Learn more! Why do PCB multilayer boards have an even number of layers?
PCBs are available in single-sided, double-sided and multi-layer versions. The number of layers in a multi-layer version is unlimited. Currently, there are PCBs with more than 100 layers, and the most common multi-layer PCBs are four-layer and six-layer versions. So why do people have this question, “Why are all multi-layer PCBs even-numbered?” Relatively speaking, PCBs with even layers do have more layers than PCBs with odd layers, and they have more advantages.
- Lower cost
Because there is one less dielectric layer and foil, the cost of raw materials for odd-numbered PCBs is slightly lower than that for even-numbered PCBs. However, the processing cost of odd-numbered PCBs is significantly higher than that of even-numbered PCBs. 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-numbered PCBs require a non-standard laminated core layer bonding process based on the core structure process. Compared with the core structure, the factory production efficiency will decrease when the foil is added to the outside of the core structure. Before lamination and bonding, the outer core requires additional process treatment, which increases the risk of scratches and etching errors on the outer layer.
- Balanced structure to avoid bending
The best reason not to design PCBs with odd layers is that odd-numbered circuit boards are prone to bending. When the PCB cools after the multi-layer circuit bonding process, the different lamination tensions of the core structure and the foil structure during cooling will cause the PCB to bend. As the thickness of the circuit board increases, the risk of bending of a composite PCB with two different structures increases. The key to eliminating circuit board bending is to use a balanced stacking. Although a PCB with a certain degree of bending meets the specification requirements, the subsequent processing efficiency will be reduced, resulting in increased costs. Because special equipment and processes are required for assembly, the accuracy of component placement is reduced, which will damage the quality.
Another easier to understand statement 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 three-layer boards is large, the warpage will exceed this standard, which will affect the reliability of SMT patches and the entire product. Therefore, general designers do not design odd-numbered boards. Even if odd-numbered layers are used to achieve functions, they will be designed as pseudo-even-numbered layers, that is, 5 layers are designed as 6 layers, and 7 layers are designed as 8 layers
Based on the above reasons, PCB multi-layer boards are mostly designed as even-numbered layers, and odd-numbered layers are less common.
How to balance the stacking and reduce the cost of odd-numbered PCB?
What should we do if an odd-numbered PCB appears in the design? The following methods can achieve balanced stacking, reduce PCB production costs, and avoid PCB bending.
1) Use one signal layer. This method can be used if the power layer of the designed PCB is an even number and the signal layer is an odd number. The added layer does not increase the cost, but can shorten the delivery time and improve the quality of the PCB.
2) Add an additional power layer. This method can be used if the power layer of the designed PCB is an odd number and the signal layer is an even number. A simple method is to add a ground layer in the middle of the stacking without changing other settings. First, route according to the odd-numbered PCB, then copy the ground layer in the middle, and mark the remaining layers. This is the same as the electrical characteristics of the thickened ground layer foil.
3) Add a blank signal layer close to the center of the PCB stacking. This method minimizes the imbalance of the stacking and improves the quality of the PCB. First, route according to the odd-numbered layers, then add a blank signal layer, and mark the remaining layers. Used in microwave circuits and mixed dielectric (media with different dielectric constants) circuits.
