Flex PCB design guidelines are important because flex PCBs are necessities in today’s dynamic technological landscape. This printed circuit board (PCB) type has positively impacted many industries with its unmatched flexibility and adaptability.
You may not realize how much flex PCBs impact your daily life. When you enter the office, you encounter various devices that rely on flex PCB technology. The sleek and compact keyboard on your desk, featuring a flexible circuit layer beneath the keys, ensures reliable input for your computer tasks.
Advantages and Applications of Flex PCBs
What about your state-of-the-art monitor? Its thin and lightweight design is made possible by flex PCBs. These flexible circuits enable the seamless integration of the display panel and the driver electronics, providing a rich visual experience.
As you grab your morning coffee, take note of the coffee machine’s touch-sensitive control panel. This user-friendly interface relies on flexible circuits for its responsive touch functionality.
Even the smart thermostat on the office wall, with its programmable features and energy efficiency, owes its compactness to flex PCBs. This device’s intricate wiring and sensors are seamlessly integrated into a flexible circuit, optimizing space and performance.
As you can see, flex PCBs offer advantages such as:
- Space savings
- Improved reliability
- Enhanced design flexibility
Their applications extend beyond the office, impacting industries like healthcare, automotive, aerospace, and more.
Approach of this blog
One intriguing application is the development of a flex PCB-based instrument cluster for cars. This blog will use the flex PCB design guidelines for creating a flex PCB using the instrument cluster as our illustrative case. We hope this will help concretize the concepts of the guidelines for you.
Bending, Flexing & Stress
Flex PCB Design Guidelines for Bending, Flexing, and Dynamic Stress
These elements need attention. To give you an example, you can design a flex PCB for an instrument cluster in a car by following these design guidelines:
Bending
Determine the appropriate bend radius for your flex PCB based on the specific requirements of the instrument cluster. Aim for a minimum bend radius of 10-20 times the thickness of the flex PCB material. For example, if the flex PCB thickness is 0.2mm, the recommended bend radius would be 2-4mm.
Flexing
Consider the expected flexing frequency of the instrument cluster to select materials and components that can withstand the anticipated number of flex cycles. Choose flex PCB materials with a high flex life rating, typically exceeding 10,000 flex cycles, to ensure long-term durability in the automotive environment.
Dynamic Stress
Evaluate the potential dynamic stresses the instrument cluster may experience during operation, such as vibrations and shocks from the vehicle’s movements. Opt for flex PCB materials with good mechanical strength and flexibility, with tensile strength values ranging from 150 to 300 MPa, to withstand dynamic stresses without compromising performance.
Remember that this is for our sample case only. Know what bending, flexing, and dynamic stress your project needs before researching your device’s exact flex PCB design guidelines.
Flex PCB Design Guidelines for Managing Temperature
If you are designing a flex PCB-based instrument cluster for cars and following the flex PCB design guidelines, consider the impact of temperature variations on the performance. Manage temperature-related challenges by:
Determine the expected temperature range
What will the instrument cluster encounter during normal operation? Select flex PCB materials that can withstand those extremes. For automotive applications, materials with a wide temperature range, such as -40°C to 125°C, are commonly used to ensure reliable performance in various climates.
Thermal Expansion Coefficient
Choose flex PCB materials with a thermal expansion coefficient closely matched that of the components and substrates they interface with. This minimizes stress and prevents delamination or component damage caused by differential expansion or contraction due to temperature variations.
Thermal Dissipation
Use thermal vias, heat sinks, and other thermal management techniques to dissipate heat generated by power components or high-speed circuits. Ensure efficient thermal transfer and consider the thermal conductivity of the flex PCB materials, aiming for values above 1 W/mK.
Substrates
Choosing the Substrates in Consideration of Flex PCB Design Guidelines
Choosing the right substrate material is a critical aspect of flex PCB design. Let’s evaluate the suitability of each material in our illustrative case of the flex PCB-based instrument cluster for cars.
For our example, the choice of materials and their distribution will depend on various factors, such as the specific design requirements alongside our material evaluation; we give a hypothetical example of material distribution:
Polyimide (PI)
This material could be used for the flexible areas where the PCB needs to bend and conform to the curved surface of the instrument cluster. It allows the PCB to bend and conform to the curved surface of the dashboard. PI may constitute around 70% of the overall PCB area, ensuring flexibility and reliability.
PET (Polyethylene Terephthalate)
Particularly good for areas that require moderate flexibility, PET is suitable for sections that require moderate flexibility and cost-effectiveness. It may be used for around 20% of the PCB area, such as areas with less curvature or where higher temperature resistance is not crucial.
LCP (Liquid Crystal Polymer)
Its excellent dimensional stability and high-temperature resistance make it suitable for critical sections that require precise component positioning and fine-pitch trace routing. LCP may be allocated around 5% of the PCB area, focusing on areas with high-density components or complex routing.
PEN (Polyethylene Naphthalate)
Balancing flexibility and rigidity, good thermal stability, and mechanical properties, PEN is suitable for areas requiring resistance to vibration and environmental stress. It may be used for the remaining 5% of the PCB area.
It’s important to note that the specific material distribution will depend on evaluating the intended application. Now, let’s proceed to the other guidelines on our list.
Bend Radius
Bend Radius Flex PCB Design Guidelines
Discussing this topic entails posting the values of minimum bend radius for different substrate materials. Here they are:
Polyimide (PI)
The minimum bend radius for polyimide substrates typically ranges from 10 to 20 times the substrate thickness. For example, if the substrate thickness is 0.1mm, the minimum bend radius would be around 1mm to 2mm.
PET (Polyethylene Terephthalate)
It has higher flexibility compared to polyimide. The minimum bend radius for PET can be around 5 to 10 times the substrate thickness. The minimum bend radius for a substrate thickness of 0.1mm would be approximately 0.5mm to 1mm.
PTFE (Polytetrafluoroethylene)
These substrates, known for their high-temperature resistance, have a minimum bend radius ranging from 20 to 30 times the substrate thickness. If the substrate thickness is 0.1mm, the minimum bend radius would be around 2mm to 3mm.
LCP (Liquid Crystal Polymer)
They offer excellent flexibility and dimensional stability. The minimum bend radius for LCP can be around 5 to 10 times the substrate thickness. The minimum bend radius for a substrate thickness of 0.1mm would be approximately 0.5mm to 1mm.
PEN (Polyethylene Naphthalate)
The minimum bend radius for PEN can range from 5 to 15 times the substrate thickness. If the substrate thickness is 0.1mm, the minimum bend radius would be around 0.5mm to 1.5mm.
How to determine the correct bend radius by factoring material thickness and the number of layers
Here’s a general approach to knowing the correct bend radius for whatever material you use.
- Determine the thickness of the flex PCB material (T).
- Identify the material factor (MF) the manufacturer provides for the specific substrate material.
- Know the number of layers factor (LF) based on recommendations from the PCB manufacturer.
- Multiply the material thickness (T) by the material factor (MF) to obtain the adjusted material thickness (T_adj = T * MF).
- Next, multiply the adjusted material thickness (Tadj) by the number of layers factor (LF) to obtain the calculated bend radius (R = T_adj * LF).
- Lastly, add a safety margin (SM) to the calculated bend radius (R) to ensure the design’s reliability and durability. Safety margin is typically around 10-20% of the calculated bend radius.
Here’s an informative video about Bend radius in Printed Circuit Boards by a reputable software company:
Flex PCB Design Guidelines for Trace Routing
There are important considerations regarding trace routing in flex PCB designs. We will again use the illustrative case of the car instrument cluster. Let’s explore the key guidelines:
Minimize Crosstalk
To reduce electromagnetic coupling and interference, we follow a guideline that suggests maintaining a spacing between adjacent signal traces at least 3 times the trace width. For example, if the trace width is 0.2mm, we ensure a minimum spacing of 0.6mm.
Attain Impedance Control
Achieve the desired characteristic impedance. Refer to guidelines that provide trace width and spacing recommendations. For instance, if we target a characteristic impedance of 50 ohms, we might use a trace width of 0.25mm and a spacing of 0.2mm.
In addition, incorporate shielding techniques such as ground planes and differential pair routing to minimize crosstalk and preserve signal integrity.
EMI/EMC Mitigation and Signal Routing
Ensuring reliable and interference-free operation in our sample case of the car instrument cluster is crucial. Here are the flex PCB design guidelines we follow:
Ground Planes
Incorporate solid ground planes into the PCB design to provide a low-impedance path for return currents and minimize ground loops.
Signal Routing
To mitigate EMI, route sensitive or high-speed signal traces away from potential sources of electromagnetic radiation, such as power lines or components generating significant EMI.
Filtering Components
Deliberately place filtering components like capacitors and ferrite beads at critical points in the circuit to suppress high-frequency noise and harmonics.
Layouts
Flex PCB Design Guidelines for Layouts
For flex PCB design layouts, follow the guidelines to make your design safe. In our case of the flex PCB-based instrument cluster for cars, you should, for example, pay close attention to the following aspects:
Group components based on mechanical stress tolerance
Components on a flex PCB may experience varying levels of mechanical stress during bending or flexing. It is crucial to group components based on their mechanical stress tolerance. Components that are more susceptible to stress or prone to failure under mechanical strain should be placed in areas of lower stress. This helps to ensure the longevity and overall reliability of the flex PCB.
Engage in techniques to accommodate component movement
During bending or flexing, components may undergo slight movement or displacement. To accommodate this movement, use design techniques such as using flexible component leads or allowing additional clearance between components. Doing so prevents stress concentration and potential damage to the components or the flex PCB.
Speaking of designs, let’s focus on an advanced topic.y
Flex PCB design guidelines & Design for Manufacturability (DFM)
Designing a flex PCB involves more than just the circuit layout and functionality. It also requires careful consideration of manufacturability to ensure smooth and efficient production processes. By following design for manufacturability (DFM) principles, you can optimize your design for streamlined assembly, reduced costs, and improved overall quality. Let’s discuss the key aspects of DFM in flex PCB assembly.
Component selection and placement for easy assembly
Component Footprint Standardization: Standardizing component footprints across your design simplifies the assembly process. By using commonly available and widely used components, you enhance compatibility with automated assembly equipment, reduce procurement lead times, and lower costs.
Component Spacing and Orientation: Maintaining sufficient spacing between components and ensuring clearances for assembly equipment access is crucial. Consider the accessibility of soldering points, the use of pick-and-place machines, and the overall efficiency of the assembly process.
Design optimization for the manufacturing process
Design Rule Check (DRC): Running a comprehensive DRC ensures that your design adheres to manufacturing constraints and specifications. This step helps identify potential issues early on, such as incorrect pad sizes, clearance violations, or trace widths that are difficult to manufacture.
Design Simplification: Streamlining your design by reducing unnecessary complexity improves manufacturability. Simplified designs with optimized routing, reduced vias, and minimized layer changes lead to faster and more cost-effective production.
Design validation and prototyping
Design Verification: Before moving into full-scale production, it’s crucial to validate your design through prototyping. Prototyping allows you to identify and address potential issues or design flaws early on, reducing the risk of costly rework or product failures during assembly.
Design for Testability (DFT): Implementing DFT techniques in your design enables easier testing and fault diagnosis during assembly. Incorporate test points, built-in self-test (BIST) features, and other testing provisions to facilitate efficient testing and troubleshooting.
Integrating DFM principles into your flex PCB design allows you to streamline the assembly process, reduce manufacturing complexities, and ensure a smoother transition from design to production. This ultimately results in improved product quality, reduced costs, and faster time-to-market for your electronic devices.
A discussion of supply chain management and component sourcing will be covered in the section after this one.
Coverlay and Solder Mask
Coverlay and Solder Mask Flex PCB Design Guidelines
Lastly, let’s talk about the final layer of flex PCBs, the coverlay, and the solder mask. In our example case of the flex PCB-based instrument cluster for cars, the guidelines are often used:
Coverlay thickness
The coverlay, also known as a flexible solder mask, is applied to protect the flexible circuit and provide insulation. The typical range for coverlay thickness is between 0.025mm to 0.075mm (25µm to 75µm). The thickness is determined based on the required insulation properties and the mechanical requirements of the flex PCB.
Coverlay opening dimensions
To allow for component soldering and expose specific areas of the circuit, create openings in the coverlay. The dimensions of these openings should be carefully determined to ensure proper alignment with component pads and sufficient solder mask coverage. For example, the recommended minimum solder mask web width between two adjacent openings can be around 0.15mm.
Solder mask application
A solder mask is applied to protect the conductive traces and prevent solder bridging during assembly. It is important that the solder mask is applied uniformly and covers all the required areas. For flexible PCBs, the solder mask thickness typically ranges from 0.015mm to 0.05mm (15µm to 50µm).
Find Your Authoritative Partner for Flex PCB design Guidelines: MV Flex Circuit
At MV Flex Circuit, we understand that navigating the complex world of flex PCB design guidelines can be overwhelming. That’s why we offer guided design support tailored to your specific needs. Our experienced engineers and designers will work closely with you to ensure that your flex PCB design meets all the requirements for optimal performance, reliability, and manufacturability.
We are experts in Flex PCBs and more
With our expertise in designing and manufacturing flex PCBs, we have successfully implemented numerous projects, including automotive instrument clusters. We can bring our 15+ years of expertise to your project.
Simplify the complexities
When you partner with MV Flex Circuit, you can leave the complexities of flex PCB design guidelines to us. Our team will handle the technical aspects, ensuring your design complies with industry standards and best practices. So now, you can focus on your core competencies and bring your innovative ideas to life.
We provide personalized support every step of the way. Work together with us and achieving exceptional results.
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