mils) between adjacent BGA components to prevent short circuits.
So far, these are the general rigid-flex PCB design guidelines. For more info, refer to specific component datasheets and manufacturer recommendations.
Flip Chip Components
Minimum Bump Pitch: Follow the component datasheet guidelines but typically maintain a minimum bump pitch of 0.4mm (16 mils) for flip chip components.
Clearance Requirements: Maintain a minimum clearance of 0.3mm (12 mils) between adjacent flip chip components to prevent short circuits.
Microcontrollers and Microprocessors
Maintain a minimum pitch of 0.8mm (32 mils) between pins of microcontrollers and microprocessors to ensure proper soldering and prevent solder bridges.
Leave a clearance of at least 2mm (80 mils) between the components and adjacent traces or other components to avoid electrical interference.
For a helpful video about what microprocessor to choose, check out his video:
Surface Mount Components (SMT)
Integrated Circuits (ICs): Place ICs closer to the rigid sections with a recommended minimum distance of 2-3 mm from the flex area.
Passive Components (Resistors, Capacitors, Inductors): Maintain a minimum spacing of 0.25 mm between SMT passive components to prevent solder bridging and ensure proper assembly.
Small Outline Transistors (SOTs) and Small Outline Diodes (SODs): Position these components within 0.5 mm of the rigid area to minimize stress on the flex region during assembly and operation.
Through-Hole Components
Large Components (Transformers, Relays): Place these components in the rigid sections with a minimum distance of 3-5 mm from the nearest flex area to avoid mechanical stress on the flex portion.
Small Components (Diodes, LEDs): Position these components within 1-2 mm of the rigid area to ensure mechanical stability during assembly and operation.
Trace Routing Rigid-flex PCB Design Guidelines
Sure signal integrity and power distribution are affected by trace routing Rigid-flex PCB design guidelines. Let’s explore some important rigid-flex PCB design guidelines for trace routing. For illustration, we will continue using our smartwatch example mentioned in the Layer Stackup section.
Signal Traces
The smartwatch should have signal traces with a minimum width of 0.15mm (6 mils) to maintain impedance control and reduce resistance.
Minimum Trace Spacing: Maintain a minimum spacing of 0.15mm (6 mils) between signal traces to prevent crosstalk and electromagnetic interference.
Power and Ground Traces
Power Trace Width: Increase the width of power traces to accommodate higher current requirements. Generally, a width of 0.25mm (10 mils) or more is recommended for the power traces of our smartwatch example.
Ground Plane: Incorporate solid ground planes to provide low-impedance signal return paths and reduce noise. Connect all ground pins to the ground plane using multiple vias for improved grounding.
Differential Pair Routing
Differential Pair Spacing: Follow industry guidelines for differential pair routing, such as the IPC recommendations. Typically, maintain a spacing of 0.15mm to 0.25mm (6 mils to 10 mils) between differential pairs for a smartwatch.
Length Matching: Ensure that the traces within a differential pair are carefully routed to have equal lengths to maintain signal integrity.
Avoidance of Sharp Corners
When changing trace direction, incorporate rounded corners with a minimum radius of 0.15mm (6 mils) to prevent signal reflections and maintain signal integrity.
Mechanical Stability
Mechanical Stability Rigid-Flex PCB Design Guidelines
Proper considerations for mechanical aspects help prevent issues like PCB bow and twist, warping, and stress concentration. Some essential guidelines follow. Again, we will use our smartwatch example as a reference:
Determine the minimum bend radius based on the materials’ flexibility.
For example, if using polyimide, ensure a minimum bend radius of 10 times the board thickness (e.g., 0.5mm board thickness requires a minimum bend radius of 5mm).
Mounting Holes and Pads
Maintain a minimum distance of 1.5 times the hole diameter between mounting holes and the board edge for better mechanical strength.
Use appropriate pad sizes and shapes for mounting holes to accommodate mechanical stresses and provide secure mounting points.
Keep-Out Zones
Define keep-out zones around components to prevent interference with mechanical structures or neighboring components.
Component-to-Edge Clearance
Maintain a minimum distance of 2 times the component height between components and the board edge to prevent potential damage during flexing.
Reinforcement Structures
Utilize stiffeners of materials like FR4 or polyimide to reinforce areas requiring additional rigidity or support.
Apply adhesive uniformly when attaching stiffeners to distribute stress evenly and enhance mechanical stability.
Fiducial Markers
Position fiducial markers on both the rigid and flex sections for accurate assembly and alignment
Thermal Management Rigid-flex PCB Design Guidelines
Some key guidelines cover thermal vias, heat sinks, copper pads, and ventilation.
Thermal Vias
Via Density: Incorporate an adequate number of thermal vias to enhance heat dissipation. Depending on the thermal requirements, the recommended via density is typically 1 to 2 vias per square centimeter.
Via Size and Spacing: Use vias with a sufficient diameter, such as 0.3mm or larger, and maintain appropriate spacing to facilitate heat transfer.
Heat Sinks and Copper Pads
Heat Sink Placement: Position heat sinks strategically over power components or areas generating high heat to draw away thermal energy.
Copper Pad Size: Ensure adequate copper pad sizes for power components to promote efficient heat dissipation.
Copper Pour Relief
For Thermal Relief using Copper pours, implement thermal relief patterns in copper pours connected to thermal vias to reduce thermal impedance and facilitate heat flow.
Heat-Generating Component Placement
Provide sufficient space between heat-generating components to prevent heat buildup and ensure proper airflow.
Clearance from Enclosures: Maintain appropriate clearance between components and enclosures to allow for adequate air circulation and avoid trapping heat.
Thermal Material Considerations
Choose materials with higher thermal conductivity, such as ceramic-filled materials, to enhance heat dissipation capabilities.
Make sure that the CTE (Coefficient of Thermal Expansion) values of different materials in the rigid-flex stackup are compatible to minimize the risk of thermal stress and cracking.
Airflow and Ventilation
Design appropriate openings or slots in the enclosure or rigid sections to promote natural convection and allow for airflow. Let’s say that for the smartwatch we are making, identify the areas where heat is likely to accumulate. These are areas around the processor, battery, or other heat-generating components. Make slots or openings in these places to let cool air in and hot air out.
IPC Rules for Surface Finish
Rigid-flex PCB design guidelines from the IPC for Surface Finishes
The IPC (Association Connecting Electronics Industries) provides guidelines for various surface finishes commonly used in the industry. They are the following:
ENIG (Electroless Nickel Immersion Gold)
The recommended gold thickness for ENIG is typically 1-3µm (micro-meters), while the nickel thickness ranges from 3-5µm.
ENIPIG (Electroless Nickel Immersion Palladium Immersion Gold)
The recommended palladium thickness for ENIPIG is typically 0.05-0.15µm, while the gold thickness ranges from 0.05-0.15µm.
Immersion Silver
The silver thickness for immersion silver surface finish should be around 0.15-0.3µm.
HASL (Hot Air Solder Leveling)
HASL is a widely used surface finish but only for the rigid parts of the rigid-flex PCB.
The recommended thickness for HASL solder should be around 1.6-3.2µm.
OSP (Organic Solderability Preservatives)
The OSP coating thickness should be approximately 0.2-0.5µm.
Rigid-flex PCB design guidelines from the IPC for Conductive Materials
We discuss the IPC-recommended conductive materials for rigid-flex PCBs, along with their corresponding specifications:
Copper Foils
Standard Electrodeposited Copper: Recommended thickness ranges from 12 μm to 105 μm.
High-Temperature Copper: Suitable for elevated temperature applications, with a recommended thickness of 12 μm to 70 μm.
Low-Profile Copper: Offers reduced thickness for space-constrained designs, with a recommended range of 5 μm to 18 μm.
Silver Inks
Conductive silver inks should have a minimum conductivity of 3 × 10^6 S/m and a surface resistivity of less than 0.1 Ω/sq.
Conductive Polymers
Conductive polymers typically have a less than 1 Ω·cm volume resistivity.
Gold Plating
Selective gold plating should have a minimum gold thickness of 0.05 μm for reliable contact performance.
Carbon-Based Materials
Conductive carbon inks and coatings should have a sheet resistance of less than 100 Ω/sq for effective EMI/RF shielding.
Your Rigid-Flex PCB Authority
Summary of the Blog on Rigid-flex PCB Design Guidelines
In this comprehensive blog on rigid-flex PCB design guidelines, we have explored various crucial aspects to consider. We discussed the importance of material selection, layer stackup, bend radius, component placement, trace routing, mechanical stability, thermal management, and quality control.
We used the example of a smartwatch to illustrate how following the Rigid-flex PCB design guidelines and IPC guidelines can optimize their designs for performance and reliability.
And now that you have a solid understanding of Rigid-Flex PCB design guidelines, it’s time to explore a trusted source for obtaining the necessary materials and support.
Supporting your Rigid-Flex PCB Designs: MV Flex Circuit
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MV Flex Circuit has an extensive range of rigid-flex PCBs that use the PCB materials described in this blog. Our PCBs have copper foils, conductive polymers, and gold plating options, and you can choose the right components for your specific application.
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