Layout Principle And Common Layer Structure Of PCB Laminated Design

Before designing multi-layer PCB circuit boards, the designer must first determine the circuit board structure according to the scale of the circuit, the size of the circuit boards and the electromagnetic compatibility (EMC) requirements, that is to say, the 4 layer, the 6 tier or the more layers of circuit boards are decided. After determining the number of layers, the position of the inner layer and how to distribute different signals on these layers are determined. This is the choice of multilayer PCB cascading structures. Stacked structure is an important factor affecting the performance of PCB board EMC, and is also an important means to suppress electromagnetic interference.


The relative placement between the power supply, the number of ground layers and the number of signal layers is a topic that every PCB engineer can't avoid.


The general principle of layer arrangement:


1. There are many factors to be taken into consideration in determining the laminated structure of the multilayer PCB board. In terms of wiring, the more layers are more conducive to wiring, but the cost and difficulty of the board will increase as well. For manufacturers, stacking structure symmetry is the focus that should be paid attention to when making PCB boards, so the choice of layers needs to consider all kinds of needs to achieve the best balance. For experienced designers, the PCB wiring bottlenecks will be focused on after the pre layout of the components is completed. The wiring density in combination with other EDA tools to analyze the circuit board; the number and variety of comprehensive special wiring required signal line such as differential and sensitive signal line to determine the signal layer number; then according to the type of power supply, isolation and anti-jamming requirements to determine the number of the inner layer. In this way, the number of the board layer of the whole circuit board is basically determined.


2, under the component surface (second level) is the ground plane, providing the device shielding layer and providing the reference plane for the top layer wiring. The sensitive signal layer should be adjacent to an inner layer (internal power supply / ground), and use the large copper film of the inner layer to provide shielding for the signal level. The high speed signal transmission layer in the circuit should be the middle layer of the signal and is sandwiched between the two inner layers. In this way, the two inner layers of copper film can provide electromagnetic shielding for high-speed signal transmission, and at the same time, it can effectively limit the radiation of high-speed signals to two internal electrical layers, and do not cause interference.


3. All signal layers are adjacent to the surface as far as possible.


4, avoid the two signal layer to be directly adjacent; the crosstalk is easy to be introduced between the adjacent signal layers, which leads to the failure of the circuit function. The addition of the ground plane between the two signal layers can effectively avoid crosstalk.

5, the main power supply is adjacent to it as much as possible.

6, the structure of laminating is symmetrical.

7, the motherboard motherboard existing layer arrangement, it is difficult to control the parallel wiring board for long distance, working at frequencies above 50MHZ (50MHZ can refer to the following situation, appropriate to relax), proposed arrangement principle:


The component surface and welding surface are complete flat surface (shielding).

No adjacent parallel wiring layer;

All signal layers are adjacent to the surface as far as possible.

The key signal is adjacent to the stratum and does not cross the division area.


8, Multiple Earthing inner electric layers can effectively reduce the grounding impedance. For example, the A signal layer and the B signal layer use their respective horizontal plane surface, which can effectively reduce the common mode interference.


 

(Source: internal information)