Allegro---cascading structure settings

 PCB Stacking Structure

Cascading structure is a very important issue, can not be ignored, the general choice of cascading structure to consider the following principles:

• Below the component surface (the second layer) is the ground plane, providing the device shielding layer and providing a reference plane for the top layer cabling;

• All signal layers are adjacent to the ground plane as far as possible;

• Try to avoid the two signal layers directly adjacent;

• The main power supply is as close as possible to its counterpart;

• Balance the symmetry of the laminated structure.

For the layer arrangement of the motherboard, the existing motherboard is difficult to control the parallel long-distance wiring, for the plate-level operating frequency of more than 50MHZ (under 50MHZ conditions can be referenced, appropriate relaxation), the proposed arrangement principle:

• Component surface, welded surface is a complete ground plane (shielding);

• No adjacent parallel routing layer;

• All signal layers are adjacent to the ground plane as far as possible;

• The key signal is adjacent to the stratum and does not span the partition area.

Based on the above principles, for a four-layer board, the preferred cascade structure should be:

· s← Signal

· g← Ground plane

· p← Power supply Layer

· s← Signal

For a six-layer board, the optimal stacking structure is:

· s1← Signal

· g1← Ground plane

· s2← Signal

· g2← Ground plane

· p← Power supply Layer

· s4← Signal

For a eight-layer board, there are two options:

Scenario 1: Scenario 2:

· s1← Signal s1← Signal

· g1← plane g1← Ground plane

· s2← Signal s2← Signal

· g2← Ground plane p1← power layer

· p← power layer g2← ground plane

· s3← Signal s3← Signal

· g3← Ground plane p2← power layer

· s4← Signal s4← Signal

Scenario 2 is mainly more than scenario 1 a power layer, in the case of more power supply can choose Scenario 2. For more layers of the structure also according to the above principles, can refer to other information.

The following is a SMDK6410 core board (designed to eight layers) to set up a cascading structure, including rules, PCB cabling and so on.

Open the program->cadence SPB 16.2->PCB Editor and open the PCB file in the 3rd chapter of the layout. Click the icon button on the toolbar, or select the Setup->cross-section menu, as shown in Figure 4.1.

cascading structure Settings

The Layout cross section dialog box pops up, as shown in Figure 4.2.

Layout Cross Section dialog box

Since the circuit board is created by hand, there are only top and bottom layers in the Corss section, and you need to manually add 6 layers and adjust the cascading structure. Right-click the sequence number in front of the subclass Name column to bring up a menu, as shown in Figure 4.3.

Add Layer

You can choose Add layer above to add a level above the layer, optionally add layer below to add a level below the layer, and optionally remove layer to delete it. A layer of isolation is also required between the route layers. The cascade structure of the last set eight-layer board is shown in Figure 4.4, and the cascade structure of Scenario 2 is used.

set up eight-layer laminate structure

The Subclass Name column is the title of the layer and can be filled in as needed. The type column selects the level of the layer, with three types:

· Conductor: Walk the line layer;

· PLANE: A plane layer, such as a GND plane;

· Dielectric: Dielectric layer, that is, the isolation layer.

The

    material column sets the material for this layer, which is usually set according to the information provided by the actual PCB board factory. Thickness set the thickness of the layer, if the line layer and the plane layer is the thickness of the copper skin. The CONDUCTIVTL is set to the resistivity of the copper peel. The dielectric layer of the dielectric constant column is normally used, and the parameters of the thickness column are the necessary parameters to calculate the impedance. Loss Tangent column sets the tangent loss of the dielectric layer. Negtive artwork Sets whether the layer is output negative as a negative,  indicates that the output negative,  indicates the output positive. In this board, POWER1 and GND2 are used in negative form. When set, click OK to close the dialog box.