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Why do Multi-layer PCBs Always Have an Even Number of Layers?

by: Jan 29,2024 1545 Views 0 Comments Posted in PCB Basic Information

Multi-layer PCB Layers Even number

The number of layers in a printed circuit board (PCB) can be classified into single-layer (1 layer), double-layer (2 layers), and multi-layer. The number of layers in multi-layer PCBs is not restricted and can range from a few layers to as many as 100 layers. Commonly used configurations for multi-layer PCBs include 4 layers, 6 layers, and 8 layers.

Some customers may notice that regardless of the PCB manufacturer chosen, the majority of available options for multi-layer boards involve an even number of layers rather than an odd number. Why does this situation arise?

In theory, as long as it falls within the capabilities of the manufacturing process, PCBs can be fabricated with either an even or odd number of layers. However, both online and offline, manufacturers are generally less inclined to process orders for odd-layered PCBs unless a substantial amount is invested to persuade them to do so.

This reluctance stems from the fact that, relatively speaking, PCBs with an even number of layers offer more advantages than those with an odd number of layers. The following three aspects highlight the advantages of even-layered PCBs over odd-layered PCBs.

Low Cost

Due to the absence of one layer of dielectric and foil, the raw material cost for odd-layer PCBs is slightly lower than that for even-layer PCBs. However, the processing cost for odd-layer PCBs is significantly higher than that for even-layer PCBs. The processing cost for inner layers remains the same, but the foil/core structure noticeably increases the processing cost for outer layers.

Odd-layer PCBs require a non-standard lamination core bonding process on top of the core structure process. Factories adding foil to the core structure lose productivity compared to core structure alone. The outer core requires additional processing before lamination, increasing the risk of outer layer scratches and etching errors.

Moreover, for many manufacturers, even-layer designs fall under common practices, while odd-layer designs are considered special. Special designs imply the need for unique production processes to match specific or even entirely different production parameters, incurring substantial trial-and-error costs.

Manufacturers do not view special designs as part of routine production processes because specialized engineers must handle the process, naturally leading to increased labor costs. More importantly, successful implementation requires collaborative efforts involving production, technical, quality, and other departments to discuss and collectively address the challenges of special designs.

All these factors contribute to the expensive nature of odd-layer PCBs.

Balanced Structure to Avoid Warping

PCB boards with an odd number of layers are prone to warping. During the lamination process in multilayer circuits, the different lamination tensions between the cooling of the core structure and foil-clad structure may result in PCB warping. As the board thickness increases, the risk of warping in composite PCBs with two different structures also rises. The key to eliminating board warping lies in a balanced lamination structure. Although PCBs with a certain degree of warping may still meet specifications, subsequent processing efficiency decreases, leading to increased costs. This is because assembly requires special equipment and processes, reducing the precision of component placement and affecting overall quality.

In other words, in our PCB manufacturing process, 4-layer PCBs are easier to control compared to 3-layer boards, primarily in terms of symmetry. The warpage of 4-layer boards can be controlled to below 0.7% (IPC600 standard). However, when the size of a 3-layer board is larger, warpage will exceed this standard, impacting the reliability of SMT soldering and the entire product. Therefore, conventional PCB designers typically avoid odd-layer designs. Even if odd-layer designs achieve functionality, they are often designed to appear as false even-layer designs, such as representing a 5-layer design as 6 layers or a 7-layer design as 8 layers.

Even-layered Stack Design Enhances PCB Interference Resistance

The physical structure of a PCB multilayer board consists of copper foil and substrate. In a PCB multilayer board, each layer has its own copper foil traces and substrate. When the number of layers in a PCB multilayer board is odd, the distance between the copper foil and substrate of each layer increases, leading to an increase in signal transmission delay and interference. In contrast, when the number of layers in a PCB multilayer board is even, the distance between the copper foil and substrate of each layer remains relatively stable, resulting in lower signal transmission delay and interference. Therefore, to ensure the quality and stability of signal transmission, PCB multilayer boards are typically designed with an even number of layers.

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