Why are all PCB multilayer boards even-numbered?
PCB boards are available in single-sided, double-sided and multilayer versions. The number of layers of multilayer boards is unlimited. Currently, there are PCBs with more than 100 layers, and the most common multilayer PCBs are four-layer and six-layer boards. So why do people have the question “Why are all PCB multilayer boards even-numbered?”? Relatively speaking, PCBs with even layers do have more layers than PCBs with odd layers, and they are more advantageous.
Learn more! Why are all PCB multilayer boards even-numbered?
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 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 production efficiency of the factory that adds foil outside the core structure will decrease. Before lamination and bonding, the outer core requires additional process treatment, 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 PCBs with odd layers is that odd-layer 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 can 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 stack. Although a PCB with a certain degree of bending meets the specification requirements, the efficiency of subsequent processing will be reduced, resulting in increased costs. Because special equipment and processes are required during assembly, the accuracy of component placement is reduced, which will compromise 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-layer boards. Even if the odd-numbered layers realize functions, they will be designed as pseudo-even 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 layers, and odd layers are less.
Knowledge! Why are PCB multi-layer boards all even-numbered?
How to balance the stacking and reduce the cost of odd-numbered PCBs?
What should I 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) One signal layer and use it. 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 additional layers do 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 odd and the signal layer is even. A simple method is to add a ground layer in the middle of the stack without changing other settings. First route the PCB according to the odd number of layers, then copy the ground layer in the middle and mark the remaining layers. This is the same as the electrical characteristics of the foil with thickened ground layer.
3) Add a blank signal layer near the center of the PCB stack. This method minimizes the imbalance of the stack and improves the quality of the PCB. First route the odd number of layers, then add a blank signal layer and mark the remaining layers. It is used in microwave circuits and mixed medium (medium has different dielectric constants) circuits.