PCB Technical - Stack-up design in PCB design and layout

Before designing a multi-layer PCB circuit board, the designer needs to first determine the circuit board structure used according to the circuit scale, circuit board size and electromagnetic compatibility (EMC) requirements, that is, to decide whether to use 4 layers, 6 layers, or More layers of circuit boards. After determining the number of layers, determine where to place the internal electrical layers and how to distribute different signals on these layers. This is the choice of multilayer PCB stack structure. The laminated structure is an important factor that affects the EMC performance of the PCB board, and it is also an important means to suppress electromagnetic interference. This section will introduce the relevant content of the multilayer PCB board stack structure. After determining the number of power and ground layers and the number of signal layers, the relative arrangement of them is a topic that every PCB engineer cannot avoid;

General principles of layer arrangement:

1. To determine the laminated structure of the multilayer PCB board, many factors need to be considered. From the perspective of wiring, the more layers, the better the wiring, but the cost and difficulty of board manufacturing will also increase. For manufacturers, whether the laminated structure is symmetrical or not is the focus that needs to be paid attention to when PCB boards are manufactured, so the choice of the number of layers needs to consider the needs of all aspects to achieve the best balance. For experienced designers, after completing the pre-layout of the components, they will focus on the analysis of the PCB wiring bottleneck. Combine with other EDA tools to analyze the wiring density of the circuit board; then synthesize the number and types of signal lines with special wiring requirements, such as differential lines, sensitive signal lines, etc., to determine the number of signal layers; then according to the type of power supply, isolation and anti-interference The requirements to determine the number of internal electrical layers. In this way, the number of layers of the entire circuit board is basically determined.

2. The bottom of the component surface (the second layer) is the ground plane, which provides the device shielding layer and the reference plane for the top wiring; the sensitive signal layer should be adjacent to an internal electrical layer (internal power/ground layer), using the large internal electrical layer Copper film to provide shielding for the signal layer. 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 film of the two inner electric layers can provide electromagnetic shielding for high-speed signal transmission, and at the same time, it can effectively limit the radiation of the high-speed signal between the two inner electric layers without causing external interference.

3. All signal layers are as close as possible to the ground plane;

4. Try to avoid two signal layers directly adjacent to each other; it is easy to introduce crosstalk between adjacent signal layers, resulting in circuit function failure. Adding a ground plane between the two signal layers can effectively avoid crosstalk. 5. The main power supply is as close as possible to it correspondingly;

6. Take into account the symmetry of the laminated structure.

7. For the layer layout of the motherboard, it is difficult for the existing motherboard to control parallel long-distance wiring. For the board-level operating frequency above 50MHZ (refer to the situation below 50MHZ, appropriately relax), the layout principle is recommended:

The component surface and welding surface are a complete ground plane (shield);

No adjacent parallel wiring layers;

All signal layers are as close as possible to the ground plane;

The key signal is adjacent to the ground and does not cross the partition.

Note: When setting up the specific PCB layers, the above principles should be flexibly mastered. Based on the understanding of the above principles, according to the actual requirements of the single board, such as: whether a key wiring layer, power supply, ground plane division is required, etc., Determine the arrangement of the layers, and don't just copy it or stick it at all.

8. 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.