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Balanced PCB stack design method

writer: G January 22, 2019

The board has two different structures: a core structure and a foil-clad structure.

In the core structure, all the 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 applied to the core material, and the outer conductive layer is coated with the foil dielectric plate. All of the conductive layers are bonded together by a dielectric using a multilayer lamination process.

Nuclear material is a double-sided foil board in the factory. Because each core has two faces, the total number of conductive layers of the PCB is even when fully utilized. Why not use the foil while the other uses the nuclear structure? The main reasons are: the cost of the PCB and the curvature of the PCB.

The cost advantage of even-numbered boards

Because of the lack of a layer of media and foil, the cost of odd-numbered PCB board raw materials is slightly lower than that of even-numbered layers. However, the processing cost of odd-numbered layers of PCB is significantly higher than that of even-numbered layers. 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 PCBs require a non-standard stacked core layer bonding process based on the core structure process. Compared with the nuclear structure, the efficiency of the factory where the foil is added outside the nuclear structure will decrease. Prior to lamination bonding, the outer core requires additional processing, which increases the risk of scratches and etch errors in the outer layer.

Balanced structure avoids bending

The best reason to design PCBs without odd layers is that odd-numbered 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 structure structure cause the PCB to bend. As the thickness of the board increases, the risk of bending a composite PCB with two different structures increases. The key to eliminating board bending is to use a balanced stack. Although a certain degree of bending of the PCB meets the specification requirements, the subsequent processing efficiency will decrease, resulting in an increase in cost. Because special equipment and processes are required for assembly, the accuracy of component placement is reduced, which will damage the quality.

Use even layer PCB

When odd-numbered PCBs appear in the design, the following methods can be used to achieve balanced stacking, reduce PCB fabrication costs, and avoid PCB bending. The following methods are arranged in a preferred order.

1. A layer of signal layer is utilized. This method can be used if the power plane 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 PCB quality.

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

3. Add a blank signal layer near the center of the PCB stack. This approach minimizes stacking imbalance and improves PCB quality. First, route the odd layers, add a blank signal layer, and mark the remaining layers. It is used in microwave circuits and mixed media (medium media have different dielectric constants).

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

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