1. Pay attention to the definition of pin and package name when making the schematic library
When making the package library, pay attention to the corresponding principle diagram pin; If the pins do not correspond, the components will be isolated when the PCB diagram is obtained.
2. Any routing on the PCB will cause time delay to the high frequency signal when it passes through it. The main function of serpentine routing is to compensate the part with smaller delay in the “same set of related” signal lines, which usually have no or less logic processing than other signals. The most typical one is the clock line, usually it does not need to go through any other logic processing, so its delay will be less than other related signals.
Because different applications have different effects, if the snake line in the computer board, it mainly plays a filtering inductor role, improve the anti-interference ability of the circuit, computer motherboard in the snake line, mainly used in some clock signals, such as PCIClk,AGPClk, its role has two points: 1, impedance matching; 2. 2. Filter inductance. The use of snaking line is helpful to improve the stability of the motherboard and the graphics card, to eliminate the inductance phenomenon caused by the long direct wiring when the current passes through, and to reduce the crosstalk between lines, which is especially obvious at high frequencies.
3. When wave soldering production process is adopted for the surface mount components, the axial resistance and capacitance components should be perpendicular to the transmission direction of wave soldering, and the axial resistance and SOP(PIN spacing greater than or equal to 1.27mm) components should be parallel to the transmission direction. Active components such as IC, SOJ, PLCC and QFP whose axial spacing is less than 1.27mm(50mil) should avoid using wave soldering.
4. The distance between BGA and adjacent components is 5mm >. The distance between other SMT components is 0.7mm >. The outer distance between the soldering pad of the mounting element and the adjacent mounting element is greater than 2mm; For PCB with pressing parts, no mounting elements or devices can be found within 5mm around the pressing connector, and no mounting elements or devices can be found within 5mm around the welding surface.
5. The layout of IC decoupling capacitor should be as close as possible to the power pin of IC, and the circuit formed between it and the power source and the ground should be the shortest.
6. In the layout of components, appropriate consideration should be given to the devices that use the same power source as far as possible, so as to facilitate the separation of future power sources.
7. The layout of resistance containers for impedance matching purposes shall be reasonably arranged according to their properties. The layout of the series matching resistor should be close to the driving end of the signal and the distance is generally not more than 500mil. The layout of the matching resistor and capacitor must distinguish the source end and the terminal of the signal, and the matching of the multi-load terminal must be at the farthest end of the signal.
8. After the layout is completed, print out the assembly drawing for the schematic diagram designer to check the correctness of device packaging, and confirm the signal correspondence relationship between veneer, backboard and connector. After confirmation, wiring can be started.
9. Set wiring constraints
1) report design parameters
After the layout is basically determined, the statistical function of PCB design tool is applied to report basic parameters such as network number, network density, average pin density, etc., so as to determine the required number of signal wiring layers.
The determination of the number of signal layers can be referred to the following empirical data:
Pin density, signal layer number, plate layer number
Note: PIN density is defined as: plate area (in square inches)/(total number of pins on the board /14)
10. Setting of holes
Thread hole: the definition of the minimum aperture of the plate shall depend on the thickness of the plate, and the thickness to aperture ratio shall be less than 5-8.
The preferred aperture series is as follows:
Aperture: 24mil 20mil 16mil 12mil 8mil
Pad diameter: 40mil 35mil 28mil 25mil 20mil
Inner hot pad size: 50mil 45mil 40mil 35mil 30mil
Relationship between plate thickness and minimum aperture:
Plate thickness: 3.0mm 2.5mm 2.0mm 1.6mm 1.0mm
Minimum aperture: 24mil 20mil 16mil 12mil 8mil
11. Blind hole and buried hole:
Blind hole is a guide hole that connects the surface layer and inner layer but does not connect the whole plate, and buried hole is a guide hole that connects the inner layer but is not visible on the surface of finished plate. The size setting of these two types of holes can be referred to as a cross line hole. Be fully aware of PCB process when applying blind hole and buried-hole design to avoid unnecessary problems in PCB process, and consult with PCB supplier when necessary.
Test hole: test hole refers to the through hole used for ICT testing, which can also be used as the conducting hole. In principle, the hole diameter is not limited, the diameter of the welding plate should not be less than 25mil, and the center distance between test holes should not be less than 50mil. It is not recommended to use component welding holes as test holes.