What is the principle of PCB multilayer board selection?

What is the principle of PCB multilayer board selection?

PCB stack structure design has a direct impact on product cost and product EMC. The increase of the board layer facilitates the wiring, but also increases the cost. When designing, it is necessary to consider the needs of all aspects in order to achieve the best balance.

After completing the pre-layout of components, it is generally necessary to focus on the PCB layout bottleneck. Combining with other EDA tools to analyze the wiring density of the circuit board; then integrate the number and types of signal lines with special wiring requirements such as differential lines, sensitive signal lines, etc. to determine the number of layers of the signal layer; then according to the type of power supply, isolation and anti-interference The requirements to determine the number of inner electrical layers.

01 Cascading selection factors to consider

The more layers the circuit board has, the more types there are permutations and combinations of special signal layers, ground layers, and power layers.

(1) The signal layer should be adjacent to an internal electrical layer (internal power supply/ground layer), and use the large copper film of the internal electrical layer to provide shielding for the signal layer.

(2) The internal power layer and the ground layer should be tightly coupled, that is, the dielectric thickness between the internal power layer and the ground layer should take a smaller value.

(3) The high-speed signal transmission layer in the circuit should be a signal intermediate layer and sandwiched between two inner electrical layers. In this way, the copper films of the two inner electrical layers can provide electromagnetic shielding for high-speed signal transmission, and at the same time can effectively limit the radiation of high-speed signals between the two inner electrical layers, without causing external interference.

(4) Avoid direct adjoining of two signal layers. Crosstalk is easily introduced between adjacent signal layers, resulting in circuit function failure. Adding a ground plane between two signal layers can effectively avoid crosstalk.

(5) Multiple grounded internal electrical layers can effectively reduce grounding impedance. For example, the A signal layer and the B signal layer use separate ground planes, which can effectively reduce common-mode interference.

(6) Taking into account the symmetry of the layer structure.

02 Lamination design method of one to eight-layer circuit board

1) Lamination of single-panel and double-panel

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

The problem of electromagnetic compatibility of single-layer boards and double-layer boards is becoming more and more prominent. The main reason for this phenomenon is that the area of the signal loop 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 signals: 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-order signals of clocks or addresses. Signals sensitive to interference are those analog signals with low 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 sum of the lengths of the lines is minimized;

2. When running the power supply and ground wires, they should be close to each other; lay a ground wire next to the key signal line, and this ground wire should be as close as possible to the signal line. This forms a smaller loop area and reduces the sensitivity of differential mode radiation to external interference. When a ground wire 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 wire paths.

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

2) Lamination of four-layer boards

Recommended stacking method:

2.1 SIG－GND (PWR)－PWR (GND)－SIG;

2.2 GND－SIG(PWR)－SIG(PWR)－GND;

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

For the first solution, it is usually applied to the situation where there are many chips on the board. This solution can get better SI performance, but it is not very good for EMI performance, and it is mainly controlled by routing and other details. Main attention: 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 reflects the 20H rule.

3) Lamination of six-layer boards

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

Recommended stacking method:

3.1 SIG－GND－SIG－PWR－GND－SIG;

For this scheme, this stacking scheme can get better signal integrity, the signal layer is adjacent to the ground plane, the power plane and the ground plane are paired, the impedance of each trace layer can be well controlled, and the two The formations are good at absorbing magnetic field lines. And it can provide a better return path for each signal layer when the power supply and the ground layer are complete.

3.2 GND－SIG－GND－PWR－SIG－GND;

For this kind of scheme, this kind of 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 planes of the top and bottom layers are 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 bottom plane will be more complete. Therefore, the EMI performance is better than the first solution.

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

4) Lamination of eight-layer boards

Eight-layer boards usually use the following three stacking methods

4.1 Due to poor electromagnetic absorption capability and large power supply impedance, this is not a good stacking method. Its structure is as follows:

1 Signal 1 component surface, microstrip routing layer

2 Signal 2 Internal microstrip wiring layer, better wiring layer (X direction)

3 Ground

4 Signal 3 Stripline wiring layer, better wiring layer (Y direction)

5 Signal 4 Stripline Routing Layer

6 Power

7 Signal 5 internal microstrip routing layer

8 Signal 6 Microstrip routing layer

4.2 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 formation, better electromagnetic wave absorption capacity

3 Signal 2 stripline routing layer, good routing layer

4 Power power supply layer, which forms excellent electromagnetic absorption with the underlying stratum

5 Ground

6 Signal 3 stripline routing layer, good routing layer

7 Power ground plane with large power impedance

8 Signal 4 microstrip routing layer, good routing layer

4.3 The best stacking method, because the use of multi-layer ground reference plane has very good geomagnetic absorption ability.

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

2 Ground formation, better electromagnetic wave absorption capacity

3 Signal 2 stripline routing layer, good routing layer

4 Power power supply layer, which forms excellent electromagnetic absorption with the underlying stratum

5 Ground

6 Signal 3 stripline routing layer, good routing layer

7 Ground formation, better electromagnetic wave absorption capacity

8 Signal 4 microstrip routing layer, good routing layer

How to choose how many layers of boards to use and what kind of stacking method to use depends on many factors such as the number of signal networks on the circuit board, device density, PIN density, signal frequency, and board size. For these factors we have to consider comprehensively. 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. For good EMI performance it is best to ensure that each signal layer has its own reference layer.