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How are the layers of a circuit board assigned?

Nov 21 - 2023


PCBs (Printed Circuit Boards) can be categorized into single-sided circuit boards, double-sided PCBs, and multilayer circuit boards. Common multilayer PCBs include four-layer PCBs, six-layer PCBs and eight-layer PCBs. Simple four-layer board in the Top Layer and Bottom Layer based on the addition of power and ground layers, greatly solving the problem of electromagnetic interference, improve the reliability of the system, but also to improve the wires through the rate of reduction of the area of the PCB circuit board.

In fact, the number of layers of PCB multilayer board is very flexible, designers can be based on the actual needs of reasonable settings.2 oz copper thickness in mm So, what kind of layering principles of PCB multilayer board, PCB layer settings should meet the following points:

1. The power supply plane should be close to the ground plane, and closely coupled with the ground plane, arranged below the ground plane. Below the component layer is the ground layer, which provides the device shielding layer and a reference layer for the top wiring.

2. the signal layer should be adjacent to the internal electrical layer and should not be directly adjacent to other signal layers, and all signal layers should be adjacent to the ground plane whenever possible.

3. isolate digital and analog circuits. If conditions permit, lay out the analog and digital signal lines in separate layers and use shielding measures; if it is necessary to lay out the same signal layer, it is necessary to use isolation strips and ground lines to reduce interference; the power supply and ground of the analog and digital circuits should be isolated from each other and should not be mixed.

4. Multilayer boards should be designed with symmetry between layers, preferably with an even number of copper layers. If it is not symmetrical, it is easy to cause distortion.

Why do common PCB multilayers have even number of layers instead of odd number of layers?

Currently there are more than 100 layers of PCB, we usually encountered are basically an even number of layers of PCB. so why is it common to have an even number rather than an odd number of layers?

Multi-layer PCB circuit boards are stacked for the following reasons:

1. Lower cost: due to one less layer of media and laying foil, the cost of raw materials for odd-layer PCB is slightly lower than even-layer PCB, but the processing cost of odd-layer PCB is significantly higher than even-layer PCB. when the processing cost of the inner layer is the same, the laying of the odd layer of the foil / core structure significantly increase the cost of the outer layer of the processing.plated through hole vs via Odd-layer PCBs require the addition of a non-standard laminated core layer bonding process to the core structure process. Factory productivity is reduced by adding foils outside the core structure compared to the core structure. The outer core requires additional processing before lamination bonding, which increases the risk of scratches and etching errors on the outer layer.

2. Balanced structure to avoid bending: PCBs are not designed with an odd number of layers because odd-layer boards tend to bend. When the PCB is cooled after the multilayer circuit bonding process, the different lamination tensions between the core structure and the foiled structure as they cool can cause the PCB to bend. As board thickness increases, the risk of bending a composite PCB with two different structures increases. The key to eliminating board bending is to use a balanced laminate stack. Although a certain degree of PCB bending to meet the specification requirements, but the subsequent processing efficiency will be reduced, resulting in increased costs. Quality will be compromised because special equipment and processes are required for assembly and component placement accuracy is reduced. Simply put: in the PCB process technology, four-layer board than the three-layer board good control, mainly in the symmetry, four-layer board can be controlled in the degree of warping below 0.7% (IPC600 standard), but the three-layer board size is larger, the warpage will exceed this standard, which will affect the SMT patch and the reliability of the entire product, so the general designer does not design an odd-numbered layers of the board, even if the Need an odd number of layers to achieve the function, will also be designed as a false even number of layers, that is, the five-layer design into a six-layer, seven-layer design into an eight-layer board.

For the above reasons, PCB multilayer boards are mostly designed as an even number of layers, fewer odd layers.

But if there is an odd number of layers in the design of the PCB, how to balance the stack, reduce costs? The following are several ways to achieve balanced stacking, reduce PCB production costs, to avoid PCB bending:

1) The use of a signal layer: If the design of the PCB power layer for the even number and signal layer for the odd number can be used in this way. The additional layer does not increase the cost, but 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 design PCB has an odd number of power layers and an even number of signal layers. A simple method is to add a ground layer in the middle of the stack without changing other settings. Route the PCB in the odd-numbered layers first, then duplicate the ground layer in the middle and mark the remaining layers. This has the same electrical characteristics as laying foil with a thicker ground layer.

3) Add a blank signal layer near the center of the PCB stack: This method minimizes stacking imbalance and improves the quality of the PCB. Route the PCB in odd-numbered layers first, then add a blank signal layer to mark the remaining layers. This method is suitable for microwave circuits and mixed media (media with different dielectric constants) circuits.