Why are all PCB multilayers even? We can not live in an odd number of layers?

PCB boards have single-sided, double-sided and multilayer, which uses multilayer boards can be unlimited number of layers, at present our country has the ability to more than 100 layers of PCB, and common multilayer PCB is a four-layer and six-layer board. Then why we all may have "PCB multilayer board why these are an even number of layers?" This kind of no doubt about it? Relatively simple, even number of layers of multilayer pcb fabrication is indeed more than two odd number of layers of PCB, but also more development advantages.

01, lower cost

Because there is less than one layer of dielectric and foil, the raw material cost of an odd-numbered PCB board is slightly lower than that of an even-numbered layer PCB. but the processing cost of a single-layer PCB is higher than that of an 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-numbered PCBs require a non-standard laminate core layer bonding process added to the core structure process. Factory productivity is reduced with a foil coating on the outside of the core structure compared to the core structure. The outer core requires additional processing before lamination and bonding, which increases the risk of scratches and etching errors on the outer layer.

Maintain structural balance and avoid bending

The best reason not to design odd-layer PCBs is that odd layers of PCBs tend to bend. As the PCB cools after the multilayer circuit bonding process, the different lamination tensions of the core and foil structures will cause the PCB to bend as it cools. As the board thickness increases, the risk of bending a composite PCB with two different structures is even greater. The key to eliminating board bending is the use of balanced laminations. PCB bending to a certain extent meets the specification requirements, but the subsequent processing efficiency will be reduced, resulting in increased costs. Quality is compromised because special equipment and processes are required for assembly, reducing the precision of component placement.

To change a more easily understood statement is: in the PCB factory PCB process as well as the process, four-layer board than the three-layer board is good control, mainly in the symmetrical structure, four-layer board warping degree we can control the degree of warping in the following 0.7% (IPC600 standards), but a three-layer board size is large, the warping degree will be more than the national standard of China, which affects the SMT patch and the whole enterprise product reliability, so for the general need to design the board, the warping degree will exceed the Chinese national standard, which will affect SMT placement and product reliability. Reliability, so for the general need for designers, do not understand the design requirements of odd-layer boards, even if the two odd layers to achieve social functions, will be designed as a false even layer, that is, 5 layers designed as 6 layers, 7 layers designed as 8 layers of board.

For these reasons, PCB multilayers are mostly designed with even layers and fewer odd layers.

How to balance stacking and reduce the cost of odd PCBs?

What if there are an odd number of PCB layers in the design? The following methods can be used to achieve balanced stacking, reduce PCB production cost and avoid PCB bending.

1) Signal layer and usage.

If we design PCB with even number of power layers and odd number of signal layers can be used through this teaching method. The added layer does not increase the cost of the enterprise, but no can effectively shorten the delivery time and improve the quality of the PCB.

2) Add additional power supply layer.

If the power supply layer of the PCB is an odd number of layers, the signal layer is an even number of layers, you can use this method. An easy way is to add a layer in the middle of the stack without changing other settings. First route the PCB by the odd numbered layers, then copy the middle ground layer and mark the remaining layers. This is the same as thickening the electrical characteristics of the foil coating in the middle ground layer.

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

This method minimizes stack imbalance and improves the quality of the PCB. First, connect an odd number of layers, then add a blank signal layer and mark the remaining layers. It is used for microwave circuits and mixed media (media with different dielectric constants) circuits.

odd number of layers PCB multilayers

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