PCB design is not a random matter. There are many specifications that designers need to comply with. The following are some commonly used PCB design specifications collected by 2PCB, which are worth learning.
Basic principles of layout
1. Communicate with relevant personnel to meet special requirements in structure, SI, DFM, DFT, and EMC.
2. According to the structural element diagram, place connectors, mounting holes, indicator lights and other devices that need to be positioned, assign immovable attributes to these devices, and mark the dimensions.
3. Set prohibited wiring areas and prohibited layout areas according to the structural element diagram and the special requirements of certain devices.
4. Comprehensively consider PCB performance and processing efficiency to select the process flow (single-sided SMT; single-sided SMT + plug-in; double-sided SMT; double-sided SMT + plug-in), and layout according to different processing technology characteristics.
5. When laying out, refer to the results of the pre-layout, and follow the layout principle of “first big, then small, first difficult, then easy”.
6. The layout should try to meet the following requirements: the total wiring should be as short as possible, and the key signal lines should be the shortest; high voltage and large current signals should be completely separated from low voltage and small current signals; analog signals should be separated from digital signals; high frequency signals should be separated. Signals should be separated from low-frequency signals; high-frequency components should be adequately spaced. Make local adjustments while meeting the requirements of simulation and timing analysis.
7. Use symmetrical modular layout for the same circuit parts as much as possible.
8. The recommended grid for layout settings is 50 mil. For IC device layout, the recommended grid is 25 25 25 25 mil. When the layout density is high, for small surface mount devices, the grid setting is recommended to be no less than 5mil.
9. When laying out, consider the location of the fanout and test points, move with the center point of the device as a reference, and consider running two traces between the two vias.
PCB wiring general rules to follow
1. The number of printed wire wiring layers is determined as needed. The wiring occupied channel ratio should generally be above 50%;
2. According to the process conditions and wiring density, the wire width and wire spacing should be reasonably selected, and the wiring within the layer should be uniform and the wiring density of each layer should be similar. If necessary, auxiliary non-functional connecting pads or printed wires should be added to the missing wire area;
3. Two adjacent layers of wires should be laid out as oblique or bent paths perpendicular to each other to reduce parasitic capacitance;
4. Printed wire wiring should be as short as possible, especially for high-frequency signals and highly sensitive signal lines; for important signal lines such as clocks, equal delay wiring should also be considered if necessary;
5. When multiple power supplies (layers) or grounds (layers) are laid out on the same layer, the separation distance should be no less than 1mm;
6. For large-area conductive patterns larger than 5×5mm2, windows should be partially opened;
7. Thermal isolation design should be carried out between the large-area graphics of the power layer and ground layer and their connection pads.
Trace line width/line spacing
1. The recommended line width/space for PCB processing is ≥5mil/5mil, and the minimum usable line width/space is 4mil/4mil.
2. The distance between the trace and the pad: The distance between the outer trace and the pad is consistent with the distance between the inner trace and the annular ring.
3. The distance between the outer trace and the pad must be ≥ 2 mil from the edge of the solder mask window.
Safe distance for wiring
1. The distance between the trace and the edge of the board is >20mil. The distance between the inner power/ground layer and the edge of the board is >20mil.
2. The distance between the grounding bus wire and the grounding copper foil must be >20mil from the edge of the board.
3. No wiring is allowed in areas where metal shells (such as radiators, power modules, metal handles, horizontal voltage regulators, crystal oscillators, ferrite inductors, etc.) are in direct contact with the PCB. The contact area between the device metal shell and the PCB extends outward, and the 1.5mm area is the forbidden area for surface wiring.
General principles for multilayer PCB layer arrangement
1. The ground plane below the device surface (the second layer) provides a device shielding layer and a reference plane for device surface wiring;
2. All signal layers should be as adjacent to the ground plane as possible;
3. Try to avoid two signal layers being directly adjacent;
4. The main power supply should be adjacent to it as much as possible;
5. In principle, symmetrical structure design should be adopted. The meaning of symmetry includes: dielectric layer thickness and type, copper foil thickness, graphics
Symmetry of distribution type (large copper foil layer, line layer).
General requirements for silk screen design
1. In order to ensure that all letters, numbers and symbols are easy to identify on the PCB, the line width of the silk screen must be greater than 5mil, and the height of the silk screen must be at least 50mil.
2. Silk screen printing is not allowed to overlap with pads or reference points.
3. White is the default screen printing ink color. If there are special requirements, they need to be stated in the PCB drilling diagram file.
4. In high-density PCB design, the silk screen content can be selected according to needs.
5. The arrangement direction of silk screen strings is from left to right and from bottom to top.
