Complete flexible PCB design guide to help build projects better

Although different types of printed circuit boards are used in different electronics, they essentially perform the same function. While rigid PCB have been the norm, the advantages of flexible circuits in terms of cost-effectiveness, size, and weight have made them increasingly popular in electronic projects. However, designing flex PCB requires more attention to detail than a rigid board. TechSparks has compiled comprehensive design guidelines encompassing rules and tips to ensure successful flexible PCB design.

Flexible PCB Design Rules

9 Trace Layout Rules

Traces are an essential component of flexible PCB, serving as the means to connect electronic components and devices. Poorly designed trace layouts can result in electromagnetic interference, leading to subpar circuit performance. It’s important to note that every 2.5cm of trace creates an inductance of around 20nH during board operation. The question is, how much deviation can occur when voltage is applied to a poorly designed flex circuit?

Assuming a trace length of 2.5cm (20nH), an output current of 5A, and a switching transition time of 30ns, the circuit will generate a voltage offset of 3.33V at that point. This indicates that even a 2.5cm trace inductance can produce a significant voltage shift, which can cause the power supply to fail entirely. Therefore, designers must adhere to the following 9 flexible PCB trace design rules:

  1. Line width and spacing ≥ 3mil;
  2. Plated through hole (PTH) aperture ≥ 0.2mm, annular ring ≥ 0.15mm;
  3. Line margin≥0.2mm;
  4. The distance between the PTH hole and the broken line is ≥2mm;
  5. Avoid wiring between pads for better cover film window design;
  6. The wiring avoids hard corners and adopts arc design;
  7. Double-sided flexible PCB circuits need to be staggered rather than overlapped;
  8. Circuit orientation perpendicular to or at 45° to the fold;
  9. Line spacing is distributed evenly rather than concentrating the folds.

Current Load Rules

The engineer should base the actual circuit specifications on their understanding and analysis of the project. It is important to avoid applying historical experience to a brand new project. The performance of circuits can be influenced by temperature, so temperature considerations must be included in the design. The design largely depends on factors such as the thickness of the insulation layer, the number of power lines, the specific construction design, air circulation, and others. Rectangular circuit designs are typically thought to achieve better heat dissipation, allowing traces with the same cross-section to carry more current. The table below displays the current carrying capacity and resistance of flexible PCB copper traces.

                 1.4mil                2.8mil
Width (mil)Load (A)Resistance (mΩ/ft)Load (A)Resistance (mΩ/ft)
5  0.25          1200   0.5          600
10  0.37          600  0.65          300
15  0.5          400  0.8          200
20  0.7          300  1         150
25  0.76          240  1.2         120
50 1.4         120  2.1         60
100  2         60  3.45        30

Flex PCB Design Guidelines and Tips

When to Design a Flexible PCB

There are different types of PCBs used in electronics, and selecting the best type for your project requires an understanding of its characteristics, functions, and intended use, which can provide ideas for the design. Compared with other PCBs, flex circuits can provide compact and lightweight packaging for better space utilization. However, they tend to be more expensive, so designers typically combine them with rigid boards to create a rigid-flex PCB that offers the benefits of both types. Additionally, flexible PCBs can be bent and folded to make them suitable for various complex three-dimensional space structures. If your project falls within the field of microelectronics, then designing a flex PCB correctly can provide you with numerous benefits.

Establish a flexible PCB model analysis circuit

Creating a circuit model is the most effective way to verify the feasibility of a flex circuit design. When analyzing circuits, considering all physical properties can make analysis difficult. Idealizing the actual circuit, ignoring its secondary properties, and replacing it with an ideal circuit can approximately represent the circuit.

  1. Identify the system points to be electrically connected via flexible circuits and termination methods;
  2. Determine the approximate circuit trace that will provide conductor routing for each terminal location;
  3. Review the schematic or netlist details to determine the approximate number of layers;
  4. Check whether the flexibility of similarly designed flex PCB samples matches the requirements;
  5. Review the mechanical requirements to ensure that the bend radius is within the acceptable values for circuit thickness and the number of layers. For guidance on acceptable bend radii, see IPC-2223;
  6. Use 0.25 mm polyester film to reconstruct the prototype and mount it in the prototype housing.

Testing mechanical samples

Before prototyping a flex PCB, it is important to test a mechanical sample to ensure that the flex circuit has the correct shape and fit. The shape refers to the physical size, shape, and weight of a part, while fit refers to its interface with the environment. Mechanical samples can help prevent installation problems or potential mechanical issues that could lead to failure.

Avoid complex designs lead to higher manufacturing costs

Factors that affect PCB manufacturing costs include size, number of layers, and features. Sophisticated designs usually come with higher manufacturing costs. When designing flexible PCB, it’s recommended to follow the recommended tolerances for line width, spacing, and pad size, and to avoid designing unnecessary non-bonding areas. If your project doesn’t require the assembly of surface-mount devices (SMDs), consider using a stiffener board instead of a rigid board. This can help reduce costs while maintaining the necessary level of functionality.

Don't ignore the minimum bend radius

When a flex circuit bends too much, it can cause several problems. The inner diameter may compress and cause the lid to wrinkle, while stretching on the outside bends can tear the covering material and damage the conductor. 

Bendability in design has two meanings: the number of bends and the degree of bend.

Dynamic bending: refers to the repeated folding or vibration during electronic use, which requires a stronger and more durable design to withstand tens of thousands of bending and folding.

Static bending: Occurs during manufacturing or assembly, where the board needs to be bent into a specific shape when it is secured to the device.

Bending radius: Refers to the minimum radius that the PCB board can withstand during the bending process. Designers need to determine this range during the design phase, and the value needs to match the application, otherwise it may lead to broken or damaged wires or components. It depends on several factors such as substrate thickness and material type, please refer to the corresponding industry standards and manufacturer’s recommendations when designing.

Bending ratio: The ratio of the radius of curvature (R) to the thickness (T) of the PCB, which is used to evaluate the reliability and durability of the flexible PCB during bending. The smaller the bending ratio, the easier it is for the board to break. According to the IPC-2223 standard recommendation, for a flexible PCB with a thickness of 0.1mm, the bending ratio should be greater than 4:1; for a flexible PCB with a thickness of 0.2mm, the bending ratio should be greater than 6:1.

Layer Separation

When manufacturing and using multilayer flexible boards, layer separation is a common risk that can lead to indirect damage or direct failure of the circuit. To mitigate this risk, it is necessary to strictly adhere to the solder paste usage specifications, such as usage amount, activity, and type. Despite taking measures to prevent most risks, quality inspection of PCB is still necessary during production. The following industry standards provide guidelines for testing and ensuring the efficiency of flex PCB design:

  • IPC-2223: This industry standard provides guidelines for material selection, adhesive selection, and interface selection for segmented flex PCB design. It also presents guidelines for vias.
  • IPC-FC-234: Includes testing guidelines for FPC.
  • IPC-6013C: Provides guidelines for the performance specifications.

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