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Why do most PCB multilayer circuit boards have an even number of layers?

PCB circuit boards are single-sided, double-sided and multi-layered. The number of layers of multi-layer boards is not limited. Currently, there are more than 100-layer PCBs. The common multi-layer PCBs are four-layer and six-layer boards. So why do PCB multi-layer boards have an even number of layers? Relatively speaking, even-numbered PCB circuit boards do have more than odd-numbered layer PCB circuit boards, and they have more advantages.

1. Lower cost

Because there is one less layer of dielectric and foil, the cost of raw materials for odd-numbered PCB boards is slightly lower than even-numbered layer PCBs. However, the processing cost of odd-numbered layer PCB is significantly higher than that of even-numbered layer PCB. The processing cost of the inner layer is the same, but the foil/core structure significantly increases the processing cost of the outer layer.

Odd-layer PCB requires a non-standard laminated core layer bonding process based on the core structure process. Plants that add foil outside the core structure will experience reduced productivity compared to the core structure. The outer core requires additional processing before lamination can be bonded, which increases the risk of scratches and etching errors on the outer layer.

2. Balanced structure to avoid bending

The best reason not to design a PCB with odd layers is that boards with odd layers tend to bend. When the PCB cools after the multilayer circuit bonding process, the different lamination tensions of the core and foil structures can cause the PCB to warp. As the board thickness increases, the risk of bending of a composite PCB with two different structures increases. The key to eliminating board flex is to use a balanced stackup. Although a PCB that is bent to a certain extent meets the specification requirements, subsequent processing efficiency will be reduced, resulting in increased costs. Because assembly requires special equipment and processes, the accuracy of component placement is reduced, thus compromising quality.

Another way to put it more easily understandable is: in the PCB process, four-layer boards are easier to control than three-layer boards, mainly in terms of symmetry. The warpage of four-layer boards can be controlled below 0.7% (IPC600 standard), but When the size of the three-layer board is large, the warpage will exceed this standard, which will affect the reliability of the SMT patch and the entire product. Therefore, generally designers do not design odd-numbered boards. Even if the odd-numbered layers implement functions, they will Designed as a false even number of layers, that is, the 5th layer is designed into a 6th layer, and the 7th layer is designed into an 8-layer board.

For the above reasons, most PCB multi-layer boards are designed with even-numbered layers and fewer odd-numbered layers.

What should we do if there are odd-numbered layers of PCB circuit boards in the design? The following methods can be used to achieve balanced stacking, reduce PCB production costs, and avoid PCB bending.

1) A signal layer and exploit it. This method can be used if the PCB is designed with an even number of power layers and an odd number of signal layers. The added layers do not increase costs, but can shorten delivery times and improve PCB quality.

2) Add an additional power layer. This method can be used if the PCB is designed with an odd number of power layers and an even number of signal layers. An easy way to do this is to add a ground layer in the middle of the stack without changing other settings. First lay out the odd-numbered PCB layers, then copy the ground layers in the middle and mark the remaining layers. This is the same electrical characteristic as a thickened foil.

3) Add a blank signal layer near the center of the PCB stackup. This approach minimizes stackup imbalance and improves PCB quality. Route the odd layers first, then add a blank signal layer and label the remaining layers. Used in microwave circuits and mixed media (mediums with different dielectric constants) circuits

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