How to choose how many layers of boards are used for design and how to stack them, Today we are going to talk about this topic.
The following are some common stacked structures, let’s analyze them：
PCB Layer stackup of single-sided and double-sided boards
For two-layer boards, there is no stacking problem due to the small number of layers. Controlling EMI radiation is mainly considered from wiring and layout.The electromagnetic compatibility of single-layer and double-layer board is becoming more and more prominent.The main reason for this phenomenon is that the signal loop area is too large.It not only produces strong electromagnetic radiation, but also makes the circuit sensitive to external interference.The simplest way to improve the electromagnetic compatibility of the line is to reduce the loop area of key signals.
Key signals: from the perspective of electromagnetic compatibility, key signals mainly refer to signals that produce strong radiation and signals sensitive to the outside world.Signals that can produce strong radiation are generally periodic signals, such as low-order signals of clocks or addresses.Interference sensitive signals refer to those analog signals with low level.
Single and double-layer plates are usually used in low-frequency analog design below 10kHz:
(1.) The power lines on the same floor shall be routed radially, and the total length of the lines shall be minimized.
(2.)When layout the power and ground wires, close to each other.Lay a ground wire beside the key signal line,This ground wire should be as close as possible to the signal wire.then appear a smaller loop area,reduce the sensitivity of differential mode radiation to external interference.When a ground wire is added next to the signal line,It forms a circuit loop with the smallest area.The signal current will take this loop instead of other ground paths.
(3.)If it is a double-layer circuit board, a ground wire can be laid along the signal line on the other side of the circuit board, close to under of signal line, and the line shall be wider as possible,then the loop area thus formed is equal to the thickness of the circuit board multiplied by the length of the signal line.
PCB Layer stack up design of 4 layer PCB:
There are two types of layer stack up design:
- SIG－GND(PWR)－PWR (GND)－SIG.
- GND－SIG(PWR)－SIG(PWR)－GND.
For the above two stack up order designs, the potential problem is for the traditional 1.6mm (62mil) PCB thickness. The layer spacing will become very large, which is not only unfavorable for impedance control, interlayer coupling and shielding.In particular, the distance between the power ground layers is large, which reduces the board capacitance and is not conducive to filtering out noise.
For the first solution, it is usually used in the case of more chips on the board.This solution can get better SI performance, but it is not very good for EMI performance. It is mainly controlled by wiring and other details.
Note: The ground layer is placed on the connecting layer of the signal layer with the densest signal, which is conducive to absorbing and suppressing radiation; increase the board area to reflect the 20H rule.
For the second solution, it is usually used when the chip density on the board is low enough and there is enough area around the chip (where the required power supply copper layer is placed),In this scheme, the outer layers of the circuit board are all ground layers, and the two middle layers are signal/power layers.The power supply on the signal layer is routed with a wide wire, which can make the path impedance of the power supply current low, and the impedance of the signal microstrip path is also low, and can also shield the inner layer signal radiation through the outer ground. From an EMI control standpoint, this is the best 4-layer board structure available.
Note: The distance between the middle two layers of signal and power mixed layers should be widened, and the wiring direction should be vertical to avoid crosstalk. appropriate control panel area, reflecting the 20H rule.If you want to control the trace impedance, the above scheme is very careful to route the traces under the power and ground copper islands.In addition, the copper layers on the power or ground plane should be interconnected as much as possible to ensure DC and low frequency connectivity.
Layer stack up design of 6 layer PCB
1.SIG－GND－SIG－PWR－GND－SIG
For the design with high chip density and high clock frequency, the design of 6-layer board should be considered, and the stacking method is recommended.
For this scheme, this stack-up scheme can achieve better signal integrity, the signal layer is adjacent to the ground layer, the power layer and the ground layer are paired, the impedance of each trace layer can be well controlled, and the two The formation is able to absorb the magnetic field lines well.And in the case of complete power supply and ground layer, it can provide a better return path for each signal layer.
2.GND－SIG－GND－PWR－SIG －GND
For this scheme, this scheme is only suitable for the case where the device density is not very high. This kind of stack has all the advantages of the above stack, and the ground plane of the top and bottom layers is relatively complete and can be used as a better shielding layer. to use. It should be noted that the power layer should be close to the layer that is not the main component side, because the bottom plane will be more complete. Therefore, the EMI performance is better than the first solution.
Summary: For the scheme of the six-layer board, the distance between the power layer and the ground layer should be minimized to obtain good power and ground coupling. However, with a thickness of 62 mil, although the layer spacing has been reduced, it is not easy to control the distance between the main power supply and the ground layer to be very small. Comparing the first scheme with the second scheme, the cost of the second scheme is greatly increased. Therefore, we usually choose the first solution when stacking. When designing, follow the 20H rule and the mirror layer rule design
Layer stack up design of 8 layer PCB
This is not a good stacking method due to poor electromagnetic absorption capability and large source impedance. Its structure is as follows:
1.Signal 1
2.Signal 2
3.Ground
4.Signal 3
5.Signal 4
6.Power
7.Signal 5
8.Signal 6
It is a variant of the third stacking method. Due to the addition of the reference layer, it has better EMI performance, and the characteristic impedance of each signal layer can be well controlled.
1.Signal 1
2.Ground
3.Signal 2
4.Power
6.Signal 3
7.Power
8.Signal 4
The best stacking method, due to the use of multiple ground reference planes, has a very good geomagnetic absorption ability.
1.Signal 1
2.Ground
3.Signal 2
4.Power
6.Signal 3
7.Ground
8.Signal 4
How to choose how many layers of boards are used for design and how to stack them depends on many factors such as the number of signal networks on the board, device density, PIN density, signal frequency, board size and so on.We have to take these factors into consideration. For the more the number of signal networks, the greater the device density, the greater the PIN density, and the higher the frequency of the signal, the multi-layer board design should be used as much as possible.In order to get good EMI performance, it is best to ensure that each signal layer has its own reference layer.