PCB Blog - PCB board stackup design considerations

What problems should be paid attention to in the design of PCB boardstack-up? Let the engineers tell you the following. When doing stack-up design, be sure to follow two rules:

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

2) The adjacent main power supply layer and ground layer should be kept apart to provide larger coupling capacitance.

Let's take an example of two, four and six-layer boards to illustrate:

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

For two-layer boards, controlling EMI emissions is primarily a matter of routing and layout. The electromagnetic compatibility problem of single-layer board and double-layer board 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 line, the simple method is to reduce the loop area of the key signal; the key signal mainly refers to the signal that generates strong radiation and the signal that is sensitive to the outside world. Single- and double-layer boards are commonly used in low-frequency analog designs below 10KHz:

1) The power supply on the same layer is routed radially, and the sum of the lengths of the parallelized lines;

2) When running the power and ground wires, they should be close to each other; lay a ground wire on the side of the key signal wire, and this ground wire should be as close to the signal wire as possible. In this way, a smaller loop area is formed and the sensitivity of differential mode radiation to external interference is reduced.

3) If it is a double-layer circuit board, you can lay a ground wire 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.

2. Lamination of four-layer boards

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

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

For the above two stack-up designs, the potential problem is 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 large spacing between the power ground layers reduces the board capacitance and is not conducive to filtering noise. Usually used in the case of more chips on the board. This scheme can get better SI performance, for

EMI performance is not very good, mainly controlled by traces and other details. The second solution is usually used when the chip density on the board is low enough and there is enough area around the chip. In this scheme, the outer layer of the PCB board is the ground layer, and the two middle layers are the signal/power layer. From an EMI control perspective, this is an existing 4-layer PCB structure. Main attention: The distance between the two middle layers of signal and power mixed layers should be widened, and the wiring direction should be vertical to avoid crosstalk; the area of the board should be properly controlled to reflect the 20H rule.

3. Lamination of six-layer boards

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

1) SIG-GND-SIG-PWR-GND-SIG; this stacking scheme can obtain better signal integrity, the signal layer is adjacent to the ground layer, the power layer and the ground layer are paired, and the impedance of each trace layer Both can be well controlled, and both formations can absorb magnetic field lines well.

2) GND-SIG-GND-PWR-SIG-GND; this solution 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, Can be used as a better shielding layer. Therefore, the EMI performance is better than this scheme. Comparing the first scheme with the second scheme, the cost of the second scheme is greatly increased on PCB board.