1. Layout ideas
In the PCB layout process, the first consideration is the size of the PCB. Next, consider the devices and areas with structural positioning requirements, such as whether the height limit, width limit and punching, slotted areas. Then according to the circuit signal and power flow, pre-layout of each circuit module, and finally according to the design principles of each circuit module for the layout of all components work.
Basic principles of layout.
1) .Communicate with relevant personnel to meet special requirements in structure, SI, DFM, DFT, EMC.
2). Placement of connectors, mounting holes, indicators and other devices that need to be positioned according to the structural element diagram, and assigning immovable attributes to these devices and dimensioning them.
3). According to the structure element diagram and the special requirements of certain devices, set the no-wiring area and no-layout area.
4). Comprehensive consideration of PCB performance and the efficiency of processing to select the processing flow (priority use single-sided SMT; single-sided SMT + through hole components plug-in.
5). Double-sided SMT; double-sided SMT+ through hole components plug-in, according to the different processing characteristics to layout.
6). The layout refers to the results of pre-layout, according to the layout principle of "first big, then small, first difficult, then easy".
7). The layout should try to meet the following requirements: the total line as short as possible, the shortest key signal line; high voltage, high current signal and low voltage, small current signal weak signal completely separate; analog signal and digital signal separate; high frequency signal and low frequency signal separate; high frequency components spacing to be adequate. Local adjustment under the premise of meeting the requirements of simulation and timing analysis.
8). The same circuit part adopts symmetrical modular layout as far as possible.
9). Layout settings recommended grid for 50 mil, IC device layout, grid recommended for 25 25 25 25 25 mil. layout density is higher, small surface mount devices, grid settings recommended not less than 5 mil.
2. Special components of the layout principles
1). as much as possible to shorten the length of the connection between FM components. Susceptible to interference components can not be too close to each other, try to reduce their distribution parameters and mutual electromagnetic interference.
2). for the possible existence of higher potential difference between the device and the wire, should increase the distance between them to prevent accidental short circuit. Devices with strong electricity, try to arrange in places that are not easily accessible to the human body.
3). weight more than 15g components, should be added bracket fixed, and then welding. For large and heavy, heat-generating components should not be installed on the PCB, installed in the entire housing should consider the issue of heat dissipation, heat-sensitive devices should be far away from heat-generating devices.
4). for potentiometers, adjustable inductor coils, variable capacitors, micro switches and other adjustable components layout should consider the structural requirements of the machine, such as height limits, hole size, center coordinates, etc.
5). reserved PCB positioning holes and fixed bracket occupied by the location.
3. After layout inspection
Engineers need to strictly check the following after the layout is completed.
1. PCB size markings, whether the device layout is consistent with the structure drawings, whether it meets the PCB manufacturing process requirements, such as minimum hole size, minimum line width.
2. Whether the components interfere with each other in two-dimensional and three-dimensional space, and whether they will interfere with the structure housing.
Whether the components are all placed.
3. Need to often plug or replace the components are convenient plug and replace.
4. Whether there is a suitable distance between the thermal device and the heat-generating components.
5. Whether it is convenient to adjust the adjustable devices and press the button.
6. The location of the heat sink is installed whether the air is clear.
7. Whether the signal flow is smooth and the shortest interconnection.
8. Whether the problem of line interference has been considered.
9. Are the plugs and sockets contradictory to the mechanical design.