The lamination arrangement of PCB factory is the basis of PCB design of the whole system. If the laminate design is defective, it will eventually affect the EMC performance of the whole machine. Generally speaking, there are two main rules for laminated design:
1. Each wiring layer must have an adjacent reference layer (power supply or ground layer);
2. Keep the minimum distance between the adjacent main power layer and the stratum to provide larger coupling capacitance;
The following is a list of laminates from single-layer board to eight-layer board:
1 Lamination of single panel and double panel For two-layer panels, there is no problem of lamination because of the small number of layers. Controlling EMI radiation is mainly considered from wiring and layout; The electromagnetic compatibility of single-layer and double-layer plates is becoming more and more prominent. The main reason for this phenomenon is that the signal circuit 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 lines, the simplest way is to reduce the loop area of key signals. 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 refer to those analog signals with low level. Single-layer and double-layer boards are usually used in low-frequency analog design below 10KHz:
1. The power lines on the same floor are run in a radial way, and the sum of the lengths of the lines is minimized;
2. Stay close to each other when taking the power supply and ground wire; Lay a ground wire beside the key signal line, which should be as close as possible to the signal line. Thus, 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, a ground wire can be laid along the signal line on the other side of the circuit board, just below the signal line, and the line should be as wide as possible. The formed loop area is equal to the thickness of the pcb board multiplied by the length of the signal line.
Recommended lamination method of 4-layer board:
2.1 SIG－GND（PWR）－PWR （GND）－SIG；
2.2 GND－SIG（PWR）－SIG（PWR）－GND； For the above two kinds of circuit board laminated designs, the potential problem is the traditional 1.6mm(62mil) board thickness. The interlayer spacing will become very large, which is not only unfavorable for controlling impedance, interlayer coupling and shielding; Especially, the distance between power supply strata is very large, which reduces the board capacitance and is not conducive to filtering noise. For the first scheme, it is usually applied in the case of more chip on board. This scheme can obtain better SI performance, which is not very good for EMI performance, and it is mainly controlled by wiring and other details. Note: The stratum is placed on the adjacent layer of the signal layer with the densest signal, which is conducive to absorbing and suppressing radiation; Increase the board area and reflect the 20H rule. For the second scheme, it is usually applied to the situation where the density of chip on board is low enough and there is enough area around the chip (to place the required copper-clad layer of power supply). In this scheme, the outer layers of PCB are all strata, and the middle two 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 the inner signal radiation can also be shielded by the outer layer. From the point of view of EMI control, this is the best existing 4-layer PCB structure. Main note: the distance between the signal and power mixed layers of the middle two layers should be widened, and the wiring direction should be vertical to avoid crosstalk; Appropriate control board area, reflecting the 20H rule; If the impedance of the wiring is to be controlled, the above scheme should carefully arrange the wiring under the copper island of power supply and grounding. In addition, the power supply or copper plating on the ground should be interconnected as much as possible to ensure the connectivity between DC and low frequency.
Lamination of 6-layer board For the design with high chip density and high clock frequency, the design of 6-layer board should be considered. Recommended lamination method:
3.1 SIG－GND－SIG－PWR－GND－SIG；
For this scheme, this laminated scheme can obtain better signal integrity. The signal layer is adjacent to the grounding layer, and the power layer is paired with the grounding layer. The impedance of each wiring layer can be well controlled, and both layers can absorb magnetic lines of force well. And it can provide a better backflow path for each signal layer under the condition of complete power supply and stratum.
3.2 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 laminate has all the advantages of the above laminate, and the ground planes of the top and bottom layers are relatively complete, so that it can be used as a better shielding layer. It is important to note that the power supply 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 that of the first scheme. Summary: For the six-layer board scheme, the distance between the power supply layer and the ground layer should be reduced as much as possible to obtain good coupling between power supply and ground. However, although the plate thickness of 62mil and the layer spacing are reduced, it is not easy to control the spacing between the main power supply and the stratum to be very small. Compared with the first scheme and the second scheme, the cost of the second scheme will be greatly increased. Therefore, we usually choose the first option when laminating. When designing, follow the 20H rule and mirror layer rule.
Four-layer laminate Eight-layer laminate usually uses the following three lamination methods. 4.1 This is not a good lamination method due to poor electromagnetic absorption capacity and large power impedance. Its structure is as follows:
1signa1 element plane, microstrip wiring layer
2 Signal 2 internal microstrip wiring layer, better wiring 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 wiring layer 4.2 is a variant of the third lamination mode. Because of the addition of reference layer, it has good EMI performance, and the characteristic impedance of each signal layer can be well controlled.
1signa1 element plane, microstrip wiring layer, good wiring layer
Ground formation, with good electromagnetic wave absorption capacity
3 Signal 2 stripline routing layer, good routing layer
4 Power supply layer, which forms excellent electromagnetic absorption with the stratum below.
5 Ground formation
6 Signal 3 stripline routing layer, good routing layer
7 Power formation, with large power impedance
8 Signal 4 microstrip wiring layer, good wiring layer 4.3 best stacking method, because of the use of multi-layer ground reference plane, it has very good geomagnetic absorption capacity.
1signa1 element plane, microstrip wiring layer, good wiring layer
Ground formation, with good electromagnetic wave absorption capacity
3 Signal 2 stripline routing layer, good routing layer
4 Power supply layer, which forms excellent electromagnetic absorption with the stratum below.
5 Ground formation
6 Signal 3 stripline routing layer, good routing layer
7 Ground formation, with good electromagnetic wave absorption capacity
8 Signal 4 microstrip routing layer, good routing layer
5. Summary How to choose which layers of boards to design and how to stack them depends on the number of signal networks on the circuit board, device density, PIN density, signal frequency, board size and many other factors. We should consider these factors comprehensively. For the design with more signal networks, higher device density, higher PIN density and higher signal frequency, multilayer board design should be adopted as far as possible. In order to obtain good EMI performance, it is best to ensure that each signal layer has its own reference layer.
