PCB stack design

- Aug 05, 2016-

Before designing a multi-layer PCB board, circuit designers need to be based on the size of the circuit board size and electromagnetic compatibility (EMC) requirements to determine the structure of the board used, that is, it decided to adopt 4 layers, 6 layers, or more layers of the circuit board. After determining the number of layers, and then determine the placement of the inner layer and how the distribution of the different signals on these layers. This is the choice of a multilayer PCB laminated structure. Laminated structure is an important factor affecting the PCB EMC performance is also an important means of electromagnetic interference suppression. This section describes the content of the multi-layer PCB board laminated structure. For the power, after the manner of layers and signal layers to determine the relative position of the arrangement between them is that each PCB engineer can not avoid the topic;

Layer arrangement of the general principles:

1, to determine the structure of a multilayer PCB laminated board needs to consider many factors. From the wiring side, the more layers the more conducive to wiring, but the system board will increase the cost and difficulty. For manufacturers, the laminated structure is symmetrical or not the focus of the PCB board manufacturing concern, so the choice of the number of layers required to consider the needs of all sides, in order to achieve the best balance. For experienced designers, after the completion of the pre-layout components, PCB routing bottleneck will be the focus of analysis. In combination with other EDA tools to analyze wiring density circuit board; and then integrated with special wiring requirements for signal lines such as differential lines, sensitive signal lines to determine the number and types of layers of signal layers; and then depending on the type of power supply, isolation and anti-jamming requirements to determine the number of the inner layer. Thus, the number of plies across the board basically determined.

2, below the surface element (second layer) of a ground plane, and a shield device provides a reference plane for the top layer wiring;-sensitive layer should be a signal of the adjacent layer (internal power / ground), the use of large electric layer copper film layer to provide shielding for the signal. The high-speed signal transmission circuit layer must be the signal of the intermediate layer, and sandwiched between the two inner layer. Copper film so that the two inner layer may provide electromagnetic shielding for the high-speed signal transmission, but also can effectively radiate the high-speed signal is limited between two inner layer and not to cause interference.

3, all signal layers adjacent to the ground plane as much as possible

4. Try to avoid immediately adjacent two signal layers; easy to introduce crosstalk between adjacent signal layers, resulting in failure of the circuit functions. Layer was added between the two signal ground plane crosstalk can be effectively avoided. 5, the main power corresponding adjacent as possible

6, both symmetrical laminate structure

7, the motherboard layer arrangement, the existing motherboard difficult to control the long-distance parallel wiring for board-level operating frequency above 50MHZ (50MHZ or less can refer appropriate relaxation), the proposed arrangement principles:

Component side, solder side for a complete ground plane (the shield);

No parallel wiring layer adjacent;

All signal layers adjacent to the ground plane as much as possible;

The key signal and the adjacent formations, does not cross the partition.

NOTE: When you set a specific PCB layer to the above principles of flexibility, understanding the basis of the above principles, according to the actual needs of the board, such as: the need for a critical layer wiring, power supply, and so the ground plane split to determine the layer arrangement, should not be applied mechanically, or grasped it tightly.

8, a plurality of ground inner layer can effectively reduce the grounding resistance. For example, A and B signals signal layer of each layer using a separate ground plane, can effectively reduce common mode interference.

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