The arrangement principle of PCB stackup design layer

- Nov 10, 2017-

Before the design of multilayer PCB circuit board, designers need to according to the size and scale of the circuit board circuit, electromagnetic compatibility (EMC) circuit board structure to determine the requirements of the, is decided to adopt 4 layers, 6 layers or more layers of circuit board. After determining the number of layers, determine the placement of the inner layer and how to distribute different signals on these layers. This is the choice of multilayer PCB stack structure.

Stack structure is an important factor that affects the EMC performance of PCB board, and it is also an important means to suppress electromagnetic interference. This paper introduces the related contents of stacked structure of multilayer PCB board.

For the power, ground number and the number of signal layers, the relative arrangement between them is a topic that every PCB engineer can't avoid.

General principles of arrangement of layers

1, it is necessary to consider many factors to determine the stacking structure of multilayer PCB plates. From the wiring, the more layers, the more conducive to wiring, but the cost and difficulty of the board will increase. For manufacturers, the symmetry of laminated structure is the focus of PCB manufacturing, so the selection of layers needs to consider the needs of various aspects in order to achieve the best balance. For experienced designers, after the completion of the pre layout of components, PCB wiring bottlenecks will focus on analysis. 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 layers of the entire circuit board is basically determined.

The following 2 components (the second layer) as a ground plane, providing device shielding layer and provide a reference plane for the top-level wiring; should be adjacent to an inner layer sensitive signal layer (internal power / ground), to provide shielding for the signal layer using a large copper film of the inner layer.  The high speed signal transmission layer in the circuit should be the signal intermediate layer and sandwiched between the two internal electrical layers. Thus, the copper film of the two inner layers can provide electromagnetic shielding for high speed signal transmission, and also effectively restrict the radiation of the high-speed signal between the two inner electric layers without causing external interference.

3. All signal layers are adjacent to ground plane as far as possible.

4, as far as possible to avoid the two signal layer directly adjacent; adjacent signal layer easy to introduce crosstalk, resulting in circuit failure. Adding 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, taking into account the symmetry of laminated structure.

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:

(1) the component surface and the welding surface are the whole horizon (shielding);

(2) there is no adjacent parallel routing layer;

(3) all signal layers are adjacent to the ground plane as far as possible;

(4) the key signal is adjacent to the stratum, without crossing the section.

Note: the specific PCB layer, to the above principles of flexibility, basic principles in the understanding of the above, according to the actual needs of the single board, such as: whether you need a key wiring layer, power supply, ground plane segmentation, the arrangement, all students move hard set, or pull live a little put.

8. Multiple grounding electric layers can effectively reduce the grounding impedance. For example, the A signal layer and the B signal layer adopt separate ground planes, which can effectively reduce the common mode interference.

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