Why do PCB multilayers have an even number of layers? Can’t an odd number of layers work?

PCB is divided into a single panel, double-sided boards and multilayer boards, which is unlimited number of layers of multilayer boards, there are more than 100 layers of PCB, and the common multilayer PCB is a four-layer board and six-layer board.

We usually encountered are basically an even number of layers of PCB, so do you have “why PCB multilayer board are an even number of layers,” this kind of doubt? In general, the even number of layers of PCB is indeed more advantageous than the odd number of layers of PCB.

1. lower cost

Due to less than one layer of media and foil, odd-layer PCB raw material costs slightly lower than even-layer PCB, but odd-layer PCB processing costs are significantly higher than even-layer PCB. when the inner layer of the same processing costs, odd-layer foil/core structure significantly increase the cost of the outer layer of processing.

Odd-layer PCBs require a non-standard laminated core bonding process on top of the core structure process. Factory productivity is reduced by adding foils outside the core structure compared to the core structure. Before the laminated bonding, the outside of the core requires additional processing, which increases the risk of scratching and etching errors in the outer layer.

2. balanced structure to avoid bending

The best reason not to use an odd number of layers to design PCBs is that odd-layer circuit boards are prone to bending. When the PCB is cooled after the multilayer circuit bonding process, the different lamination tensions between the core structure and the foil structure when cooling can cause the PCB to bend. As board thickness increases, the risk of bending a composite PCB with two different structures increases.

The key to eliminating board bending is to use a balanced laminate stack. Although a certain degree of PCB bending to meet the specification requirements, but the subsequent processing efficiency will be reduced, resulting in increased costs. Quality will be compromised because special equipment and processes are required for assembly and component placement is less accurate.

To change a more understandable statement is: in the PCB process technology, four-layer board than three-layer board is good control, mainly in the symmetry, four-layer board can be controlled in the degree of warping below 0.7% (IPC600 standard), but the three-layer board size is larger, the warping degree will exceed the standard, which affects the SMT patch and the reliability of the entire product, so the general designers, do not design the odd-numbered Layer board, even if the need for an odd number of layers to achieve the function, will also be designed as a false even number of layers, that is, the five-layer design into a six-layer, seven-layer design into an eight-layer board.

For the above reasons, PCB multilayer boards are mostly designed as an even number of layers, fewer odd layers.

But how to balance the odd-layer PCB stacking, cost reduction?

If when the design of an odd number of layers PCB, what to do? With the following methods can be achieved to balance the layer stacking, reduce PCB production costs, to avoid PCB bending.

  1. A signal layer and utilize. If the power supply layer of the design PCB for an even number and the signal layer for an odd number can be used in this way. The additional layer does not increase the cost, but can shorten the delivery time, improve PCB quality.
  2. Add an additional power supply layer. This method can be used if the design PCB has an odd number of power layers and an even number of signal layers. A simple method is to add a ground layer in the middle of the stack without changing other settings. Route the PCB in the odd-numbered layers first, then duplicate the ground layer in the middle and mark the remaining layers. This has the same electrical characteristics as laying foil with a thicker ground layer.
  3. Add a blank signal layer near the center of the PCB stack. This method minimizes layer stack imbalance and improves the quality of the PCB. Route the PCB in odd-numbered layers first, then add a blank signal layer to mark the remaining layers. Used in microwave circuits and mixed media (media with different dielectric constants) circuits.
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