Section 3 multi-board design principles

In this chapter and the previous chapters, we have already emphasized some principles that need to be followed in PCB design. Here we will summarize these principles, it can be used as a reference for readers during design and also as a reference for post-design check.

1. PCB component library requirements

(1) The encapsulation of the components used on the PCB must be correct, including the size and size of the components pins, pin spacing, pin numbers, frame size and direction representation.

(2) Polarity components (electrolytic capacitor, diode, transistor, etc.) Positive and negative poles or PIN numbers should be marked in the PCB Component Library and PCB Board.

(3) the PIN numbers of components in the PCB library should be consistent with those of the schematic components, for example, the problem of inconsistent PIN numbers in components of the diode PCB library and those in the principle library is described in the previous chapter.

(4) components that need to use heat sink should be taken into account when drawing the components for encapsulation. The components and heat sink can be drawn together into the form of overall encapsulation.

(5) the pins of the component and the diameter of the pad must match. The diameter of the pad must be slightly larger than the Pin Size of the component for installation.
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2. PCB Component layout requirements

(1) components are evenly arranged. Components of the same functional module should be arranged as close as possible.
(2) components using the same type of power supply and local network should be arranged together as much as possible, which is conducive to completing the electrical connection between each other through the inner electrical layer.
(3) The interface components should be placed by the side and the interface type should be indicated with a string. The wiring direction should usually leave the circuit board.
(4) power conversion components (such as transformer, DC/DC converter, and three-end voltage regulator) should have enough space for heat dissipation.
(5) The pin or reference point of the component should be placed on the grid point, which is conducive to wiring and aesthetics.
(6) The filter capacitor can be placed on the back of the chip, close to the power supply and ground pins of the chip.

(7) The first pin of the component or the logo direction should be indicated on the PCB and cannot be covered by the component.
(8) The component label should be close to the component border, with uniform size and neat direction. It does not overlap with the pad and the pass-through, and cannot be placed in the area covered after the component is installed.

3. PCB cabling requirements

(1) The power supply of different voltage levels should be isolated, and the power supply line should not cross.
(2) A 45 ° corner or an arc corner is used for cabling. a corner with a sharp angle is not allowed.
(3) The PCB is directly connected to the center of the pad. The width of the wire connecting to the pad cannot exceed the outer diameter of the pad.
(4) The line width of the high-frequency signal line is no less than 20 mil. It is enclosed by a ground wire and isolated from other ground wires.
(5) do not wiring the bottom of the interference source (DC/DC converter, crystal oscillator, transformer, etc.) to avoid interference.
(6) use the power cord and ground wire as much as possible. The width of the power cord shall not be less than 50mil if space permits.
(7) low voltage, low current signal line width 9 ~ 30mil, which is as bold as possible when space is allowed.
(8) the spacing between signal lines should be greater than 10mil, and the gap between power lines should be greater than 20mil.
(9) the width of the large current signal line must be greater than 40mil, and the spacing should be greater than 30mil.
(10) The minimum size of the passing hole is 40mil, and the diameter is 28mil. When connecting the top and bottom layers with wires, the pad is preferred.
(11) signal lines cannot be arranged on the inner electrical layer.
(12) The interval width between different areas of the inner electrical layer is not less than 40mil.
(13) Try not to allow the border through the pad of the area to be connected when drawing the border.
(14) copper is laid on the top and bottom layers. We recommend that you set a line width value greater than the grid width to completely cover free space without dead copper, and keep it 30mil (0.762) with other lines) the above spacing (you can set the safe spacing before applying copper, and change it back to the original safe spacing value after applying copper ).
(15) after the wiring is completed, the solder tray will be treated with tears.
(16) External grounding of metal shell devices and modules.
(17) pad for installation and welding.
(18) Check whether DRC is correct.

4. Requirements for PCB layering

(1) The power supply plane should be close to the ground plane, closely coupled with the ground plane, and arranged below the ground plane.
(2) The signal layer should be adjacent to the inner electrical layer and should not be directly adjacent to other signal layers.
(3) isolate the digital and analog circuits. If conditions permit, the analog and digital signal lines are arranged hierarchically and shielded. If the analog signal lines need to be arranged on the same signal layer, the isolation band and ground line must be used to reduce interference; the power and ground of analog and digital circuits should be isolated from each other and cannot be mixed.
(4) the high-frequency circuit has great external interference. It is best to arrange it separately. The upper and lower layers have an inner electrical layer directly adjacent intermediate signal layer for transmission, so that the copper film of the inner electrical layer can be used to reduce external interference.