PCB Technical - Circuit board factory: laminated design method of one to eight layers of circuit boards

Circuit board factory: laminated design method of one to eight layers of circuit boards

The stacking arrangement of the circuit board factory is the basis of the entire system design of the PCB multilayer board. If the laminated design is defective, it will ultimately affect the EMC performance of the whole machine. In general, the laminated design must comply with two rules:

1. Each wiring layer must have an adjacent reference layer (power or ground layer);

Second, the adjacent main power layer and ground layer should be kept at a minimum distance to provide a larger coupling capacitance;

The stackups from a single-layer PCB board to an eight-layer circuit board are listed below:

(1) Single-sided circuit board and double-sided circuit board stack

For double-sided boards, due to the small number of layers, there is no longer a problem of lamination. The control of EMI radiation is mainly considered from the wiring and layout;

The electromagnetic compatibility problem of single-layer circuit boards and double-sided circuit boards is becoming more and more prominent. The main reason for this phenomenon is that the signal loop area is too large, which not only produces strong electromagnetic radiation, but also makes the circuit sensitive to external interference. To improve the electromagnetic compatibility of the circuit, the easiest way is to reduce the loop area of the key signal.

Key signal: From the perspective of electromagnetic compatibility, the key signal mainly refers to the signal that produces strong radiation and the signal that is sensitive to the outside world. The signal that can generate strong radiation is generally a periodic signal, such as a low-order signal of a clock or an address. Signals that are sensitive to interference are analog signals with lower levels.

Single and double-layer boards are usually used in low-frequency analog designs below 10KHz:

1. The power traces on the same layer are routed radially, and the total length of the lines is minimized;

2. When the power and ground wires are connected, they should be close to each other; place a ground wire beside the key signal wire, and this ground wire should be as close as possible to the signal wire. In this way, a smaller loop area is formed and the sensitivity of differential mode radiation to external interference is reduced. When a ground wire is added next to the signal wire, a loop with the smallest area is formed, and the signal current will definitely take this loop instead of other ground wires.

3. If it is a double-layer circuit board, you can place a ground wire along the signal line on the other side of the circuit board, immediately below the signal line, and make the first line as wide as possible. The loop area formed in this way is equal to the thickness of the PCB circuit board multiplied by the length of the signal line.

(2) Stacking of four-layer circuit boards

Recommended stacking method:

1. SIG－GND(PWR)－PWR(GND)－SIG;

2. GND－SIG(PWR)－SIG(PWR)－GND;

For the above two circuit board stack designs, the potential problem is the traditional 1.6mm (62mil) board thickness. The layer spacing will become very large, which is not only unfavorable for controlling impedance, interlayer coupling and shielding; especially the large spacing between power ground planes reduces the board capacitance and is not conducive to filtering noise.

For the first scheme, it is usually applied to the situation where there are more chips on the board. This scheme can get better SI performance, which is not very good for EMI performance. It is mainly controlled by wiring and other details. Main attention: The ground layer is placed on the connecting layer of the signal layer with the densest signal, which is beneficial to absorb and suppress radiation; increase the area of the board 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 (place the required power copper layer). In this scheme, the outer layer of the PCB circuit board is the ground layer, and the middle two layers are the signal/power layer. The power supply on the signal layer is routed with a wide line, which can make the path impedance of the power supply current low, and the impedance of the signal microstrip path is also low, and the signal radiation of the inner layer can also be shielded by the outer layer. From the perspective of EMI control, this is the best 4-layer PCB structure available. Main attention: The distance between the middle two layers of signal and power mixing layers should be widened, and the wiring direction should be vertical to avoid crosstalk; the board area should be appropriately controlled to reflect the 20H rule; if the wiring impedance is to be controlled, the above solution should be very carefully routed Arranged under the copper island for power supply and grounding. In addition, the copper on the power supply or ground layer should be interconnected as much as possible to ensure DC and low-frequency connectivity.

(3) Stacking of six-layer circuit boards

For the design with higher chip density and higher clock frequency, the design of 6-layer board should be considered

Recommended stacking method:

1. SIG－GND－SIG－PWR－GND－SIG;

For this kind of scheme, this kind of laminated scheme can get 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 wiring layer can be better controlled, and two The stratum is capable of absorbing magnetic field lines well. And when the power supply and ground layer are intact, it can provide a better return path for each signal layer.

2. GND－SIG－GND－PWR－SIG－GND;

For this kind of scheme, this kind of scheme is only suitable for the situation that the device density is not very high, this kind of lamination has all the advantages of the upper lamination, and the ground plane of the top and bottom layers is relatively complete, which 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 surface, because the plane of the bottom layer will be more complete. Therefore, the EMI performance is better than the first solution.

Summary: For the six-layer circuit board scheme, the distance between the power layer and the ground layer should be minimized to obtain good power and ground coupling. However, although the thickness of the board is 62mil and the layer spacing is reduced, it is not easy to control the spacing between the main power supply and the ground layer to be small. Comparing the first scheme with the second scheme, the cost of the second scheme will increase greatly. Therefore, we usually choose the first option when stacking. When designing, follow the 20H rule and the mirror layer rule design

(4) Stacking of eight-layer circuit boards

Eight-layer circuit boards usually use the following three stacking methods

1. This is not a good lamination method due to poor electromagnetic absorption and large power supply impedance. Its structure is as follows:

1Signal1 component surface, microstrip trace layer

2Signal2 internal microstrip wiring layer, better wiring layer (X direction)

3Ground

4Signal3 stripline routing layer, better routing layer (Y direction)

5Signal4 stripline routing layer

6Power

7Signal5 internal microstrip wiring layer

8Signal6 microstrip trace layer

2. 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.

1Signal1 component surface, microstrip wiring layer, good wiring layer

2Ground formation, good electromagnetic wave absorption ability

3Signal2 stripline routing layer, good routing layer

4Power power layer, and the ground layer below form excellent electromagnetic absorption

5Ground formation

6Signal3 stripline routing layer, good routing layer

7Power stratum, with large power supply impedance

8Signal4 microstrip wiring layer, good wiring layer

3. The best stacking method, due to the use of multi-layer ground reference planes, it has very good geomagnetic absorption capacity.

1 Signal1 component surface, microstrip wiring layer, good wiring layer

2 Ground stratum, good electromagnetic wave absorption ability

3 Signal2 stripline routing layer, good routing layer

4 Power layer, and the ground layer below form excellent electromagnetic absorption

5 Ground

6 Signal3 stripline routing layer, good routing layer

7 Ground stratum, good electromagnetic wave absorption ability

8 Signal4 microstrip wiring layer, good wiring layer

Three, summary

How to choose how many layers of boards are used for design and what method of stacking depends on many factors such as the number of signal networks on the circuit board, device density, PIN density, signal frequency, PCB board size and so on. We must consider these factors comprehensively.