Electronic OEM PCB Circuit Board
Before designing multi-layer PCB boards, designer should first determine the circuit board structure based on the circuit size and electromagnetic compatibility (EMC) requirements, that is, decide whether to use 4, 6 or more layers of PCB boards.After determining layers no, determine the placement of the inner layers and how the different signals are distributed on those layers.This is the problem of choosing multilayer PCB.
Laminated structure is an important factor to affect the EMC performance of PCB board, and also an important means to restrain electromagnetic interference.
After confirming the power sources, number of ground levels and signal levels, the relative arrangement between them is a topic that every PCB engineer cannot avoid.
General principles of layout:
1. More factors should be taken into account to determine the laminated structure of multi-layer PCB boards.In terms of circuits, the more layers, the better the lines, but the cost and difficulty of making boards will also increase.For manufacturers, whether the laminated structure is symmetrical or not is the focus of attention during PCB manufacturing, so the selection of layer nos needs to consider all aspects to achieve the best balance. For experienced designers, the bottleneck of PCB lines will be analyzed after the pre-layout of components is completed.Analyze the lines density of circuit board with other EDA tools;Then the number and types of signal lines with special circuits requirements, such as difference lines and sensitive signal lines, are integrated to determine the number of signal layers.The number of internal layers is then determined according to power supply type, isolation and anti-interference requirements.In this way, the number of board layers is basically confirmed.
2. Under component surface (the second layer) is the ground plane, providing the device shielding layer and providing the reference plane for the top layer circuits;The sensitive signal layer should be near to an inner electric layer (internal power source/stratum), using the large copper film of the inner electric layer to provide shielding for the signal layer.The high speed signal transmission layer in the circuit shall be the middle layer of the signal and sandwiched between the two inner electric 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, the radiation of high-speed signals can be effectively limited between the two inner electric layers, without external interference.
3. All signal layers are as close to the ground plane as possible;
4. Try to avoid the direct proximity of the two signal layers;It is easy to introduce crosstalk between adjacent signal layers, which leads to circuit failure.Adding ground plane between two signal layers can effectively avoid crosstalk.
5. The main power supply is as close to its corresponding location as possible;
6. The lamination structure is symmetrical.
7. For the layout of the motherboard, it is difficult for the existing motherboard to control the parallel long-distance lines, and for the work frequency of the plate-level above 50MHZ (under 50MHZ, it can be referred, and can be relaxed appropriately), the layout principle is suggested:
Component surface and welding surface are complete ground plane (shielding);
No adjacent parallel lines layer;
All signal layers are as close to the ground as possible;
Key signals are adjacent to the stratum and do not cross the partition zone.
Note: for the setting of the specific PCB layer, the above principles should be flexibly mastered based on requirements of the actual veneer, such as whether need a key circuit layer, power supply, and the segmentation of the ground plane, etc., the layer layout should be determined.
8. Multiple grounded internal electrical layers can effectively reduce grounding impedance. For example, A signal layer and B signal layer adopt separate ground planes, which can effectively reduce common mode interference.
Common cascading structures:
4 layer board
The following is an example of how to optimize the arrangement and combination of various laminated structures.
For common 4-layer board, there are several ways of layering (from top to bottom).
(1)Siganl_1(Top), GND(Inner_1), POWER(Inner_2), Siganl_2(Bottom).
(2)Siganl_1(Top), POWER(Inner_1), GND(Inner_2), Siganl_2(Bottom).
(3)POWER(Top), Siganl_1(Inner_1), GND(Inner_2), Siganl_2(Bottom).
Clearly, plan 3 lack of effective coupling between the power supply layer and the stratum should not be adopted.
So how to choose between plan 1 and 2?
Generally, the designer will choose plan 1 as the structure of the 4-layer PCB.That don't means plan 2 cannot be adopted, just becuause most PCB boards only place components at the top level, so plan 1 is more appropriate.
However, when the components need to be placed at both top and bottom layer, and the medium thickness between the internal power layer and the stratum is large and the coupling is poor, it is necessary to consider which layer has less signal lines.For scheme 1, there are few signal lines at the bottom, and a large area of copper film can be used to couple with the power layer.On the contrary, if the components are mainly arranged at the bottom, plan 2 should be selected to make the PCB.
If the cascade structure is adopted, the power layer and the ground layer are already coupled. Considering the requirement of symmetry, scheme 1 is generally adopted.
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