Why are PCB multi-layer boards always double layered?

There are single-sided, double-sided and multi-layer PCBs. There is no strict limit to the number of layers in a multi-layer board, and there are even PCBs with more than 100 layers, although four- and six-layer boards are common. Many people will be curious, why PCB multilayer boards are mostly even layers?

1. Cost factor

From the cost point of view, although the singular PCB board because of one less layer of media and foil, the cost of raw materials is slightly lower than the dual-layer PCB, but the processing cost of the singular layer PCB is significantly higher than the dual-layer. When processing the inner layer, the costs are similar, but the foil/core structure increases the cost of processing the outer layer of an odd-layer PCB significantly.

Odd-layer PCBs require a non-standard laminated core bonding process in addition to the core structure process. Adding a foil to the core structure reduces factory productivity compared to the core structure. And in the laminated bonding before the outside of the core needs additional processing, which increases the risk of scratches and etching errors in the outer layer, thus increasing the processing costs.

2, to prevent bending, quality assurance

A key reason for not designing PCBs with odd layers is that odd-layer boards tend to bend. As the multilayer circuit bonding process cools down after the bonding process is complete, the different lamination tensions of the core and foil structures can cause the PCB to flex. As the board thickness increases, so does the risk of bending of this composite PCB, which consists of two different structures.

A balanced laminate structure is the key to avoiding board bending. While a PCB with a certain amount of bending may be able to meet the specification, this reduces the efficiency of subsequent processing and leads to higher costs. Special equipment and processes are also required to handle it during assembly, and it can reduce the accuracy of component placement and affect product quality.

As an example, four-layer boards are easier to control than three-layer boards in terms of symmetry in the PCB production process. Warpage can be controlled to less than 0.7% on a 4-layer board (in accordance with the IPC600 standard), whereas on a 3-layer board with a larger size, the warpage can exceed this standard. This will have an impact on the SMT patch and reduce the reliability of the whole product. Therefore, the general designers rarely design single-layer boards, even if the function of single-layer can be achieved, it will be designed as a pseudo-double-layer, such as the design of the 5-layer into 6-layer, 7-layer design into 8-layer board.

3, single layer PCB optimisation strategy

If there is a single layer PCB in the design, how to balance the layer stack, reduce production costs and avoid bending? There are several ways:

Use a signal layer: If the power layer of the PCB is even and the signal layer is odd, this method can be used. This additional layer does not increase the cost, but also reduces the lead time and improves the quality of the PCB.

Add an additional power layer: This method can be considered when the PCB has an odd number of power layers and an even number of signal layers. Simply put, add a ground layer in the middle of the stack without changing the other settings. Route the PCB according to the odd-numbered layers, then duplicate the ground layer in the middle, marking the remaining layers, which has the same electrical characteristics as a thicker ground layer foil.

Adding a Blank Signal Layer: Adding a blank signal layer near the centre of the PCB stack minimises stack imbalance and improves PCB quality. Route the layers in odd numbers, then add a blank signal layer and mark the remaining layers. This method is suitable for microwave circuits and mixed media (different dielectric constants) circuits.