Layout means placing the components on the board reasonably based on comprehensive consideration of signal quality, EMC, thermal design, DFM, DFT, structure, safety regulations and other requirements. In high-speed PCB design, reasonable layout is the first step to successful PCB design. Next, 2PCB will introduce the ideas and principles of PCB layout to you, which are all practical information~
Layout ideas
During the PCB layout process, the first thing to consider is the size of the PCB. Secondly, consider the devices and areas with structural positioning requirements, such as whether there are height limits, width limits, punching, and slotting areas. Then, according to the circuit signal and power flow, pre-layout each circuit module, and finally layout all components according to the design principles of each circuit module.
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, and give these devices immovable attributes and dimension them.
3. According to the structural element diagram and the special requirements of certain devices, set prohibited wiring areas and prohibited layout areas.
4. Comprehensively consider the PCB performance and processing efficiency to select the process processing flow (preferably 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. Refer to the results of pre-layout during layout, and follow the layout principle of “big first, small later, difficult first, easy later”.
6. The layout should try to meet the following requirements: the total connection should be as short as possible, and the key signal line should be the shortest; high voltage, high current signals and weak signals of low voltage, low current signals should be completely separated; analog signals should be separated from digital signals; high frequency signals should be separated from low frequency signals; the spacing of high frequency components should be sufficient. Under the premise of meeting the requirements of simulation and timing analysis, make local adjustments.
7. Symmetric modular layout should be used as much as possible for the same circuit part.
8. The recommended grid for layout setting is 50mil, and the recommended grid for IC device layout 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
Layout principles for special components
1. Shorten the length of the connection between FM components as much as possible. Components susceptible to interference should not be too close to each other, and try to reduce their distribution parameters and mutual electromagnetic interference.
2. For devices and wires that may have a high potential difference, the distance between them should be increased to prevent accidental short circuits. Devices with strong electricity should be placed in places that are not easily touched by the human body.
3. Components weighing more than 15g should be fixed with brackets and then welded. Large, heavy and high-heat components should not be installed on PCBs. Heat dissipation should be considered when installed on the whole machine casing. Thermistors should be kept away from heating devices.
4. The layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, micro switches, etc. should consider the structural requirements of the whole machine, such as height limit, hole size, center coordinates, etc.
5. Reserve the position occupied by PCB positioning holes and fixing brackets.
Post-layout inspection
In PCB design, reasonable layout is the first step to successful PCB design. Engineers need to strictly check the following contents after the layout is completed:
1. PCB size marking, whether the device layout is consistent with the structural drawing, and whether it meets the PCB manufacturing process requirements, such as the minimum aperture and the minimum line width.
2. Do the components interfere with each other in two-dimensional and three-dimensional space, and will they interfere with the structural shell?
3. Have all the components been placed?
4. Are the components that need to be frequently plugged in or replaced convenient to plug in and replace?
5. Is there a suitable distance between thermistors and heat-generating components?
6. Is it convenient to adjust adjustable components and press buttons?
7. Is the location where the radiator is installed unobstructed?
8. Is the signal flow unobstructed and the interconnection shortest?
9. Has the line interference problem been considered?
10. Are the plugs and sockets inconsistent with the mechanical design?