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PCB design signal layer planning

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Update time : 2021-06-08 13:49:37
A planned PCB design is naturally more convincing. The number of layers of the PCB board is generally not determined in advance, and the engineer will give a plan based on the overall board situation. The total number of layers is composed of the number of signal layers plus the number of power ground layers.
  1. Planning of the number of signal layers
The routing channel is usually an important factor in determining the number of signal layers. First of all, it is necessary to know whether there is a relatively deep BGA and connector on the board. The depth of the BGA and the PIN pitch of the BGA are the key to determining the number of BGA outlet layers. For example, two wires can pass between 1.0mm BGA vias and only one wire can pass between 0.8mm BGA vias. There is a big difference in the number of wiring layers between the two. The connector mainly considers its depth, basically a pair of differential lines pass between two vias.
Secondly, consider the wiring channels of high-speed signals on the board. Because there are many conditions required for high-speed signal processing, factors such as stubs, wiring spacing, and reference planes need to be considered. Therefore, it is necessary to give priority to whether the wiring channels are sufficient.
The last is the planning of the bottleneck area. After the basic layout is dealt with, the narrower bottleneck area needs to be focused on. Comprehensive consideration of differential lines, sensitive signal lines, special signal topologies, etc., to specifically calculate how many lines can go out of the bottleneck area, and how many layers can pass all the required lines through this area.

  1. Planning of power supply and number of stratums
The number of power layers is mainly determined by the number of power types, distribution, current-carrying capacity, performance indicators of the single board, and the cost of the single board. The setting of the power plane needs to meet two conditions: the power supplies are not interlaced with each other; and the important signals of adjacent layers are prevented from being divided across.
When setting the number of ground layers, you need to pay attention to the following points: the second layer corresponding to the main device surface should have a relatively complete ground plane; important signals such as high-speed, high-frequency, and clock should refer to the ground plane; the main power supply and the ground plane are tightly coupled , Reduce the power plane impedance and so on.
Taking the above two points into consideration, basically there will be no cases where some lines will not work, temporarily add layers, and then modify them on a large scale, which will waste time and cost. In our actual assessment, it may also be necessary to consider the influence of processing and plate thickness.

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