PCB Blog - Balanced PCB board stack-up design method

Designers may design an odd-layer PCB board. If routing doesn't require an extra layer, why use it? Wouldn't reducing layers make the board thinner? Wouldn't the cost be lower if there was one less circuit board? However, in some cases, adding a layer can actually reduce the cost. There are two different structures of circuit boards: core structure and foil structure. In the core structure, all the conductive layers in the circuit board are coated on the core material; while 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 coated with a foil-coated dielectric board. All conductive layers are bonded together through a dielectric using a multi-layer lamination process. The nuclear material is the double-sided foil in the factory. Because each core has two sides, when fully utilized, the number of conductive layers on the PCB is even. Why not use foil on one side and core structure on the rest? The main reasons are the cost of the PCB board and the curvature of the PCB board.

The cost advantage of even-layer boards

Because there is one less 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 PCB boards is significantly higher than that of even-layer PCB boards. The processing cost of the inner layer is the same; however, the foil/core structure significantly increases the processing cost of the outer layer. Odd-layer PCB boards need to add a non-standard laminated core layer bonding process on the basis of the core structure process. Factories that add foil outside the core structure will lose productivity compared to the core structure. The outer core requires additional processing prior to lamination bonding, which increases the risk of scratching and etching errors on the outer layer.

A balanced structure avoids bending

The reason for not designing a PCB with odd layers is that odd-layer boards are prone to bending. When the PCB board is cooled after the multilayer circuit bonding process, the different lamination tensions of the core structure and the foil-clad structure can cause the PCB board to buckle. As the thickness of the board increases, the risk of bending a composite PCB board with two different structures increases. The key to eliminating board flex is a balanced stack up. Although a PCB board with a certain degree of curvature meets the specification requirements, the subsequent processing efficiency will be reduced, resulting in increased costs. Because special equipment and processes are required for assembly, the accuracy of component placement is reduced, so quality will be compromised.

Use an even-layered PCB

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

1) One layer of signal layer and use. This method can be used if the power supply layer of the design PCB board is even and the signal layer is odd. The added layers do not increase the cost but can shorten the lead time and improve the quality of the PCB board.

2) Add an additional power layer. This method can be used if the power plane of the design PCB is odd and the signal plane is even. An easy way to do this is to add a formation in the middle of the stack without changing other settings. First, route the PCB according to the odd-numbered layers, then copy the ground layer in the middle, and mark the remaining layers. This is the same as the electrical characteristics of the thickened layer foil.

3) Add a blank signal layer near the center of the PCB stack. This method eliminates stacking unbalance and improves the quality of the PCB board. Route the odd-numbered layers first, then add a blank signal layer and mark the remaining layers. It is used in microwave circuits and mixed-medium (medium with different dielectric constants) circuits.

Advantages of balanced laminated PCB board: low cost, not easy to bend, shortened delivery time, and guaranteed quality.