Talking about PCB stack design

- Jul 22, 2016-

   Before the design of multi-layer PCB, designers need to first according to the size of the circuit, a circuit board size and electromagnetic compatible (EMC) requirements to determine the circuit board structure is decided by four layers, 6 layer or more layers of circuit board. Determine the number of layers, and then determine the position of the inner layer of the layer and how to distribute different signals on these layers. This is the choice of multilayer PCB stack structure.

   Stacked structure is one of the important factors that affect the performance of EMC board PCB, and it is also an important means to suppress electromagnetic interference.

This section will introduce the related content of multilayer PCB board.

For the power supply and ground layer and signal layers are identified, between them the relative arrangement is every PCB engineers can not avoid the topic.

The general principle of the layer arrangement:

   1, to determine the multi-layer PCB board structure needs to consider more factors. From the wiring, the more the number of layers is conducive to wiring, but the board cost and difficulty will be increased. For manufacturers, the laminated structure is symmetrical or not is the focus of attention of the PCB plate manufacturing, so the choice of layers need to consider all aspects of the demand, in order to achieve the best balance. For experienced designers, after the completion of the layout of the components, the PCB will be the bottleneck of the wiring analysis. Combined with other EDA tools to analyze the circuit board wiring density; comprehensive special wiring required signal lines such as poor junction, sensitive signal line and the number of species to determine the number of layers of the signal layer. Then depending on the type of power supply, isolation and anti interference the number to determine the electric layer. In this way, the number of the board layer of the whole circuit board is basically determined.

   2, element below the surface (the second layer) ground plane, providing device shielding layer and for top-level routing provides reference plane; sensitive signal layer should be with a layer of electrically adjacent (internal power / formation), using electric layer big copper film for signal layer to provide shielding. The high speed signal transmission layer in the circuit is supposed to be the middle layer of the signal, and is clamped between the two inner layers. The copper film of the two inner layer can provide electromagnetic shielding for the high speed signal transmission, and also can effectively limit the radiation of the high speed signal to the two inner electric layer without interference from the outside.

  3, all the signal layer as far as possible with the ground plane adjacent;

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

  5, the main power supply as close as possible to its corresponding;

  6, taking into account the laminated structure symmetry.

  7, for motherboard layer arrangement, existing motherboard is difficult to control parallel long distance wiring for the working frequency of the board level in more than 50MHz of (below 50MHz to refer, the appropriate relaxation), the proposed arrangement principle:

The element surface, the welding surface for complete ground plane (shield);

No adjacent parallel wiring layer;

   All the signal layer as far as possible with the adjacent ground plane;

The key signal and adjacent strata, not cross region segmentation.

Note: specific PCB layer settings, to carry the flexibility of the above principles, to comprehend the above principles based on, according to the actual demand for veneer, such as: is there a need for a key wiring layer, a power supply, a ground plane segmentation etc. determined the array, avoid by all means is born move hard set, pull or live on.

  8, multiple grounding of the inner layer can effectively reduce the grounding impedance. For example, the A signal layer and the B signal layer using the respective ground plane, can effectively reduce the common mode interference.

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