Electronic Design - Balanced PCB stackup design method

If PCB routing does not require additional layers, why use it? Wouldn't reducing the layers make the circuit board thinner? If there is one less circuit board, wouldn't the cost be lower? However, in some cases, adding a layer will reduce the cost.  

Circuit boards have two different structures: core structure and foil structure.  

In the core structure, all conductive layers in the circuit board are coated on the core material; in the foil-clad structure, only the inner conductive layer of the circuit board is coated on the core material, and the outer conductive layer is a foil-clad dielectric board. All conductive layers are bonded together through a dielectric using a multilayer lamination process.  

The nuclear material is the double-sided foil-clad board in the factory. Because each core has two sides, when fully utilized, the number of conductive layers of the PCB is an even number. Why not use foil on one side and core structure for the rest? The main reasons are: the cost of the PCB and the bending degree of the PCB.

Cost advantage of even-numbered circuit boards  

Because of the lack of a layer of dielectric and foil, the cost of raw materials for odd-numbered PCBs is slightly lower than that of even-numbered PCBs. However, the processing cost of odd-layer PCBs is significantly higher than that of even-layer PCBs. The processing cost of the inner layer is the same; but the foil/core structure obviously increases the processing cost of the outer layer.  

The odd-numbered PCB needs to add a non-standard laminated core layer bonding process on the basis of the core structure process. Compared with the nuclear structure, the production efficiency of factories that add foil to the nuclear structure will decrease. Before lamination and bonding, the outer core requires additional processing, which increases the risk of scratches and etch errors on the outer layer.  

Balance structure to avoid bending  

The best reason not to design a PCB with an odd number of layers is that an odd number of layer circuit boards are easy to bend. When the PCB is cooled after the multilayer circuit bonding process, the different lamination tensions of the core structure and the foil-clad structure 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 stack. Although a PCB with a certain degree of bending meets the specification requirements, the subsequent processing efficiency will decrease, leading to increased costs. Because special equipment and craftsmanship are required during assembly, the accuracy of component placement is reduced, which will damage the quality.  

Use even-numbered PCB  

When an odd-numbered PCB appears in the design, the following methods can be used to achieve balanced stacking, reduce PCB manufacturing costs, and avoid PCB bending. The following methods are arranged in order of preference.  

1. A signal layer and use it. This method can be used if the power layer of the design PCB is even and the signal layer is odd. The added layer does not increase the cost, but it 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 design PCB is odd and the signal layer is even. A simple method is to add a layer in the middle of the stack without changing other settings. First, follow the odd-numbered PCB layout, and then copy the ground layer in the middle to mark the remaining layers. This is the same as the electrical characteristics of a thickened layer of foil.

   3. Add a blank signal layer near the center of the PCB stack. This method minimizes the stacking imbalance and improves the quality of the PCB. First, follow the odd-numbered layers to route, then add a blank signal layer, and mark the remaining layers. Used in microwave circuits and mixed media (different dielectric constants) circuits.  

The advantages of balanced laminated PCB: low cost, not easy to bend, shorten delivery time, and ensure quality.