Electronic Design - Preparation before PCB circuit board design

Preparation before PCB design

1. The accuracy of the schematic diagram. Including a complete schematic file and a netlist of the formal BOM with component codes.

The PCB package of all devices in the schematic (for components not in the package library, the hardware engineer should provide a data sheet or physical object and specify the order of pin definitions).

2. Provide general PCB layout diagrams or important units, core circuit layout, mounting hole positions, need to limit positioning components, restricted areas and other related information.

Design requirements: The designer must read the schematic diagram in detail, fully communicate with the project engineer, understand the circuit architecture, understand the working principle of the circuit, and have clear requirements for the layout and routing of key signals.

designing process

1. PCB document standard file naming rules: use the Numbering method to control the version of the PCB file.

The file name includes: project code board name-version number-date.

Notice

Project code: For different projects represented by internal numbers, such as Anwei -aw, several Len -sl, etc. Board name: Use English as a simple description.

For example, backplane-motherboard, panel-panel, etc. The version number uses two digits uniformly, namely V10, V11, V30 ......

If there is a change in the schematic diagram, the version upgrade will change the first number, such as V10-V20. If it is only a layout change, the version upgrade will change the second number, namely v10-v11, and so on.

Date: including year and month

The entire code can only contain numbers and letters, which are connected by underscores.

example:

Take Anwei backboard as an example, the file name is: AW-mainboard-v10-20100108

2. Determine the packaging of the components

Open the netlist and browse all packages to ensure that all components are packaged correctly, especially the package size, pin sequence, aperture size and hole type, and electrical properties (layer 25) must be consistent with the specifications in the data sheet, and the pad leads The feet should be considered slightly larger than the data sheet of the given size.

The package library and BOM of the components should be managed and maintained by specialized personnel to ensure uniform version.

3. Establish the PCB board frame

According to customer needs, determine the size of the frame and the location of the interface, as well as related information such as mounting holes, forbidden areas, and copper areas.

4. Download the network table

Load the net into the PCB and check the import report to ensure that all components are properly packaged.

5. Overlay settings

Factors to consider for lamination settings:

1. Stable, low noise, low AC impedance PDS (distribution system).

2. Transmission line structure requirements, microstrip line or strip line, whether there is coating, etc.

3. The characteristic impedance requirement of the transmission line.

4. Crosstalk noise suppression.

5. Absorb and shield space electromagnetic interference.

6. The structure is symmetrical to prevent deformation. The wiring density determines the number of signal layers.

The place with the highest wiring density is usually around the CPU. The number of pins in the CPU determines the number of signal layers that need to be used.

The thickness of the laminated copper and the thickness of the dielectric layer are determined by the impedance control, so it is necessary to use simulation software (such as Hyperlynx or SI9000) to calculate the stack parameters of single-ended impedance and ohmic differential impedance. Ohm, and determine the laminate design. Power supply and formation design: Design as much as possible to form the power supply and ground, and the thickness between the power supply and the battery is as thin as possible, which can provide a good decoupling capacitance distribution, which can greatly improve the signal integrity of the system and EMC, and form stability, PDS with low noise and low AC impedance.

The ground plane should be set on the layer directly adjacent to the PCB surface where the components are mounted, and the closer the ground plane is to the surface of the main PCB components (usually the surface layer), the lower the interconnect inductance will be.

PCB laminate design also needs to consider the degree of warpage of the board, that is, the laminate is designed to be as symmetrical as possible from top to bottom.

The general rules for high-speed digital design are:

1. Power layer + number of layers = number of signal layers

2. The power and ground are designed in pairs as much as possible, and at least one pair is a "back-to-back" design.

3. Try to use a stripline structure for wiring, with better EMC shielding, and a symmetrical stripline structure should be used for key signal transmission