PCB stacking design

In general, stacking design mainly follows two rules:

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

2. The adjacent main power layer and ground layer must maintain a minimum spacing to provide a larger coupling capacitor; The following lists the stacking from two-layer boards to ten-layer boards:

3.Stacking of single-sided PCB boards and double-sided PCB boards
For two-layer boards, due to the small number of board layers, there is no stacking problem. Controlling EMI radiation is mainly considered from the perspective of wiring and layout;

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 generates strong electromagnetic radiation, but also makes the circuit sensitive to external interference. To improve the electromagnetic compatibility of the line, the simplest way is to reduce the loop area of ​​key signals.

Key signal: From the perspective of electromagnetic compatibility, key signals mainly refer to signals that generate strong radiation and signals that are sensitive to the outside world. Signals that can generate strong radiation are generally periodic signals, such as low-bit signals of clocks or addresses. Signals that are sensitive to interference refer to those analog signals with lower levels.
Single-layer and double-layer boards are usually used in low-frequency analog designs below 10KHz:

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

2 When routing the power and ground lines, they should be close to each other

a ground line should be laid next to the key signal line, and this ground line should be as close to the signal line as possible. This will form a smaller loop area and reduce the sensitivity of differential mode radiation to external interference. When a ground line is added next to the signal line, a loop with the smallest area is formed, and the signal current will definitely take this loop instead of other ground line paths.

3 If it is a double-layer circuit board, a ground line can be laid on the other side of the circuit board

close to the bottom of the signal line, along the signal line, and the line 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-layer and four-layer board stacking

Recommended stacking method:

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

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

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

For the first solution, it is usually used when there are many chips on the board. This solution can obtain better SI performance, but it is not very good for EMI performance, which mainly needs to be controlled through routing and other details. Mainly note: the ground layer is placed on the connected layer of the signal layer with the densest signal, which is conducive to absorbing and suppressing radiation; increasing the board area and reflecting the 20H rule.

For the second solution, it is usually used in situations where the chip density on the board is low enough and there is enough area around the chip (to place the required power copper layer). In this solution, 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 inner layer signal radiation can also be shielded by the outer layer. From the perspective of EMI control, this is the best 4-layer PCB structure available. Mainly note: the spacing between the two middle layers of signal and power mixed layers should be widened, and the routing direction should be vertical to avoid crosstalk; the board area should be properly controlled to reflect the 20H rule; if the routing impedance is to be controlled, the above solution should be very careful to arrange the routing under the power and ground copper islands. In addition, the copper on the power or ground layer should be interconnected as much as possible to ensure DC and low-frequency connectivity.

3.Stacking of six-layer boards

4.For designs with high chip density and high clock frequency, the design of six-layer boards should be considered

Recommended stacking methods:

4.SIG-GND-SIG-PWR-GND-SIG;

For this solution, this stacking solution can obtain 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 routing layer can be well controlled, and both ground layers can absorb magnetic lines well. And when the power supply and ground layers are complete, a good return path can be provided for each signal layer.

5.GND-SIG-GND-PWR-SIG-GND;

For this solution, this solution is only suitable for situations where the device density is not very high. This stacking has all the advantages of the above stacking, and the ground planes of the top and bottom layers are relatively complete, which 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 bottom layer will be more complete. Therefore, the EMI performance is better than the first solution.

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

6.Stacking of eight-layer boards

Eight-layer boards usually use the following three stacking methods
A: This is not a good stacking method due to poor electromagnetic absorption and large power supply impedance.

Its structure is as follows:

1.Signal 1 component surface, microstrip routing layer

2.Signal 2 internal microstrip routing layer, better routing layer (X direction)

3.Ground

4.Signal 3 stripline routing layer, better routing layer (Y direction)

5.Signal 4 stripline routing layer 6.Power 7.Signal 5 internal microstrip routing layer 8.Signal 6 microstrip routing layer

B: It is a variant of the third stacking method. Due to the addition of a reference layer, it has better EMI performance and the characteristic impedance of each signal layer can be well controlled

1.Signal 1 component surface, microstrip routing layer, good routing layer

2.Ground layer, better electromagnetic wave absorption ability

3.Signal 2 stripline routing layer, good routing layer

4.Power power layer, excellent electromagnetic absorption with the underlying layer

5.Ground layer

6.Signal 3 Stripline routing layer, good routing layer

7.Power ground layer, with large power supply impedance 8.Signal 4 microstrip routing layer, good routing layer

C: The best stacking method, due to the use of multi-layer ground reference plane, it has very good geomagnetic absorption ability.

1.Signal 1 component surface, microstrip routing layer, good routing layer

2.Ground ground layer, good electromagnetic wave absorption ability

3.Signal 2 stripline routing layer, good routing layer

4.Power power layer, with the ground layer below to form excellent electromagnetic absorption

5.Ground ground layer

6.Signal 3 stripline routing layer, good routing layer

7.Ground ground layer, good electromagnetic wave absorption ability

8.Signal 4 microstrip routing layer, good routing layer How to choose how many layers to design and what kind of stacking method to use depends on many factors such as the number of signal networks on the board, device density, PIN density, signal frequency, board size, etc. We need to consider these factors comprehensively. For designs with more signal networks, higher device density, higher PIN density, and higher signal frequency, multi-layer board design should be used as much as possible. To obtain good EMI performance, it is best to ensure that each signal layer has its own reference layer.