Why are Most of The PCB Design Layers Even

I don't know if you have found this phenomenon: most PCB design layers are even layers, such as the common four-layer board, six-layer board. Why is that? In fact, even layer PCB do have more advantages than odd-layer PCB.

Why are most of the PCB design layers even?

Advantages of even layer PCB

1. Low cost

Due to the lack of a layer of media 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 PCB is significantly higher than that of even-layer PCB. The machining cost of the inner layer is the same, but the foil/core structure significantly increases the machining cost of the outer layer.

Odd-layer PCB needs to add a non-standard laminated bonding process on the basis of the core structure process. The production efficiency of a plant that adds foil to the nuclear structure will be reduced compared to the nuclear structure. Before laminating, the outer core requires additional treatment, which increases the risk of scratching and etching errors in the outer layer.

2. Balance the structure to avoid bending

The best reason to design a PCB without odd layers is that it bends easily. When cooling the PCB after the multilayer circuit bonding process, different laminating tension will cause the PCB to bend as the core structure and foil coated structure cool. As the board thickness increases, so does the risk of bending a composite PCB with two different structures. The key to eliminating circuit board bending is to use balanced layering. Although a certain degree of bent PCB meets the specifications, the subsequent PCBA processing efficiency will be reduced, resulting in an increase in costs. Because assembly requires special equipment and processes, the placement accuracy of the parts is reduced, and therefore the quality is damaged.

The changes are easier to understand: In the PCB technology process, the four-layer board is better than the three-layer board control, mainly in terms of symmetry, the warpage of the four-layer board can be controlled below 0.7% (IPC600 standard), but the size of the three-layer board, the warpage will exceed the standard, which will affect the reliability of SMT and the entire product, so for the general PCB designer, do not design odd layer board. Even with the odd-layer function, the counterfeit design is divided into even layers, the five-layer PCB design is divided into six layers, and the seven-layer PCB design is divided into eight layers.

Based on the above reasons, the design of PCB multi-layer boards mostly uses even layers, and rarely uses odd layers.

How to balance the cascade and reduce the cost of odd-layer PCB?

When an odd number of PCB layers appear in the PCB design, the following methods can balance the cascade, reduce the cost of PCB production and avoid PCB bending.

1. Add a signal layer and use it

This method can be used if the PCB's power layer is even and the signal layer is odd. Adding layers does not increase costs, but can reduce lead times and improve PCB quality.

2. Add additional power layer

This method can be used if the PCB has an odd power layer and an even signal layer. An easy way to do this is to add a layer in the middle of the stack without changing other Settings. First, an odd number of layers are arranged on the PCB, then the middle layer is copied and the remaining layers are labeled. This is similar to the electrical properties of foils applied in thickened layers.

3. Add a blank signal layer near the center of the PCB board

This approach minimizes stack unbalance and improves PCB quality. First, the odd-numbered layers of wiring are added, and then a blank signal layer is added, marking the remaining layers. It is used in microwave circuits and mixed media (different permittivity) circuits.