PCB Tech - What rules should be followed in PCB stack design

In general, the laminated design must comply with two rules:

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

2. The adjacent main power layer and ground layer should maintain a minimum distance to provide a larger coupling capacitance;

The following lists the stack from two-layer board to eight-layer board for example explanation:

1. Stacking of single-sided PCB board and double-sided PCB board

For two-layer 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;

What are the rules for PCB stack design

The electromagnetic compatibility problems of single-layer boards and double-layer boards are 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 generates 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 running the power and ground wires, 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 wire paths.

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

Two and four-layer laminates

1. SIG-GND(PWR)-PWR(GND)-SIG; 2.GND-SIG(PWR)-SIG(PWR)-GND;

For the above two laminated designs, the potential problem is for the traditional 1.6mm (62mil) board thickness. The layer spacing will become very large, which is not only unfavorable for impedance control, 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 conducive to absorbing and suppressing radiation; increasing the board area to reflect the 20H rule.

For the second scheme, 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 layers of the PCB are all ground layers, and the middle two layers are signal/power layers. 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.

Note: The middle two layers of signal and power mixed layers should be separated, and the wiring direction should be vertical to avoid crosstalk; the board area should be properly controlled to reflect the 20H rule; if you want to control the wiring impedance, the above solution should be very careful to arrange the wiring Pave the copper under the power and ground. 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.

Three, six-layer laminate

For the design with higher chip density and higher clock frequency, the design of 6-layer board should be considered, and the stacking method is recommended:

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, each The impedance of the trace layer can be better controlled, and both ground layers can absorb 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 such top and bottom layers The ground plane is relatively complete and can be used as a better shielding layer. 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 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.

Stacking of four and eight-layer boards

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

1. Signal1 component surface, microstrip wiring layer

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

3.Ground

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

5.Signal4 stripline routing layer

6.Power

7. Signal5 internal microstrip wiring layer

8.Signal6 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.

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

2. Ground stratum, better electromagnetic wave absorption capacity

3. Signal2 strip line routing layer, good routing layer

4. Power layer, and the ground layer below form excellent electromagnetic absorption 5. Ground layer

6. Signal3 stripline routing layer, good routing layer

7. Power stratum, with large power supply impedance

8.Signal4 microstrip wiring layer, good wiring layer

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

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

2. Ground stratum, better electromagnetic wave absorption capacity

3. Signal2 strip line routing layer, good routing layer

4. Power layer, and the ground layer below form excellent electromagnetic absorption 5. Ground layer

6. Signal3 stripline routing layer, good routing layer

7. Ground stratum, better electromagnetic wave absorption capacity

8.Signal4 microstrip wiring layer, good wiring layer

How many layers of boards are used for PCB design and what method of stacking is used 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 must consider these factors comprehensively. For the more signal networks, the greater the device density, the greater the PIN density, and the higher the signal frequency, the multilayer board design should be used as much as possible. To get good EMI performance, it is best to ensure that each signal layer has its own reference layer.