Why Are PCB Multilayer Boards Usually Even-Numbered?

This Gold Phoenix PCB Knowledge Center article explains why PCB multilayer boards are typically designed with even numbers of layers and how to balance stacking for odd-layer designs.

There are single-sided, double-sided, and multilayer circuit boards, and there is no fixed limit on the number of layers in a multilayer board. Currently, there are PCBs with more than 100 layers, while four-layer and six-layer boards are common multilayer PCB configurations. Relatively speaking, even-layer PCBs do have more advantages than odd-layer PCBs.

1. Low Cost

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

Odd-layer PCBs require additional non-standard stacked-core bonding processes on top of the core structure process. Compared with the core structure, the production efficiency of factories that add foil outside the core structure will decrease. Before lamination and bonding, the outer core requires additional processing, which increases the risk of scratches and etching errors on the outer layer.

2. Balanced Structure to Avoid Bending

The main reason for avoiding odd-layer PCB designs is that odd-layer circuit boards are prone to bending. When the PCB is cooled after the multilayer circuit bonding process, differences in lamination tension between the core structure and the foil structure during cooling can cause the PCB to bend. As the thickness of the circuit board increases, the risk of bending in composite PCBs with two different structures also increases. The key to eliminating circuit board bending is to use balanced stacking. Although a PCB with a certain degree of bending may meet specification requirements, subsequent processing efficiency will decrease, resulting in increased costs. Due to the need for special equipment and processes during assembly, component placement accuracy is reduced, which will damage quality.

A more easily understandable explanation is that, in PCB process technology, four-layer boards are easier to control than three-layer boards, mainly because of their symmetry. The warping degree of four-layer boards can be controlled below 0.7% (according to the IPC600 standard), but when the size of three-layer boards is large, the warping degree will exceed this standard, which will affect the reliability of SMT mounting and the entire product. Therefore, most designers do not design odd-layer boards. Even when odd-layer boards are needed to achieve specific functions, they may be designed as false-even-layer boards; that is, 5-layer boards are designed as 6-layer boards, and 7-layer boards are designed as 8-layer boards.

Based on the above reasons, PCB multilayer boards are mostly designed with even numbers of layers, with fewer odd-layer boards.

How to Balance Stacking and Reduce Costs for Odd-Layer PCBs?

What if an odd-layer PCB is required in the design? The following methods can achieve balanced stacking, reduce PCB production costs, and avoid PCB bending.

1.Add one signal layer for utilization. If the power layer of a PCB is even and the signal layer is odd, this method can be used. The added layer does not increase costs, but it can shorten delivery time and improve PCB quality.

2.Add an additional power layer. If the power layer of a PCB is odd and the signal layer is even, this method can be used. A simple method is to add a layer in the middle of the stack without changing other settings. First, route the PCB according to the odd‑layer PCB type, then copy the middle layer and mark the remaining layers. This is the same as the electrical characteristics of foil applied to a thickened core structure.

3.Add a blank signal layer near the center of the PCB stack. This method minimizes stacking imbalance and improves PCB quality. First, route the PCB according to the odd‑layer design, then add a blank signal layer and mark the remaining layers. It is used in microwave circuits and mixed‑medium circuits with different dielectric constants.