Stacking design method for balanced PCB
If the wiring does not require an extra layer, why use it? Wouldn’t reducing the layer make the board thinner? Wouldn’t the cost be lower if the board had one less layer? However, in some cases, adding a layer would reduce the cost.
There are two different structures of PCB boards: core structure and foil structure.
In the core structure, all the conductive layers in the PCB board are coated on the core material; while in the foil structure, only the inner conductive layer of the PCB board is coated on the core material, and the outer conductive layer uses a foil dielectric board. All the conductive layers are bonded together through the dielectric using a multi-layer lamination process.
The core material is the double-sided foil board in the factory.
Because each core has two sides, when fully utilized, the number of conductive layers of the PCB board is an even number. Why not use foil on one side and the rest with core structure? The main reasons are: the cost of the PCB board and the curvature of the PCB board.
The cost advantage of even-layer PCB boards
Because there is one less layer of dielectric and foil, the cost of raw materials for odd-numbered PCB boards 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 layers is the same; but the foil/core structure significantly increases the processing cost of the outer layers.
Odd-layer PCBs require a non-standard laminated core layer bonding process on top of the core structure process. Compared with the core structure, the factory production efficiency will decrease when the foil is added to the outside of the core structure. Before lamination and bonding, the outer core requires additional processing, which increases the risk of scratches and etching errors on the outer layer.
Balanced structure to avoid bending
The best reason not to design PCBs with odd layers is that odd-layer PCBs are prone to bending. When the PCB cools after the multi-layer circuit bonding process, the different lamination tensions of the core structure and the foil structure during cooling 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 PCB 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 be reduced, resulting in increased costs. Because special equipment and processes are required during assembly, the accuracy of component placement is reduced, which will compromise quality.
When odd-numbered PCBs are used 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 priority.
- Use a signal layer. This method can be used if the power layer of the designed PCB is an even number and the signal layer is an odd number. The added layer does not increase the cost, but can shorten the delivery time and improve the quality of the PCB.
- Add an additional power layer. This method can be used if the power layer of the designed PCB is an odd number and the signal layer is an even number. A simple method is to add a ground layer in the middle of the stacking without changing other settings. First route the PCB board according to the odd number of 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 ground layer foil.
- Add a blank signal layer close to the center of the PCB stacking. This method minimizes the imbalance of the stacking and improves the quality of the PCB. First route the odd number of layers, then add a blank signal layer, and mark the remaining layers. Used in microwave circuits and mixed dielectric (media with different dielectric constants) circuits.