Detailed explanation of multilayer PCB laminate structure (3)

3 Middle layer creation and setting

The middle layer is the layer between the top and bottom layers of the PCB. Its structure is shown in Figure 11-1. Readers can refer to the labels in the figure for understanding. So how is the middle layer implemented during the production process? Simply put, multilayer boards are made by pressing multiple single-layer boards and double-layer boards, and the middle layer is the top or bottom layer of the original single-layer board and double-layer board. In the process of making a PCB board, firstly, a copper film needs to be applied to both sides of a base material (generally made of synthetic resin material), and then the connection relationship of the wires in the drawing is transferred to the board of the printed board by light drawing and other processes ( Protect the printed wires, pads and vias in the drawings to prevent the copper film in these parts from being corroded in the subsequent etching process, and then use chemical etching (with FeCl3 or H2O2 as the main component) Etching solution) Etch away the copper film without the protection of the film, and finally complete the post-processing such as drilling, printing silk screen layer, etc., so that a PCB is basically completed. Similarly, multi-layer PCB boards are pressed into a circuit board after multiple layers are completed, and in order to reduce costs and via interference, multi-layer PCB boards are often no better than double-layer boards and single-layer boards. How thick is this? Compared with ordinary double-layer and single-layer boards, the layers that make up multi-layer PCB boards tend to be smaller in thickness and lower in mechanical strength, resulting in higher processing requirements. Therefore, the manufacturing cost of multilayer PCB boards is much more expensive than that of ordinary double-layer boards and single-layer boards.

But because of the existence of the intermediate layer, the wiring of the multilayer board becomes easier, which is also the main purpose of selecting the multilayer board. However, in practical applications, multi-layer PCB boards have higher requirements for manual wiring, making designers need more help from EDA software; meanwhile, the existence of the middle layer allows power and signals to be transmitted in different board layers. The signal isolation and anti-interference performance will be better, and the large-area copper connection of the power supply and the ground network can effectively reduce the line impedance and reduce the ground potential shift caused by the common ground. Therefore, PCB boards with multi-layer board structure usually have better anti-interference performance than ordinary double-layer boards and single-layer boards.

3.1 Creation of the middle tier

The Protel system provides a special layer setting and management tool-Layer Stack Manager (Layer Stack Manager). This tool helps designers add, modify, and delete working layers, and define and modify layer properties. Select the [Design] / [Layer Stack Manager ...] command to pop up the layer stack manager property setting dialog box.

A 4-layer PCB board's layer stack manager interface. In addition to the top layer (TopLayer) and bottom layer (BottomLayer), there are two internal power layers (Power) and ground (GND). The positions of these layers are clearly shown in the figure. Double-click the name of the layer or click the Properties button to pop up the layer properties setting dialog box.

There are 3 options in this dialog box.

(1) Name: used to specify the name of the layer.

(2) Copper thickness: Specify the copper film thickness of this layer. The default value is 1.4mil. The thicker the copper film, the larger the current carrying capacity of the wires of the same width.

(3) Net name: Specify the network to which this layer is connected in the drop-down list. This option can only be used to set the internal electrical layer. This option is not available in the signal layer. If the internal electrical layer has only one network such as "+ 5V", you can specify the network name here; but if the internal electrical layer needs to be divided into several different areas, then do not specify the network name here.

There is also insulation material between the layers as a carrier of the circuit board or for electrical isolation. Among them, Core and Prepreg are insulating materials, but Core is a copper film and wires on both sides of the board, and Prepreg is only an insulating material used for interlayer isolation. The property setting dialog box of the two is the same. Double-click Core or Prepreg, or click the Properties button after selecting the insulation material to pop up the insulation layer property setting dialog box.

The thickness of the insulation layer is related to the withstand voltage between layers, signal coupling, and other factors. It has been introduced in the previous layer number selection and superposition principle. If there is no special requirement, the default value is generally selected.

In addition to the "Core" and "Prepreg" insulation layers, there are usually insulation layers on the top and bottom of the board. Click the selection box before Top Dielectric or Bottom Dielectric in the upper left corner of Figure 11-2 to select whether to display the insulation layer. Click the button next to it to set the properties of the insulation layer.

Below the options set by the top and bottom insulation layers there is a drop-down mode selection drop-down list, you can choose different stacking modes: Layer Pairs, Internal Layer Pairs and Build-up ). As mentioned earlier, multi-layer boards are actually pressed from multiple double-layer or single-layer boards. Selecting different modes means that different pressing methods are used in actual production. The "Core" and "Prepreg" The location is also different. For example, the layer-pair mode is a two-layer board sandwiching an insulating layer (Prepreg), and the internal electrical layer-pair mode is a two-layer board sandwiching a double-layer board. Usually use the default Layer Pairs mode.

There is a list of layer operation buttons on the right side of the layer stack manager property setting dialog box. The functions of each button are as follows.

(1) Add Layer: Add an intermediate signal layer. For example, if you need to add a high-speed signal layer between GND and Power, you should first select the GND layer. Click the Add Layer button, and a signal layer will be added under the GND layer. The default names are MidLayer1, MidLayer2, and so on. Double-click the layer name or click the Properties button to set the layer properties.

(2) Add Plane: Add internal electrical layer. The method of adding is the same as adding the intermediate signal layer. First select the location of the internal electrical layer you want to add, and then click this button to add the internal electrical layer below the specified layer. The default names are Internal Plane1, InternalPlane2, and so on. Double-click the layer name or click Properties The button can set the layer properties.

(3) Delete: delete a layer. Except for the top and bottom layers, which cannot be deleted, other signal layers and internal electrical layers can be deleted, but the intermediate signal layers that have been routed and the internal electrical layers that have been divided cannot be deleted. Select the layer to be deleted and click this button to pop up the dialog box shown in Figure 11-8. Click the Yes button to delete the layer.

(4) Move Up: Move up one layer. Select the layer that needs to be moved up (can be a signal layer or an internal electrical layer), click this button, the layer will move up one layer, but will not exceed the top layer.

(5) Move Down: Move down one layer. Similar to the Move Up button, click this button, the layer will move down one layer, but will not exceed the bottom layer.

(6) Properties: Properties button. Click this button to pop up the layer property setting dialog box.

3.2 Setting of the middle layer

After completing the relevant settings of the layer stack manager, click the OK button to exit the layer stack manager, and then perform related operations in the PCB editing interface. When operating the middle layer, you need to first set whether the middle layer is displayed in the PCB editing interface. Select the [Design] / [Options ...] command to pop up the option setting dialog box. Tick the internal layer options under Internal planes to display the internal layer.

After completing the settings, you can see the displayed layers below the PCB editing environment. With the mouse click on the board layer label to switch between different layers for operation. If you are not used to the default colors of the system, you can customize the colors of each layer by selecting the [Tools] / [Preferences ...] command.