Rigid PCB vs. Flex PCB Design Factors

Why Are Rigid PCB Layer Counts Usually Even?

In rigid PCB design, we typically see layer counts like 2, 4, 6, or 10—rarely odd numbers. This is due to the symmetrical lamination method used in PCB manufacturing. A PCB is mainly constructed by pressing together PP and Core layers. PP consists of a copper foil with an FR4 insulation layer, while the Core is a central board made of copper foil on both sides with an FR4 insulation core.

1. Importance of Symmetrical Structure

The stack-up of rigid PCB layers strictly follows the principle of symmetry. This requirement dictates that PCBs must have an even number of layers, except for single-layer boards, which use only one PP without needing a Core.

Even-layer stack-ups are structurally symmetrical. For example, a standard 4-layer board follows a structure like: 

[Signal Layer / Inner Power Layer / Inner Ground Layer / Signal Layer]

Such symmetry ensures balanced thermal stress during high-temperature lamination and subsequent cooling, preventing warping or bending. Odd-layer boards lack this symmetry, making them prone to deformation during production or use, which compromises assembly and reliability.

2. Optimized Manufacturing Cost

PCB fabrication processes (like lamination, drilling, and etching) are standardized for even-layer boards. When producing an odd-layer PCB, manufacturers often need to add a "dummy layer" to maintain symmetry, increasing processing steps, material waste, and ultimately, cost. Thus, odd-layer boards do not offer cost advantages; instead, they complicate production and raise pricing.

3. Superior Electrical Performance

Even-layer boards make it easier to pair power and ground planes, which is beneficial for high-speed and high-frequency signal performance. Odd-layer designs can lead to impedance mismatches and increased electromagnetic interference, negatively affecting signal integrity and electromagnetic compatibility.

4. Industry Standards and Design Tools

Most PCB design software, layer stack configurations, and auto-routing tools are optimized for even-layer structures. Manufacturers also prefer even-layer boards in terms of quoting, production, and quality control.

5. Signal Integrity Requirements In Rigid PCB Design

High-speed signal layers typically require a consistent reference ground plane. Even-layer stack-ups simplify alternating signal and power/ground layers, reducing crosstalk.

Why Can a Flex PCB Design Have Odd Layers?

While odd layers are not recommended for rigid PCBs due to material symmetry and warping risks, flex PCB design allows for both odd and even layer configurations. Though FPCs are also laminated, the stack-up process is different: layers are applied sequentially after shape cutting, and symmetry isn’t required. Therefore, 3, 5, or 7-layer flex PCBs are common.

1. Flexible Material Properties in Flex PCB Design

FPCs primarily use polyimide films as their base, which have low thermal expansion and high ductility. These films are much thinner (12.5μm to 50μm) than rigid FR4 substrates. Their softness inherently prevents warping from asymmetric copper distribution. Therefore, symmetrical stack-up isn't necessary. Flex PCB manufacturers can directly build odd-layer designs (e.g., 3-layer: copper–PI–copper–PI–copper) without compensation layers.

2. Designed for Dynamic Connections

Unlike rigid boards, Flexible PCBs aren't meant to remain flat. Their purpose is to bend, fold, and interconnect. As long as thickness, flexibility, and tensile strength meet requirements, odd-layer FPC boards work just fine. In fact, a 3-layer flex PCB has at least one less dielectric layer than a 4-layer flex PCB, making it more suitable for space-constrained designs.

3. Reinforcement Possible for Strength in Flex PCB Design

Mechanical strength in areas like solder joints or connectors can be improved by stiffeners or coverlay reinforcements, compensating for structural asymmetry even in odd-layer designs.

4. Lower SI/EMI Demands

Most flex PCB applications carry low-speed signals or sensor lines, where strict impedance control isn’t necessary. Even in high-speed cases, designers can manually balance the layout to mitigate signal integrity concerns.

5. Custom Wiring Advantages

Multilayer FPCs (e.g., 5-layer) allow flexible allocation of signal, power, and ground layers, making them ideal for complex topologies. This enables integration of differential pairs and isolated power zones within limited layer counts.